| 0/0 |
2026/05/31 15:04 |
flow |
assessment-security |
1h06m
Results: map[DenialOfService:true Explanation:**Bug Analysis:**
The bug report indicates a hung task in `blk_trace_setup`, specifically blocked
while trying to acquire `q->debugfs_mutex`.
Looking at the reproducer and the kernel source code, the issue is triggered by
calling the `BLKTRACESETUP` ioctl on a block device (in this case,
`/dev/loop2`). This ioctl allows the user to configure block tracing and specify
the size (`buf_size`) and number (`buf_nr`) of the relay buffers to be
allocated.
In `blk_trace_setup_prepare`, the kernel calls `relay_open` to allocate these
buffers. The only validation performed on the size is ensuring that `buf_size *
buf_nr` does not exceed `UINT_MAX` (4 GB). If a user provides values that
maximize this product (e.g., `buf_size = 0x100100` and `buf_nr = 0xfff` as seen
in the reproducer), `relay_open` will attempt to allocate nearly 4 GB of memory
**per CPU**. On a standard 2-CPU system, this is 8 GB; on a 64-CPU server, this
would be 256 GB.
The memory is allocated page-by-page in a loop using `alloc_page(GFP_KERNEL |
__GFP_ZERO)`. Allocating millions of pages takes a significant amount of time
and induces massive memory pressure (potentially triggering the OOM killer).
Crucially, this allocation happens while the task is holding the
`q->debugfs_mutex`. Any other tasks attempting to perform operations on the
block device that require this mutex (like other tracing setups, or
registering/unregistering queues) will be blocked indefinitely, eventually
triggering the hung task watchdog.
**Security Impact Assessment:**
* **Exploitable:** No. This is a pure resource exhaustion issue. There is no
memory corruption, use-after-free, or out-of-bounds access. If the allocation
eventually fails, the kernel gracefully frees the already allocated pages and
returns an error. Even on 32-bit systems where `PAGE_ALIGN` could theoretically
overflow to 0, the underlying `vmap` implementation safely rejects 0-sized
allocations and returns `NULL`.
* **Denial Of Service:** Yes. A user can easily exhaust system memory and cause
deadlocks by holding the `debugfs_mutex` for extended periods, leading to system
instability and hung tasks.
* **Accessible From Unprivileged Processes:** No. Triggering this bug requires
calling an ioctl on a block device node. By default, block device nodes (like
`/dev/loop2`, `/dev/sda`) are owned by `root:disk` with `rw-rw----` permissions.
A typical unprivileged user does not have the permissions required to open these
device nodes.
* **Accessible From User Namespaces:** No. User namespaces do not grant access
to block device nodes on the host. Even if a user namespace is created, device
nodes retain their original permissions and cannot be opened or recreated by
unprivileged users.
* **Other Triggers:** The bug cannot be triggered via network traffic, remote
connections, peripherals, or malicious filesystems. It strictly requires a local
process with sufficient privileges to open a block device node and issue the
specific ioctl. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/05/31 15:04 |
action |
syz-repro-to-c-repro |
0m
Results: map[SimplifiedCRepro:// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#ifndef __NR_close_range
#define __NR_close_range 436
#endif
uint64_t r[1] = {0xffffffffffffffff};
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffffffff000ul, /*len=*/0x1000ul, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200000000000ul, /*len=*/0x1000000ul, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200001000000ul, /*len=*/0x1000ul, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
intptr_t res = 0;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// openat$auto_def_blk_fops_fs arguments: [
// fd: const = 0xffffffffffffff9c (8 bytes)
// file: ptr[in, buffer] {
// buffer: {2f 64 65 76 2f 6c 6f 6f 70 32 00} (length 0xb)
// }
// flags: open_flags = 0x24040 (4 bytes)
// mode: const = 0x0 (2 bytes)
// ]
// returns fd_def_blk_fops_fs
memcpy((void*)0x200000000140, "/dev/loop2\000", 11);
res = syscall(__NR_openat, /*fd=*/0xffffffffffffff9cul, /*file=*/0x200000000140ul, /*flags=O_NOFOLLOW|O_DIRECT|O_CREAT*/0x24040, /*mode=*/0);
if (res != -1)
r[0] = res;
// ioctl$auto_BLKTRACESETUP arguments: [
// fd: fd_def_blk_fops_fs (resource)
// cmd: const = 0xc0481273 (4 bytes)
// arg: ptr[inout, blk_user_trace_setup$auto] {
// blk_user_trace_setup$auto {
// name: buffer: {ef 65 ce 6c 00 cf 81 00 00 00 ff ff ff ff ff ff ff 29 1d 00 00 00 00 07 00 00 00 00 00 03 00 00} (length 0x20)
// act_mask: int16 = 0x3ff (2 bytes)
// pad = 0x0 (2 bytes)
// buf_size: int32 = 0x100100 (4 bytes)
// buf_nr: int32 = 0xfff (4 bytes)
// pad = 0x0 (4 bytes)
// start_lba: int64 = 0x400004 (8 bytes)
// end_lba: int64 = 0x200000000040000d (8 bytes)
// pid: pid (resource)
// pad = 0x0 (4 bytes)
// }
// }
// ]
memcpy((void*)0x2000000000c0, "... [truncated large byte array] ...", 32);
*(uint16_t*)0x2000000000e0 = 0x3ff;
*(uint32_t*)0x2000000000e4 = 0x100100;
*(uint32_t*)0x2000000000e8 = 0xfff;
*(uint64_t*)0x2000000000f0 = 0x400004;
*(uint64_t*)0x2000000000f8 = 0x200000000040000d;
*(uint32_t*)0x200000000100 = 0;
syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc0481273, /*arg=*/0x2000000000c0ul);
// socket arguments: [
// domain: socket_domain = 0x2 (8 bytes)
// type: socket_type = 0x1 (8 bytes)
// proto: int32 = 0x106 (4 bytes)
// ]
// returns sock
syscall(__NR_socket, /*domain=AF_INET*/2ul, /*type=SOCK_STREAM*/1ul, /*proto=*/0x106);
// openat$auto_kernfs_file_fops_kernfs_internal arguments: [
// fd: const = 0xffffffffffffff9c (8 bytes)
// file: nil
// flags: open_flags = 0x121681 (4 bytes)
// mode: const = 0x0 (2 bytes)
// ]
// returns fd_kernfs_file_fops_kernfs_internal
syscall(__NR_openat, /*fd=*/0xffffffffffffff9cul, /*file=*/0ul, /*flags=O_TRUNC|O_SYNC|O_NOFOLLOW|O_EXCL|O_APPEND|O_WRONLY*/0x121681, /*mode=*/0);
// close_range$auto arguments: [
// fd: fd (resource)
// max_fd: fd (resource)
// flags: int32 = 0x0 (4 bytes)
// ]
syscall(__NR_close_range, /*fd=*/2, /*max_fd=*/8, /*flags=*/0);
// socket arguments: [
// domain: socket_domain = 0x2 (8 bytes)
// type: socket_type = 0x3 (8 bytes)
// proto: int32 = 0x100 (4 bytes)
// ]
// returns sock
syscall(__NR_socket, /*domain=AF_INET*/2ul, /*type=SOCK_RAW*/3ul, /*proto=*/0x100);
// socket arguments: [
// domain: socket_domain = 0x10 (8 bytes)
// type: socket_type = 0x2 (8 bytes)
// proto: int32 = 0x0 (4 bytes)
// ]
// returns sock
syscall(__NR_socket, /*domain=AF_NETLINK*/0x10ul, /*type=SOCK_DGRAM*/2ul, /*proto=*/0);
// sendmsg$auto_NL80211_CMD_GET_REG arguments: [
// fd: sock_nl_generic (resource)
// msg: nil
// f: send_flags = 0x800 (8 bytes)
// ]
syscall(__NR_sendmsg, /*fd=*/(intptr_t)-1, /*msg=*/0ul, /*f=MSG_CONFIRM*/0x800ul);
// sendmsg$auto_NL80211_CMD_GET_REG arguments: [
// fd: sock_nl_generic (resource)
// msg: nil
// f: send_flags = 0x4004 (8 bytes)
// ]
syscall(__NR_sendmsg, /*fd=*/(intptr_t)-1, /*msg=*/0ul, /*f=MSG_NOSIGNAL|MSG_DONTROUTE*/0x4004ul);
// openat$auto_console_fops_tty_io arguments: [
// fd: const = 0xffffffffffffff9c (8 bytes)
// file: ptr[in, buffer] {
// buffer: {2f 64 65 76 2f 74 74 79 30 00} (length 0xa)
// }
// flags: open_flags = 0x102 (4 bytes)
// mode: const = 0x0 (2 bytes)
// ]
// returns fd_console_fops_tty_io
memcpy((void*)0x200000000800, "/dev/tty0\000", 10);
syscall(__NR_openat, /*fd=*/0xffffffffffffff9cul, /*file=*/0x200000000800ul, /*flags=O_NOCTTY|O_RDWR*/0x102, /*mode=*/0);
return 0;
}
]
|
| 2/1 |
2026/05/31 15:04 |
action |
kernel-checkouter |
1m
Results: map[KernelSrc:/app/workdir/cache/src/467792fa1488290b3edd7716e095bf26d236d119]
|
| 3/1 |
2026/05/31 15:06 |
action |
kernel-builder |
28m
Results: map[KernelObj:/app/workdir/cache/build/918fa5a4d880dbafd59281ac42644fe10965448d]
|
| 4/1 |
2026/05/31 15:34 |
action |
codesearch-prepare |
12m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/05/31 15:47 |
agent |
expert |
23m
Model:
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Instruction: You are an experienced Linux kernel security engineer. Your task is to analyze given kernel bug report
and determine its security impact based on the following dimensions.
Use the provided tools to examine the source code, check for capability checks (e.g., capable(), ns_capable()),
and understand the nature of the bug. Analyze the given kernel build and configuration.
You can check the kernel config by grepping ".config" file; you can check kernel cmdline by grepping
".config" file for "CONFIG_CMDLINE=". Assume sysctl parameters have default values.
But analyze for the corresponding production build w/o debugging tools enabled (like KASAN, KMSAN, UBSAN).
Try different strategies when analyzing the bug:
- think of ways in which the vulnerable code is unreachable
- or the other way around: try to come up with different ideas of how an unprivileged user can reach the bug
If still unsure err on the side of the bug being non-exploitable/not-accessible.
In the final reply, provide a reasoning for your assessment.
Analysis dimensions:
* Exploitable:
Determine if the bug can result in memory corruption or elevated privileges.
Memory safety issues are almost always exploitable (KASAN or UBSAN reports for use-after-free, out-of-bounds;
refcounting issues, corrupted lists, etc). When kernel is crashing on a completely wild pointer access
(e.g. user-space address, or non-canonical address, but not on NULL or address corresponding to KASAN shadow
for NULL address), including both data accesses and control transfers, that also usually implies possibility
of exploitation. Such reports usually say "unable to handle kernel paging request".
Uses of uninitialized values detected by KMSAN may be exploitable b/c attacker frequently can affect uninit
values with spraying techniques. However, for these exploitability depends on how exactly the uninit value
is used in the code, and what it affects.
Think of what happens after the bug is triggered. Some bugs cause kernel panic and halt execution,
they are harder to exploit. For example, BUG reports halts the kernel. However, WARNING reports don't halt
execution in production builds. Debug bug detection tools (like KASAN, KMSAN, KCSAN, UBSAN) are also not enabled
in production builds, so attacker can freely exploit these bugs w/o being detected by these tools.
If you see an integer overflow, think how the overflowed value used later (if it's used as allocation size,
or an array index). If you see an out-of-bounds read, think if it's followed by an out-of-bounds write as well.
Some KCSAN data-races may be exploitable by skilled attackers as well. Think what data structures got corrupted
as the result of data races and how. However, note that kernel has lots of "benign" data races that don't lead
to any runtime misbehavior at all.
* Denial Of Service:
Determine if the bug can result in denial-of-service. Most bugs can, since they cause system crash,
hangs, deadlocks, or resource leaks. This is mostly applicable to WARNING bugs that won't cause system crash
in production. For these think what will be consequences of the violation of the kernel assumptions flagged
by the WARNING. In some cases the unexpected condition is also properly handled by the normal control flow
(e.g. with "if (WARN_ON(...))"), these won't cause denial-of-service. If the condition is not handled,
then it may or may not cause denial-of-service.
* Accessible From Unprivileged Processes:
Determine if the bug can be reached from a typical (non-root) user process that does NOT have any special capabilities
(like CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON) or access to device nodes restricted to root.
Assume that unprivileged_bpf_disabled=1, that is eBPF loading is not accessible. However, cBPF (classical BPF)
is still accessible to non-root processes.
Assume that user namespaces are not accessible, that is, the process cannot get the mentioned capabilities even
within a new user namespace (checked by ns_capable() function in the kernel sources).
* Accessible From User Namespaces:
Determine if the bug can be reached within a user-namespace where the process has all capabilities
(including CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON). Such capabilities are checked with ns_capable()
function in the kernel sources.
* VM Guest Trigger:
Determine if the bug can be triggered from the context of a typical KVM guest (e.g., set up by a QEMU VMM).
Consider accesses to standard Linux host paravirtualized features (virtio-blk, virtio-net, etc.),
and handling of VM exits in the KVM code.
* VM Host Trigger in The Confidential Computing Context:
Determine if the bug can be triggered in a confidential computing guest kernel from the context of a KVM host.
Consider access to standard Linux guest paravirtualized features (virtio-blk, virtio-net, etc.).
* Ethernet Network Trigger:
Determine if the bug can be triggered by processing ingress network Ethernet traffic, either directly (network stack)
or via drivers exposed to network data.
* Other Remote Trigger:
Determine if the bug can be triggered by processing remote traffic other than Ethernet (Wifi, Bluetooth, NFC, etc).
* Peripheral Trigger:
Determine if the bug can be triggered via an untrusted peripheral device that can be physically plugged
into a system, such as a USB device or a niche hardware driver handling external hardware inputs.
This is particularly important for mobile and desktop environments where users can plug in unknown devices.
* Malicious Filesystem Trigger:
Determine if the bug can be triggered by the kernel mounting and parsing a malicious filesystem image.
This is highly critical for Desktop and Mobile environments where external media or downloaded images
might be auto-mounted.
Don't make assumptions about the kernel source code (it may be different from what you assume it is).
Extensively use the provided code access tools (codesearch-*, git-*, grepper, etc)
to examine the actual source code, and confirm any assumptions.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The kernel bug report is:
INFO: task syz.5.22:6267 blocked for more than 143 seconds.
Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.5.22 state:D stack:28808 pid:6267 tgid:6266 ppid:6205 task_flags:0x480040 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5388 [inline]
__schedule+0x1295/0x67a0 kernel/sched/core.c:7189
__schedule_loop kernel/sched/core.c:7268 [inline]
schedule+0xdd/0x390 kernel/sched/core.c:7283
schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7340
__mutex_lock_common kernel/locking/mutex.c:726 [inline]
__mutex_lock+0xced/0x1b10 kernel/locking/mutex.c:820
blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
blk_debugfs_lock block/blk.h:770 [inline]
blk_trace_setup+0x1f2/0x440 kernel/trace/blktrace.c:788
blk_trace_ioctl+0x245/0x320 kernel/trace/blktrace.c:937
blkdev_ioctl+0x17c/0x6f0 block/ioctl.c:786
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x840 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f15b679ce59
RSP: 002b:00007f15b76cb028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f15b6a15fa0 RCX: 00007f15b679ce59
RDX: 00002000000000c0 RSI: 00000000c0481273 RDI: 0000000000000003
RBP: 00007f15b6832d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f15b6a16038 R14: 00007f15b6a15fa0 R15: 00007ffd4ebfd548
</TASK>
Showing all locks held in the system:
3 locks held by kworker/0:0/9:
#0: ffff88813fe57140 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x12d6/0x1980 kernel/workqueue.c:3289
#1: ffffc900000e7d08 ((work_completion)(&data->fib_event_work)){+.+.}-{0:0}, at: process_one_work+0x973/0x1980 kernel/workqueue.c:3290
#2: ffff888022a8f250 (&data->fib_lock){+.+.}-{4:4}, at: nsim_fib_event_work+0x1b8/0x63b0 drivers/net/netdevsim/fib.c:1490
4 locks held by kworker/u8:1/13:
1 lock held by khungtaskd/31:
#0: ffffffff8e7e53e0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#0: ffffffff8e7e53e0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#0: ffffffff8e7e53e0 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x3d/0x184 kernel/locking/lockdep.c:6775
2 locks held by getty/5396:
#0: ffff8880381500a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x24/0x80 drivers/tty/tty_ldisc.c:243
#1: ffffc9000322b2e8 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x419/0x14f0 drivers/tty/n_tty.c:2211
3 locks held by syz-execprog/5676:
7 locks held by syz.4.21/6202:
1 lock held by syz.5.22/6267:
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock block/blk.h:770 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_trace_setup+0x1f2/0x440 kernel/trace/blktrace.c:788
1 lock held by syz.6.23/6424:
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock block/blk.h:770 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_trace_setup+0x1f2/0x440 kernel/trace/blktrace.c:788
1 lock held by syz.7.24/6470:
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock block/blk.h:770 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_trace_setup+0x1f2/0x440 kernel/trace/blktrace.c:788
1 lock held by syz.8.25/6579:
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock block/blk.h:770 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_trace_setup+0x1f2/0x440 kernel/trace/blktrace.c:788
1 lock held by syz.9.26/6691:
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock block/blk.h:770 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_trace_setup+0x1f2/0x440 kernel/trace/blktrace.c:788
1 lock held by syz.0.27/6772:
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock block/blk.h:770 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_trace_setup+0x1f2/0x440 kernel/trace/blktrace.c:788
1 lock held by syz.1.28/7026:
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock block/blk.h:770 [inline]
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_trace_setup+0x1f2/0x440 kernel/trace/blktrace.c:788
1 lock held by syz-executor/7188:
#0: ffffffff8e7f0f28 (rcu_state.exp_mutex){+.+.}-{4:4}, at: exp_funnel_lock+0x27f/0x3c0 kernel/rcu/tree_exp.h:311
4 locks held by dhcpcd-run-hook/7288:
=============================================
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 31 Comm: khungtaskd Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
nmi_cpu_backtrace.cold+0x12d/0x151 lib/nmi_backtrace.c:113
nmi_trigger_cpumask_backtrace+0x1d7/0x230 lib/nmi_backtrace.c:62
trigger_all_cpu_backtrace include/linux/nmi.h:162 [inline]
__sys_info lib/sys_info.c:157 [inline]
sys_info+0x141/0x190 lib/sys_info.c:165
check_hung_uninterruptible_tasks kernel/hung_task.c:353 [inline]
watchdog+0xcb1/0x1030 kernel/hung_task.c:561
kthread+0x370/0x450 kernel/kthread.c:436
ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 6590 Comm: kworker/u8:3 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
Workqueue: events_unbound cfg80211_wiphy_work
RIP: 0010:__sanitizer_cov_trace_switch+0x3b/0x90 kernel/kcov.c:351
Code: 53 48 8b 46 08 48 83 f8 20 74 63 77 48 48 83 f8 08 74 63 41 be 03 00 00 00 48 83 f8 10 75 29 4c 8b 6d 00 31 db 4d 85 ed 74 1e <48> 8b 74 dd 10 48 8b 4c 24 28 4c 89 e2 4c 89 f7 48 83 c3 01 e8 8c
RSP: 0018:ffffc90002ea75a8 EFLAGS: 00000212
RAX: 0000000000000000 RBX: 000000000000000d RCX: ffffffff8b0b3032
RDX: 0000000000000032 RSI: 000000000000005a RDI: ffff888034371f00
RBP: ffffffff8d1cdfe0 R08: 0000000000000001 R09: 000000000000005a
R10: 0000000000000032 R11: ffff8881bf1c6000 R12: 0000000000000032
R13: 0000000000000020 R14: 0000000000000001 R15: ffff8881d7610d8d
FS: 0000000000000000(0000) GS:ffff88812438a000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f47854ac4e8 CR3: 000000022f922000 CR4: 00000000003526f0
Call Trace:
<TASK>
_ieee802_11_parse_elems_full+0x2f2/0x4d60 net/mac80211/parse.c:355
ieee802_11_parse_elems_full+0x1175/0x37f0 net/mac80211/parse.c:1120
ieee802_11_parse_elems net/mac80211/ieee80211_i.h:2518 [inline]
ieee80211_rx_mgmt_probe_beacon net/mac80211/ibss.c:1565 [inline]
ieee80211_ibss_rx_queued_mgmt+0xc8d/0x2f80 net/mac80211/ibss.c:1596
ieee80211_iface_process_skb net/mac80211/iface.c:1795 [inline]
ieee80211_iface_work+0xbff/0x13e0 net/mac80211/iface.c:1849
cfg80211_wiphy_work+0x410/0x570 net/wireless/core.c:513
process_one_work+0xa0e/0x1980 kernel/workqueue.c:3314
process_scheduled_works kernel/workqueue.c:3397 [inline]
worker_thread+0x5ef/0xe50 kernel/workqueue.c:3478
kthread+0x370/0x450 kernel/kthread.c:436
ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
It is reproducible with the followint program.
Keep in mind that it may lack the precise threading, sandboxing, and some arguments of a working reproducer.
But it should give an idea of the involved syscalls.
// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#ifndef __NR_close_range
#define __NR_close_range 436
#endif
uint64_t r[1] = {0xffffffffffffffff};
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffffffff000ul, /*len=*/0x1000ul, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200000000000ul, /*len=*/0x1000000ul, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200001000000ul, /*len=*/0x1000ul, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
intptr_t res = 0;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// openat$auto_def_blk_fops_fs arguments: [
// fd: const = 0xffffffffffffff9c (8 bytes)
// file: ptr[in, buffer] {
// buffer: {2f 64 65 76 2f 6c 6f 6f 70 32 00} (length 0xb)
// }
// flags: open_flags = 0x24040 (4 bytes)
// mode: const = 0x0 (2 bytes)
// ]
// returns fd_def_blk_fops_fs
memcpy((void*)0x200000000140, "/dev/loop2\000", 11);
res = syscall(__NR_openat, /*fd=*/0xffffffffffffff9cul, /*file=*/0x200000000140ul, /*flags=O_NOFOLLOW|O_DIRECT|O_CREAT*/0x24040, /*mode=*/0);
if (res != -1)
r[0] = res;
// ioctl$auto_BLKTRACESETUP arguments: [
// fd: fd_def_blk_fops_fs (resource)
// cmd: const = 0xc0481273 (4 bytes)
// arg: ptr[inout, blk_user_trace_setup$auto] {
// blk_user_trace_setup$auto {
// name: buffer: {ef 65 ce 6c 00 cf 81 00 00 00 ff ff ff ff ff ff ff 29 1d 00 00 00 00 07 00 00 00 00 00 03 00 00} (length 0x20)
// act_mask: int16 = 0x3ff (2 bytes)
// pad = 0x0 (2 bytes)
// buf_size: int32 = 0x100100 (4 bytes)
// buf_nr: int32 = 0xfff (4 bytes)
// pad = 0x0 (4 bytes)
// start_lba: int64 = 0x400004 (8 bytes)
// end_lba: int64 = 0x200000000040000d (8 bytes)
// pid: pid (resource)
// pad = 0x0 (4 bytes)
// }
// }
// ]
memcpy((void*)0x2000000000c0, "... [truncated large byte array] ...", 32);
*(uint16_t*)0x2000000000e0 = 0x3ff;
*(uint32_t*)0x2000000000e4 = 0x100100;
*(uint32_t*)0x2000000000e8 = 0xfff;
*(uint64_t*)0x2000000000f0 = 0x400004;
*(uint64_t*)0x2000000000f8 = 0x200000000040000d;
*(uint32_t*)0x200000000100 = 0;
syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc0481273, /*arg=*/0x2000000000c0ul);
// socket arguments: [
// domain: socket_domain = 0x2 (8 bytes)
// type: socket_type = 0x1 (8 bytes)
// proto: int32 = 0x106 (4 bytes)
// ]
// returns sock
syscall(__NR_socket, /*domain=AF_INET*/2ul, /*type=SOCK_STREAM*/1ul, /*proto=*/0x106);
// openat$auto_kernfs_file_fops_kernfs_internal arguments: [
// fd: const = 0xffffffffffffff9c (8 bytes)
// file: nil
// flags: open_flags = 0x121681 (4 bytes)
// mode: const = 0x0 (2 bytes)
// ]
// returns fd_kernfs_file_fops_kernfs_internal
syscall(__NR_openat, /*fd=*/0xffffffffffffff9cul, /*file=*/0ul, /*flags=O_TRUNC|O_SYNC|O_NOFOLLOW|O_EXCL|O_APPEND|O_WRONLY*/0x121681, /*mode=*/0);
// close_range$auto arguments: [
// fd: fd (resource)
// max_fd: fd (resource)
// flags: int32 = 0x0 (4 bytes)
// ]
syscall(__NR_close_range, /*fd=*/2, /*max_fd=*/8, /*flags=*/0);
// socket arguments: [
// domain: socket_domain = 0x2 (8 bytes)
// type: socket_type = 0x3 (8 bytes)
// proto: int32 = 0x100 (4 bytes)
// ]
// returns sock
syscall(__NR_socket, /*domain=AF_INET*/2ul, /*type=SOCK_RAW*/3ul, /*proto=*/0x100);
// socket arguments: [
// domain: socket_domain = 0x10 (8 bytes)
// type: socket_type = 0x2 (8 bytes)
// proto: int32 = 0x0 (4 bytes)
// ]
// returns sock
syscall(__NR_socket, /*domain=AF_NETLINK*/0x10ul, /*type=SOCK_DGRAM*/2ul, /*proto=*/0);
// sendmsg$auto_NL80211_CMD_GET_REG arguments: [
// fd: sock_nl_generic (resource)
// msg: nil
// f: send_flags = 0x800 (8 bytes)
// ]
syscall(__NR_sendmsg, /*fd=*/(intptr_t)-1, /*msg=*/0ul, /*f=MSG_CONFIRM*/0x800ul);
// sendmsg$auto_NL80211_CMD_GET_REG arguments: [
// fd: sock_nl_generic (resource)
// msg: nil
// f: send_flags = 0x4004 (8 bytes)
// ]
syscall(__NR_sendmsg, /*fd=*/(intptr_t)-1, /*msg=*/0ul, /*f=MSG_NOSIGNAL|MSG_DONTROUTE*/0x4004ul);
// openat$auto_console_fops_tty_io arguments: [
// fd: const = 0xffffffffffffff9c (8 bytes)
// file: ptr[in, buffer] {
// buffer: {2f 64 65 76 2f 74 74 79 30 00} (length 0xa)
// }
// flags: open_flags = 0x102 (4 bytes)
// mode: const = 0x0 (2 bytes)
// ]
// returns fd_console_fops_tty_io
memcpy((void*)0x200000000800, "/dev/tty0\000", 10);
syscall(__NR_openat, /*fd=*/0xffffffffffffff9cul, /*file=*/0x200000000800ul, /*flags=O_NOCTTY|O_RDWR*/0x102, /*mode=*/0);
return 0;
}
Reply: **Bug Analysis:**
The bug report indicates a hung task in `blk_trace_setup`, specifically blocked while trying to acquire `q->debugfs_mutex`.
Looking at the reproducer and the kernel source code, the issue is triggered by calling the `BLKTRACESETUP` ioctl on a block device (in this case, `/dev/loop2`). This ioctl allows the user to configure block tracing and specify the size (`buf_size`) and number (`buf_nr`) of the relay buffers to be allocated.
In `blk_trace_setup_prepare`, the kernel calls `relay_open` to allocate these buffers. The only validation performed on the size is ensuring that `buf_size * buf_nr` does not exceed `UINT_MAX` (4 GB). If a user provides values that maximize this product (e.g., `buf_size = 0x100100` and `buf_nr = 0xfff` as seen in the reproducer), `relay_open` will attempt to allocate nearly 4 GB of memory **per CPU**. On a standard 2-CPU system, this is 8 GB; on a 64-CPU server, this would be 256 GB.
The memory is allocated page-by-page in a loop using `alloc_page(GFP_KERNEL | __GFP_ZERO)`. Allocating millions of pages takes a significant amount of time and induces massive memory pressure (potentially triggering the OOM killer). Crucially, this allocation happens while the task is holding the `q->debugfs_mutex`. Any other tasks attempting to perform operations on the block device that require this mutex (like other tracing setups, or registering/unregistering queues) will be blocked indefinitely, eventually triggering the hung task watchdog.
**Security Impact Assessment:**
* **Exploitable:** No. This is a pure resource exhaustion issue. There is no memory corruption, use-after-free, or out-of-bounds access. If the allocation eventually fails, the kernel gracefully frees the already allocated pages and returns an error. Even on 32-bit systems where `PAGE_ALIGN` could theoretically overflow to 0, the underlying `vmap` implementation safely rejects 0-sized allocations and returns `NULL`.
* **Denial Of Service:** Yes. A user can easily exhaust system memory and cause deadlocks by holding the `debugfs_mutex` for extended periods, leading to system instability and hung tasks.
* **Accessible From Unprivileged Processes:** No. Triggering this bug requires calling an ioctl on a block device node. By default, block device nodes (like `/dev/loop2`, `/dev/sda`) are owned by `root:disk` with `rw-rw----` permissions. A typical unprivileged user does not have the permissions required to open these device nodes.
* **Accessible From User Namespaces:** No. User namespaces do not grant access to block device nodes on the host. Even if a user namespace is created, device nodes retain their original permissions and cannot be opened or recreated by unprivileged users.
* **Other Triggers:** The bug cannot be triggered via network traffic, remote connections, peripherals, or malicious filesystems. It strictly requires a local process with sufficient privileges to open a block device node and issue the specific ioctl.
|
| 6/2 |
2026/05/31 15:47 |
llm |
expert |
0m
Model:
Tokens:
input: 12735
output: 38
thoughts: 656
Thoughts: **Analyzing a Hung Task in the Block Layer**
Okay, so I've got this hung task report β "INFO: task syz.5.22:6267 blocked for more than 143 seconds." The task is stuck, and from the report, it looks like it's blocked within `blk_trace_setup`, specifically trying to grab `q->debugfs_mutex`.
First, I need to look at the stack trace to confirm my hunch: `__schedule`, `schedule`, mutex locking, right down to `blk_trace_setup`. Yep, it's clear: `blk_trace_setup` is where it's stalling, trying to get that mutex. Now I need to see *why* that mutex contention is happening in this scenario.
The reproducer shows a syscall sequence. It appears the process is first opening `/dev/loop2` with a few flags, then performs an `ioctl` on the opened file descriptor. The second call uses the `BLKTRACESETUP` ioctl. `BLKTRACESETUP` translates to `_IOWR(0x12,115,struct blk_user_trace_setup)`. So, the application is setting up a block trace on that loop device.
Okay, let's go directly to the source. I need to examine `blk_trace_setup` in `kernel/trace/blktrace.c`. I'll look at the locking strategy within that function to understand why it might be blocking. This is likely a mutex contention issue, now I need to figure out the exact sequence of events that triggers it.
|
| 7/2 |
2026/05/31 15:47 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/blktrace.c Name:blk_trace_setup]
Results: map[SourceCode: 762: int blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
763: struct block_device *bdev,
764: char __user *arg)
765: {
766: struct blk_user_trace_setup2 buts2;
767: struct blk_user_trace_setup buts;
768: struct blk_trace *bt;
769: unsigned int memflags;
770: int ret;
771:
772: ret = copy_from_user(&buts, arg, sizeof(buts));
773: if (ret)
774: return -EFAULT;
775:
776: if (!buts.buf_size || !buts.buf_nr)
777: return -EINVAL;
778:
779: buts2 = (struct blk_user_trace_setup2) {
780: .act_mask = buts.act_mask,
781: .buf_size = buts.buf_size,
782: .buf_nr = buts.buf_nr,
783: .start_lba = buts.start_lba,
784: .end_lba = buts.end_lba,
785: .pid = buts.pid,
786: };
787:
788: memflags = blk_debugfs_lock(q);
789: bt = blk_trace_setup_prepare(q, name, dev, buts.buf_size, buts.buf_nr,
790: bdev);
791: if (IS_ERR(bt)) {
792: blk_debugfs_unlock(q, memflags);
793: return PTR_ERR(bt);
794: }
795: blk_trace_setup_finalize(q, name, 1, bt, &buts2);
796: strscpy(buts.name, buts2.name, BLKTRACE_BDEV_SIZE);
797: blk_debugfs_unlock(q, memflags);
798:
799: if (copy_to_user(arg, &buts, sizeof(buts))) {
800: blk_trace_remove(q);
801: return -EFAULT;
802: }
803: return 0;
804: }
SourceFile:kernel/trace/blktrace.c]
|
| 8/2 |
2026/05/31 15:47 |
llm |
expert |
0m
Model:
Tokens:
input: 14498
output: 35
thoughts: 518
Thoughts: **Analyzing a Potential Race Condition in blk_trace_setup**
Okay, let's break this down. My initial thought was: is `blk_trace_setup` calling `blk_debugfs_lock(q)`? And, critically, could it be doing so concurrently, leading to a block on `blk_debugfs_lock(q)`? The report strongly suggests this.
The output confirms this suspicion. Specifically, the report states that `syz.5.22/6267` holds `q->debugfs_mutex`. That seems problematic. But, wait, is it actually *blocked* while holding the mutex? I need to clarify that.
Looking again, the report snippet says `syz.5.22/6267` holds `q->debugfs_mutex`, at `blk_debugfs_lock_nomemsave`, but it appears blocked. This part is a bit confusing. The stack trace shows it's blocked in `__mutex_lock_common` which is where the mutex acquisition happens, which seems to imply it's *trying* to acquire it.
Then, there's a moment of clarity. `lockdep` only reports *held* locks. So, if it's blocked trying to acquire a lock, that lock won't be listed as held. Unless... unless this is some sort of recursive locking scenario? No, the report doesn't suggest that. It's simply holding the lock and appears blocked at the acquisition point, which is very strange.
The obvious next step is to examine the `blk_debugfs_lock` function. I need to understand exactly how it interacts with the mutex. This is where the root cause probably lies.
|
| 9/2 |
2026/05/31 15:47 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:block/blk.h Name:blk_debugfs_lock]
Results: map[SourceCode: 766: static inline unsigned int __must_check blk_debugfs_lock(struct request_queue *q)
767: {
768: unsigned int memflags = memalloc_noio_save();
769:
770: blk_debugfs_lock_nomemsave(q);
771: return memflags;
772: }
SourceFile:block/blk.h]
|
| 10/2 |
2026/05/31 15:47 |
llm |
expert |
0m
Model:
Tokens:
input: 15549
output: 39
thoughts: 16
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Analyzing `blk_debugfs_lock_nomemsave`**
Okay, let's dive into this `blk_debugfs_lock_nomemsave` function. Given the naming convention, my initial hunch is that this is related to a debugfs interface, specifically a lock implementation designed to avoid memory allocation or saving. The "no memsave" suggests that this lock likely doesn't store any data or state that would need saving, or that its state is intrinsically transient.
I immediately start thinking about potential use cases. Is this used in contexts where memory allocation is highly constrained? Perhaps within an interrupt handler or during critical boot sequences? Does this suggest a specific type of lock implementation like a spinlock or a simpler atomic operation? Or, does it refer to a lock for debugfs entries, where the debugfs data itself is transient, and the lock is just for access serialization?
I need to quickly understand the design choices behind this. Iβd want to know where this function is used and how it interfaces with other locking mechanisms in the block layer, especially if itβs interacting with other debug mechanisms or more traditional locks. Are there any potential for lock-ordering issues, especially in error paths or under specific conditions that involve other locking primitives? I want to check for potential race conditions due to the non-memory-saving aspect. Whereβs the actual implementation? Is it a thin wrapper, or is there some interesting logic I need to parse? I have to look at the surrounding code to determine how it fits into the overall locking strategy for debugfs and its interaction with the block layerβs memory management. This would provide insight into the potential performance implications and scalability. I am curious to see if there are any comments or documentation in the code about its intended use and any known caveats.
|
| 11/2 |
2026/05/31 15:47 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:block/blk.h Name:blk_debugfs_lock_nomemsave]
Results: map[SourceCode: 750: /*
751: * debugfs directory and file creation can trigger fs reclaim, which can enter
752: * back into the block layer request_queue. This can cause deadlock if the
753: * queue is frozen. Use NOIO context together with debugfs_mutex to prevent fs
754: * reclaim from triggering block I/O.
755: */
756: static inline void blk_debugfs_lock_nomemsave(struct request_queue *q)
757: {
758: mutex_lock(&q->debugfs_mutex);
759: }
SourceFile:block/blk.h]
|
| 12/2 |
2026/05/31 15:47 |
llm |
expert |
0m
Model:
Tokens:
input: 16186
output: 34
thoughts: 838
Thoughts: **Analyzing the Mutex Hold**
Okay, so I see a mutex lock is blocking, specifically `q->debugfs_mutex`. The first question that pops into my head is, who actually *holds* this lock right now? I need to figure out which other tasks have a grip on it.
Looking at the output, it seems multiple tasks, syz.4.21, syz.5.22, etc., all have `q->debugfs_mutex` listed as held. That's immediately suspect. I know how lockdep *should* work β it's supposed to identify locks that are currently held.
But hold on, is lockdep reporting what tasks *are trying* to acquire a lock as if they *already* hold it? No, that can't be right. If a task is blocked on a mutex, it technically isn't holding the lock. I need to be sure. My initial understanding is that lockdep shows locks that *are held*, not those being *attempted*.
Wait a minute, how can several tasks all appear to be holding the same mutex? That's impossible for a regular mutex. If it was a read-write lock, then multiple readers could hold it. But `debugfs_mutex` is a plain `mutex`. Something's wrong with the interpretation here.
The lockdep output says:
```
1 lock held by syz.5.22/6267:
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
```
Could lockdep be showing it as held if a task is in the process of acquiring it? Does lockdep add the lock to the held list *before* actually calling `__mutex_lock_common`? I need to check the source code of `mutex_lock` to understand the exact sequence of events.
|
| 13/2 |
2026/05/31 15:48 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:kernel/locking/mutex.c Name:mutex_lock]
Results: map[SourceCode: SourceFile:]
|
| 14/2 |
2026/05/31 15:48 |
llm |
expert |
0m
Model:
Tokens:
input: 17510
output: 26
thoughts: 28
Thoughts: Okay, let's dive into this. Here's how I'd approach that kernel locking scenario, and the thought process that goes along with it:
**Tracing a Mutex Lock**
Alright, I need to understand how this mutex lock is behaving. Given my experience, the first place I'd go to investigate would be within the kernel source code itself. My initial thought is to zero in on `__mutex_lock_common` or, if that isn't the primary entry point, the higher-level wrapper `mutex_lock`. Both are going to be fundamental. I'm pretty certain they're going to reside in `kernel/locking/mutex.c`. That's where I would expect to find the core implementation and understand the critical path. From there, I'll be able to work backward, understand how this specific lock is being used in the context of the running kernel.
|
| 15/2 |
2026/05/31 15:48 |
tool |
grepper |
0m
Args: map[Expression:void __sched mutex_lock\(struct mutex \*lock\)]
Results: map[Output:kernel/locking/mutex.c=290=static void __sched __mutex_lock_slowpath(struct mutex *lock)
--
kernel/locking/mutex.c-313- */
kernel/locking/mutex.c:314:void __sched mutex_lock(struct mutex *lock)
kernel/locking/mutex.c-315-{
--
kernel/locking/rtmutex_api.c=611=EXPORT_SYMBOL(mutex_rt_init_generic);
kernel/locking/rtmutex_api.c-612-
kernel/locking/rtmutex_api.c:613:void __sched mutex_lock(struct mutex *lock)
kernel/locking/rtmutex_api.c-614-{
]
|
| 16/2 |
2026/05/31 15:48 |
llm |
expert |
1m
Model:
Tokens:
input: 17939
output: 35
thoughts: 19
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Diving into the Mutex Implementation**
Okay, so I want to refresh my memory on the inner workings of `mutex_lock`, specifically the implementation. Let's start by digging into `kernel/locking/mutex.c`. I need to understand the fundamental locking mechanism at play here, not just at a conceptual level, but down to the nitty-gritty details of how it interacts with the scheduler, wait queues, and potentially, preemption. I'm looking for any clever optimizations or potential gotchas that might be lurking within the code. Are they using spinlocks under the hood? What are the implications of the different mutex flags and what edge cases are being handled (contention, priority inheritance, etc.)? I'm anticipating possibly encountering some interesting code that will need careful thought. I also want to make sure the implementation hasn't changed drastically over the recent kernel releases.
|
| 17/2 |
2026/05/31 15:49 |
tool |
read-file |
0m
Args: map[File:kernel/locking/mutex.c FirstLine:314 LineCount:20]
Results: map[Contents: 314: void __sched mutex_lock(struct mutex *lock)
315: {
316: might_sleep();
317:
318: if (!__mutex_trylock_fast(lock))
319: __mutex_lock_slowpath(lock);
320: }
321: EXPORT_SYMBOL(mutex_lock);
322: #endif
323:
324: #include "ww_mutex.h"
325:
326: #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
327:
328: /*
329: * Trylock variant that returns the owning task on failure.
330: */
331: static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock)
332: {
333: return __mutex_trylock_common(lock, false);
]
|
| 18/2 |
2026/05/31 15:49 |
llm |
expert |
0m
Model:
Tokens:
input: 18472
output: 31
thoughts: 13
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience and written in the first person:
**Diving into `__mutex_lock_slowpath`**
Okay, so we're hitting a slow path. Let's see what's happening under the hood in `__mutex_lock_slowpath`. I need to understand why this isn't a fast path acquisition. Is there contention? Are we dealing with something more complex than a straightforward, uncontended lock? My initial thought is to quickly evaluate the likely causes, like the mutex already being held, possibly by another process or task. Is the caller blocked? We need to determine the blocking mechanism used. I'll need to trace the code flow, looking for likely scenarios like spinning, or, more likely, a wait queue involved. Then, let's look at the implementation's specifics: the use of atomic operations, wait queues. I will check any relevant flags and state variables associated with the mutex to identify the bottlenecks. I want to build a clear mental model of the sequence of operations within this path, from the initial check to the point of potentially sleeping or resuming, in order to diagnose where the performance issues lie.
|
| 19/2 |
2026/05/31 15:49 |
tool |
grepper |
0m
Args: map[Expression:static void __sched __mutex_lock_slowpath\(struct mutex \*lock\)]
Results: map[Output:kernel/locking/mutex.c=263=static void __mutex_handoff(struct mutex *lock, struct task_struct *task)
--
kernel/locking/mutex.c-289- */
kernel/locking/mutex.c:290:static void __sched __mutex_lock_slowpath(struct mutex *lock)
kernel/locking/mutex.c-291- __acquires(lock);
]
|
| 20/2 |
2026/05/31 15:49 |
llm |
expert |
0m
Model:
Tokens:
input: 18878
output: 35
thoughts: 13
Thoughts: Okay, here's my interpretation of that request, formatted as requested:
**Diving into the `__mutex_lock_slowpath` Code**
Alright, let's get into this. I'm taking a look at `__mutex_lock_slowpath`. I immediately recognize this name; it implies a more involved locking scenario, likely when the straightforward, optimized path isn't enough. It tells me we're dealing with contention or some form of complex state, perhaps involving sleeping or queuing. I'm expecting to see the core logic for handling a mutex lock when the initial attempt fails β the fallback path, if you will. I anticipate this function will contain the mechanisms for waiting, managing waiters, and ensuring fairness, or perhaps even implementing some form of priority inheritance. I'll be looking closely at any interactions with the scheduler or wait queues. I need to understand how it deals with potential race conditions and any optimizations made to reduce latency in congested scenarios. The devil's always in the details, so I'll be meticulously examining the use of any atomic operations, spinlocks, or other synchronization primitives within its implementation to ensure they are correct. I'm interested in how it interacts with the underlying architecture as well. I hope to come away with a comprehensive understanding of how a mutex is implemented in a challenging context.
|
| 21/2 |
2026/05/31 15:49 |
tool |
read-file |
0m
Args: map[File:kernel/locking/mutex.c FirstLine:290 LineCount:20]
Results: map[Contents: 290: static void __sched __mutex_lock_slowpath(struct mutex *lock)
291: __acquires(lock);
292:
293: /**
294: * mutex_lock - acquire the mutex
295: * @lock: the mutex to be acquired
296: *
297: * Lock the mutex exclusively for this task. If the mutex is not
298: * available right now, it will sleep until it can get it.
299: *
300: * The mutex must later on be released by the same task that
301: * acquired it. Recursive locking is not allowed. The task
302: * may not exit without first unlocking the mutex. Also, kernel
303: * memory where the mutex resides must not be freed with
304: * the mutex still locked. The mutex must first be initialized
305: * (or statically defined) before it can be locked. memset()-ing
306: * the mutex to 0 is not allowed.
307: *
308: * (The CONFIG_DEBUG_MUTEXES .config option turns on debugging
309: * checks that will enforce the restrictions and will also do
]
|
| 22/2 |
2026/05/31 15:49 |
llm |
expert |
0m
Model:
Tokens:
input: 19539
output: 27
thoughts: 24
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Examining `__mutex_lock_slowpath`**
Okay, so I'm looking at this code and immediately my mental parsing process latches onto `__mutex_lock_slowpath`. Hmmm, it's just *declared* here, isn't it? My intuition immediately kicks in β it's likely defined further down the file, or maybe even in a separate compilation unit. I need to keep that in mind as I follow the execution path. Definitely won't find the meat here, and will need to chase down its definition to understand its function. I'm already anticipating the typical slowpath strategies that this suggests.
|
| 23/2 |
2026/05/31 15:49 |
tool |
grepper |
0m
Args: map[Expression:__mutex_lock_slowpath\(struct mutex \*lock\)]
Results: map[Output:kernel/locking/mutex.c=263=static void __mutex_handoff(struct mutex *lock, struct task_struct *task)
--
kernel/locking/mutex.c-289- */
kernel/locking/mutex.c:290:static void __sched __mutex_lock_slowpath(struct mutex *lock)
kernel/locking/mutex.c-291- __acquires(lock);
--
kernel/locking/mutex.c=1110=static noinline void __sched
kernel/locking/mutex.c:1111:__mutex_lock_slowpath(struct mutex *lock)
kernel/locking/mutex.c-1112- __acquires(lock)
]
|
| 24/2 |
2026/05/31 15:49 |
llm |
expert |
0m
Model:
Tokens:
input: 19924
output: 36
thoughts: 21
Thoughts: Here's my summary, as though I'm thinking through this myself, given my expertise:
**Diving into the `__mutex_lock_slowpath` Function**
Okay, so we're looking into locking mechanisms in the kernel, and the function of interest is `__mutex_lock_slowpath`, starting at line 1110. I know this is the slow-path implementation, which means it handles the more complex scenarios when a simple, fast-path lock acquisition isn't possible. My immediate thought is, what are the conditions that trigger this slow path? Contention, obviously, but what other edge cases? I'm expecting to see a lot of memory barriers and potentially sleep/wake-up logic in here. I want to see how this function handles acquiring the mutex when it's already held by another task. We'll be looking for how it handles timeouts, priority inheritance (if applicable), and any other optimization attempts within a contended locking scenario. This is where the real nuance of the mutex implementation lies. Let's see how it orchestrates waiting for the lock, handling interrupt contexts (if any), and ensuring the lock is acquired safely in a multi-threaded, potentially preemptive environment. This will provide more in-depth knowledge of how the locking works, how it's implemented, and how performance is impacted.
|
| 25/2 |
2026/05/31 15:49 |
tool |
read-file |
0m
Args: map[File:kernel/locking/mutex.c FirstLine:1110 LineCount:20]
Results: map[Contents:1110: static noinline void __sched
1111: __mutex_lock_slowpath(struct mutex *lock)
1112: __acquires(lock)
1113: {
1114: __mutex_lock(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
1115: __acquire(lock);
1116: }
1117:
1118: static noinline int __sched
1119: __mutex_lock_killable_slowpath(struct mutex *lock)
1120: __cond_acquires(0, lock)
1121: {
1122: return __mutex_lock(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
1123: }
1124:
1125: static noinline int __sched
1126: __mutex_lock_interruptible_slowpath(struct mutex *lock)
1127: __cond_acquires(0, lock)
1128: {
1129: return __mutex_lock(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
]
|
| 26/2 |
2026/05/31 15:49 |
llm |
expert |
0m
Model:
Tokens:
input: 20588
output: 27
thoughts: 10
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and written in the first person:
**Diving into `__mutex_lock`**
Alright, time to get to the core of this. When I see that thought, my mind immediately jumps to the kernel's internal locking mechanisms. Specifically, the function `__mutex_lock`. It's likely the foundational piece, the unsung hero, that underpins much of our concurrent code. I need to refresh my understanding of its intricacies. What are the key aspects I should scrutinize first? I need to consider: What are the low-level primitives it relies upon? Spinlocks? Atomic operations? How does it handle contention? Does it employ a specific queueing strategy? Does it incorporate any optimization techniques like adaptive locking or priority inheritance? The devil is in the details, and with `__mutex_lock`, those details are going to shape the performance and reliability of the whole system. This function is critical for concurrency so I need to understand it fully.
|
| 27/2 |
2026/05/31 15:49 |
tool |
grepper |
0m
Error:
Args: map[Expression:static int __sched __mutex_lock\(struct mutex \*lock]
Results:
|
| 28/2 |
2026/05/31 15:49 |
llm |
expert |
0m
Model:
Tokens:
input: 20855
output: 23
thoughts: 20
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Diving into the Mutex Implementation**
Okay, so I need to refresh my memory on the specifics of mutex locking within the Linux kernel. Let's start with the fundamental building block: `__mutex_lock`. I'll navigate to `kernel/locking/mutex.c` - that's where the magic happens. I want to see how the core logic is implemented. I'm primarily interested in the spinlock interactions, the waiter queue management (likely using linked lists or something similar), the handling of potential contention, and, of course, the fastpath vs. slowpath optimization strategies. I suspect there will be some architecture-specific considerations in there too, especially around atomic operations and memory barriers, which I'll be sure to identify. Looking forward to see what's actually there.
|
| 29/2 |
2026/05/31 15:49 |
tool |
grepper |
0m
Args: map[Expression:__mutex_lock\(struct mutex \*lock]
Results: map[Output:kernel/locking/mutex.c=815=static int __sched
kernel/locking/mutex.c:816:__mutex_lock(struct mutex *lock, unsigned int state, unsigned int subclass,
kernel/locking/mutex.c-817- struct lockdep_map *nest_lock, unsigned long ip)
]
|
| 30/2 |
2026/05/31 15:49 |
llm |
expert |
0m
Model:
Tokens:
input: 21172
output: 35
thoughts: 10
Thoughts: Okay, here's my interpretation of what's going through my mind as I approach the `__mutex_lock` function, summarized in the first person, assuming I'm an expert in this area:
**Analyzing the Mutex Lock Implementation**
Alright, let's dive into this `__mutex_lock` function. Immediately, I'm expecting to see the core synchronization logic for a mutex. Given my experience, I know the potential pitfalls and the common approaches. I'm looking for the use of atomic operations β that's the first thing my eye will be scanning for. Is it a spinlock? A sleep-wait mechanism? Or perhaps a hybrid approach? I'm anticipating some form of busy-waiting if the mutex is contended, followed by a graceful transition to a wait queue if the contention persists.
I need to understand the memory model implications. Are they using memory barriers to ensure the correct ordering of reads and writes to the mutex state? That's crucial, and I'll be scrutinizing that part carefully. Then there are the potential race conditions β are they properly handled? What about the implementation details? Are there priority inheritance mechanisms or deadlock detection implemented? I'll probably be looking to see if there are any specific platform-specific optimizations or adaptations. Finally, I'll be looking for any potential performance bottlenecks. Memory alignment, the efficiency of the wait queue implementation, and overhead of entering kernel mode if we're dealing with a kernel mutex are all factors that are going to be examined closely. Let's see how well they've navigated these challenges.
|
| 31/2 |
2026/05/31 15:50 |
tool |
read-file |
0m
Args: map[File:kernel/locking/mutex.c FirstLine:815 LineCount:20]
Results: map[Contents: 815: static int __sched
816: __mutex_lock(struct mutex *lock, unsigned int state, unsigned int subclass,
817: struct lockdep_map *nest_lock, unsigned long ip)
818: __cond_acquires(0, lock)
819: {
820: return __mutex_lock_common(lock, state, subclass, nest_lock, ip, NULL, false);
821: }
822:
823: static int __sched
824: __ww_mutex_lock(struct mutex *lock, unsigned int state, unsigned int subclass,
825: unsigned long ip, struct ww_acquire_ctx *ww_ctx)
826: __cond_acquires(0, lock)
827: {
828: return __mutex_lock_common(lock, state, subclass, NULL, ip, ww_ctx, true);
829: }
830:
831: /**
832: * ww_mutex_trylock - tries to acquire the w/w mutex with optional acquire context
833: * @ww: mutex to lock
834: * @ww_ctx: optional w/w acquire context
]
|
| 32/2 |
2026/05/31 15:50 |
llm |
expert |
0m
Model:
Tokens:
input: 21887
output: 25
thoughts: 12
Thoughts: Okay, let's dive into this `__mutex_lock_common` function. I recognize this; it's a critical piece of the kernel's locking infrastructure. My immediate thought is, what's the architectural context? Is this being called from interrupt context, process context, or both? The locking behavior will be vastly different depending on the caller's environment. I'll need to keep an eye out for how it handles priority inheritance, especially if this is a recursive mutex or if it's involved in deadlock avoidance mechanisms.
**Analyzing `__mutex_lock_common` - Diving into the details**
First, I need to understand the data structures it manipulates. Specifically, I'm interested in the mutex itself and any related wait queues. How are waiters added and removed? Are there any spinlocks involved internally to protect the mutex's state, and if so, what's their scope and contention characteristics? I need to analyze how the atomic operations are utilized. What barriers or memory ordering constraints are employed? This is crucial for understanding how the lock guarantees data consistency across multiple cores. Are there any speculative locking tricks or hand-crafted lock implementations being employed?
Beyond the basic lock/unlock semantics, I'm going to look for any special handling for priority inversion scenarios. I'd be looking for the inclusion of priority inheritance logic. How does it ensure that a low-priority thread holding the mutex doesn't indefinitely block higher-priority threads that need it? If priority inheritance is used, how is the inherited priority propagated? Does this function manage any debugging or tracing capabilities? Are there any hooks for lockdep or other lock validation tools? I need to see if it is integrated with the general tracing infrastructure.
Finally, I'll be scrutinizing error handling. How does it handle potential deadlock situations? What are the failure cases and what recovery mechanisms are implemented? Are there any timeout mechanisms? If a lock acquisition fails, does it return an error code or does it block? What's the strategy for handling lock contention? Is there any backoff mechanism implemented? This will be very interesting to review.
|
| 33/2 |
2026/05/31 15:50 |
tool |
grepper |
0m
Args: map[Expression:__mutex_lock_common\(struct mutex \*lock]
Results: map[Output:kernel/locking/mutex.c=608=static __always_inline int __sched
kernel/locking/mutex.c:609:__mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclass,
kernel/locking/mutex.c-610- struct lockdep_map *nest_lock, unsigned long ip,
--
kernel/locking/rtmutex_api.c=518=static void __mutex_rt_init_generic(struct mutex *mutex)
--
kernel/locking/rtmutex_api.c-523-
kernel/locking/rtmutex_api.c:524:static __always_inline int __mutex_lock_common(struct mutex *lock,
kernel/locking/rtmutex_api.c-525- unsigned int state,
]
|
| 34/2 |
2026/05/31 15:50 |
llm |
expert |
0m
Model:
Tokens:
input: 22565
output: 35
thoughts: 22
Thoughts: Here's my summary, as though I'm thinking through this process:
**Diving into the Mutex Lock Core**
Okay, time to get my hands dirty. I need to understand how mutex locks are implemented in the kernel, so let's start dissecting `__mutex_lock_common` in `kernel/locking/mutex.c`. This is the low-level, critical function that *should* be at the heart of the mutex locking mechanism. I'm expecting this to be where the real work of atomic operations, wait queues, and perhaps even some spinlock interaction, is done. Let's see if my mental model lines up with the actual implementation. I'll be looking for things like how contention is detected, how the process is put to sleep (if necessary), and how the lock is finally acquired. I'll be paying particularly close attention to any optimizations or special cases that are handled within this core function. This is where the rubber meets the road.
|
| 35/2 |
2026/05/31 15:50 |
tool |
read-file |
0m
Args: map[File:kernel/locking/mutex.c FirstLine:608 LineCount:20]
Results: map[Contents: 608: static __always_inline int __sched
609: __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclass,
610: struct lockdep_map *nest_lock, unsigned long ip,
611: struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
612: __cond_acquires(0, lock)
613: {
614: DEFINE_WAKE_Q(wake_q);
615: struct mutex_waiter waiter;
616: struct ww_mutex *ww;
617: unsigned long flags;
618: int ret;
619:
620: if (!use_ww_ctx)
621: ww_ctx = NULL;
622:
623: might_sleep();
624:
625: MUTEX_WARN_ON(lock->magic != lock);
626:
627: ww = container_of(lock, struct ww_mutex, base);
]
|
| 36/2 |
2026/05/31 15:50 |
llm |
expert |
0m
Model:
Tokens:
input: 23133
output: 35
thoughts: 135
Thoughts: **Analyzing Lockdep Output**
Okay, so I'm looking at this `lockdep` output and my initial thought is, "Wait, `lockdep` prints the locks *held* by a task, right?" That's the key. So, if I see something like:
```
1 lock held by syz.5.22/6267:
#0: ffff888026f1ade8 (&q->debugfs_mutex){+.+.}-{4:4}, at: blk_debugfs_lock_nomemsave block/blk.h:758 [inline]
```
...does this *explicitly* mean the task acquired the `debugfs_mutex`? It's a fundamental question of what the output *signifies*. I need to confirm this.
To be sure, I'm going to quickly check how `mutex_lock` interacts with lockdep. Specifically, I need to understand the sequence of operations. My assumption is that the lockdep calls occur *before* the actual wait or sleep within the lock acquisition process. Let me quickly scan through `__mutex_lock_common` and look for lockdep calls. That's the important piece here -- I need to see if lockdep is invoked before or after the acquisition is *attempted*. Finding that spot will clarify if this "held" statement represents a confirmed acquisition or just the *attempt* to acquire.
|
| 37/2 |
2026/05/31 15:50 |
tool |
read-file |
0m
Args: map[File:kernel/locking/mutex.c FirstLine:645 LineCount:20]
Results: map[Contents: 645: preempt_disable();
646: mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
647:
648: trace_contention_begin(lock, LCB_F_MUTEX | LCB_F_SPIN);
649: if (__mutex_trylock(lock) ||
650: mutex_optimistic_spin(lock, ww_ctx, NULL)) {
651: /* got the lock, yay! */
652: lock_acquired(&lock->dep_map, ip);
653: if (ww_ctx)
654: ww_mutex_set_context_fastpath(ww, ww_ctx);
655: trace_contention_end(lock, 0);
656: preempt_enable();
657: return 0;
658: }
659:
660: raw_spin_lock_irqsave(&lock->wait_lock, flags);
661: /*
662: * After waiting to acquire the wait_lock, try again.
663: */
664: if (__mutex_trylock(lock)) {
]
|
| 38/2 |
2026/05/31 15:50 |
llm |
expert |
1m
Model:
Tokens:
input: 23951
output: 51
thoughts: 1523
Thoughts: **Debugging a Potential Lock Contention Issue**
Okay, so I've got this `mutex_acquire_nest` call at line 646. That tells me that when a task tries to acquire a lock, the system immediately marks it as *held* by that task, even if it has to sleep and wait. Effectively, lockdep flags the lock as held even while the task is blocked waiting for it. This is interesting. It means the tasks are all blocked waiting for `q->debugfs_mutex`. Who actually *has* it?
Let's check the global lock status. Hmm, looking at the lock output shows that none of the listed tasks hold the `q->debugfs_mutex` *except* those that are blocked on it. So if a task *did* have it, but wasn't blocked, it wouldn't show up here, since the report only shows tasks holding locks. No, the output says "Showing all locks held in the system".
Ah, wait... `syz.4.21/6202` holds *7* locks, but the output is truncated. That's not helpful. I need to see what those locks are.
Okay, let's step back. I'm seeing a Non-Maskable Interrupt (NMI) backtrace on CPU 0. It's running `cfg80211_wiphy_work`, deep inside `_ieee802_11_parse_elems_full`. Could this be an infinite loop? If CPU 0 is stuck, it *could* be holding a lock, or just causing a lockup. Wait, the hung task is `syz.5.22:6267` and it's been blocked for 143 seconds.
But if CPU 0 is stuck in an infinite loop, could it be holding `q->debugfs_mutex`? Probably not. `cfg80211_wiphy_work` is wireless workqueue stuff, and that shouldn't be holding a block-layer `debugfs_mutex`. But, if the system only has two CPUs, and CPU 0 is hogged, the lock-holder could be preempted! But `syz.5.22` is blocked on a mutex.
Okay, I need to investigate `_ieee802_11_parse_elems_full` in `net/mac80211/parse.c` to see if there's any obvious looping or blocking behavior that could be contributing to this problem.
|
| 39/2 |
2026/05/31 15:51 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:net/mac80211/parse.c Name:_ieee802_11_parse_elems_full]
Results: map[SourceCode: 312: static u32
313: _ieee802_11_parse_elems_full(struct ieee80211_elems_parse_params *params,
314: struct ieee80211_elems_parse *elems_parse,
315: const struct element *check_inherit)
316: {
317: struct ieee802_11_elems *elems = &elems_parse->elems;
318: const struct element *elem;
319: bool calc_crc = params->filter != 0;
320: DECLARE_BITMAP(seen_elems, 256);
321: u32 crc = params->crc;
322:
323: bitmap_zero(seen_elems, 256);
324:
325: switch (params->type) {
326: /* we don't need to parse assoc request, luckily (it's value 0) */
327: case IEEE80211_FTYPE_MGMT | IEEE80211_STYPE_ASSOC_REQ:
328: case IEEE80211_FTYPE_MGMT | IEEE80211_STYPE_REASSOC_REQ:
329: default:
330: WARN(1, "invalid frame type 0x%x for element parsing\n",
331: params->type);
332: break;
333: case IEEE80211_FTYPE_MGMT | IEEE80211_STYPE_ASSOC_RESP:
334: case IEEE80211_FTYPE_MGMT | IEEE80211_STYPE_REASSOC_RESP:
335: case IEEE80211_FTYPE_MGMT | IEEE80211_STYPE_PROBE_REQ:
336: case IEEE80211_FTYPE_MGMT | IEEE80211_STYPE_PROBE_RESP:
337: case IEEE80211_FTYPE_MGMT | IEEE80211_STYPE_BEACON:
338: case IEEE80211_FTYPE_MGMT | IEEE80211_STYPE_ACTION:
339: case IEEE80211_FTYPE_EXT | IEEE80211_STYPE_S1G_BEACON:
340: break;
341: }
342:
343: for_each_element(elem, params->start, params->len) {
344: const struct element *subelem;
345: u8 elem_parse_failed;
346: u8 id = elem->id;
347: u8 elen = elem->datalen;
348: const u8 *pos = elem->data;
349:
350: if (check_inherit &&
351: !cfg80211_is_element_inherited(elem,
352: check_inherit))
353: continue;
354:
355: switch (id) {
356: case WLAN_EID_SSID:
357: case WLAN_EID_SUPP_RATES:
358: case WLAN_EID_FH_PARAMS:
359: case WLAN_EID_DS_PARAMS:
360: case WLAN_EID_CF_PARAMS:
361: case WLAN_EID_TIM:
362: case WLAN_EID_IBSS_PARAMS:
363: case WLAN_EID_CHALLENGE:
364: case WLAN_EID_RSN:
365: case WLAN_EID_ERP_INFO:
366: case WLAN_EID_EXT_SUPP_RATES:
367: case WLAN_EID_HT_CAPABILITY:
368: case WLAN_EID_HT_OPERATION:
369: case WLAN_EID_VHT_CAPABILITY:
370: case WLAN_EID_VHT_OPERATION:
371: case WLAN_EID_MESH_ID:
372: case WLAN_EID_MESH_CONFIG:
373: case WLAN_EID_PEER_MGMT:
374: case WLAN_EID_PREQ:
375: case WLAN_EID_PREP:
376: case WLAN_EID_PERR:
377: case WLAN_EID_RANN:
378: case WLAN_EID_CHANNEL_SWITCH:
379: case WLAN_EID_EXT_CHANSWITCH_ANN:
380: case WLAN_EID_COUNTRY:
381: case WLAN_EID_PWR_CONSTRAINT:
382: case WLAN_EID_TIMEOUT_INTERVAL:
383: case WLAN_EID_SECONDARY_CHANNEL_OFFSET:
384: case WLAN_EID_WIDE_BW_CHANNEL_SWITCH:
385: case WLAN_EID_CHAN_SWITCH_PARAM:
386: case WLAN_EID_EXT_CAPABILITY:
387: case WLAN_EID_CHAN_SWITCH_TIMING:
388: case WLAN_EID_LINK_ID:
389: case WLAN_EID_BSS_MAX_IDLE_PERIOD:
390: case WLAN_EID_RSNX:
391: case WLAN_EID_S1G_BCN_COMPAT:
392: case WLAN_EID_S1G_CAPABILITIES:
393: case WLAN_EID_S1G_OPERATION:
394: case WLAN_EID_AID_RESPONSE:
395: case WLAN_EID_S1G_SHORT_BCN_INTERVAL:
396: /*
397: * not listing WLAN_EID_CHANNEL_SWITCH_WRAPPER -- it seems possible
398: * that if the content gets bigger it might be needed more than once
399: */
400: if (test_bit(id, seen_elems)) {
401: elems->parse_error |=
402: IEEE80211_PARSE_ERR_DUP_ELEM;
403: continue;
404: }
405: break;
406: }
407:
408: if (calc_crc && id < 64 && (params->filter & (1ULL << id)))
409: crc = crc32_be(crc, pos - 2, elen + 2);
410:
411: elem_parse_failed = 0;
412:
413: switch (id) {
414: case WLAN_EID_LINK_ID:
415: if (elen + 2 < sizeof(struct ieee80211_tdls_lnkie)) {
416: elem_parse_failed =
417: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
418: break;
419: }
420: elems->lnk_id = (void *)(pos - 2);
421: break;
422: case WLAN_EID_CHAN_SWITCH_TIMING:
423: if (elen < sizeof(struct ieee80211_ch_switch_timing)) {
424: elem_parse_failed =
425: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
426: break;
427: }
428: elems->ch_sw_timing = (void *)pos;
429: break;
430: case WLAN_EID_EXT_CAPABILITY:
431: elems->ext_capab = pos;
432: elems->ext_capab_len = elen;
433: break;
434: case WLAN_EID_SSID:
435: elems->ssid = pos;
436: elems->ssid_len = elen;
437: break;
438: case WLAN_EID_SUPP_RATES:
439: elems->supp_rates = pos;
440: elems->supp_rates_len = elen;
441: break;
442: case WLAN_EID_DS_PARAMS:
443: if (elen >= 1)
444: elems->ds_params = pos;
445: else
446: elem_parse_failed =
447: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
448: break;
449: case WLAN_EID_TIM:
450: if (elen >= sizeof(struct ieee80211_tim_ie)) {
451: elems->tim = (void *)pos;
452: elems->tim_len = elen;
453: } else
454: elem_parse_failed =
455: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
456: break;
457: case WLAN_EID_VENDOR_SPECIFIC:
458: if (elems_parse->skip_vendor)
459: break;
460:
461: if (elen >= 4 && pos[0] == 0x00 && pos[1] == 0x50 &&
462: pos[2] == 0xf2) {
463: /* Microsoft OUI (00:50:F2) */
464:
465: if (calc_crc)
466: crc = crc32_be(crc, pos - 2, elen + 2);
467:
468: if (elen >= 5 && pos[3] == 2) {
469: /* OUI Type 2 - WMM IE */
470: if (pos[4] == 0) {
471: elems->wmm_info = pos;
472: elems->wmm_info_len = elen;
473: } else if (pos[4] == 1) {
474: elems->wmm_param = pos;
475: elems->wmm_param_len = elen;
476: }
477: }
478: }
479: break;
480: case WLAN_EID_RSN:
481: elems->rsn = pos;
482: elems->rsn_len = elen;
483: break;
484: case WLAN_EID_ERP_INFO:
485: if (elen >= 1)
486: elems->erp_info = pos;
487: else
488: elem_parse_failed =
489: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
490: break;
491: case WLAN_EID_EXT_SUPP_RATES:
492: elems->ext_supp_rates = pos;
493: elems->ext_supp_rates_len = elen;
494: break;
495: case WLAN_EID_HT_CAPABILITY:
496: if (params->mode < IEEE80211_CONN_MODE_HT)
497: break;
498: if (elen >= sizeof(struct ieee80211_ht_cap))
499: elems->ht_cap_elem = (void *)pos;
500: else
501: elem_parse_failed =
502: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
503: break;
504: case WLAN_EID_HT_OPERATION:
505: if (params->mode < IEEE80211_CONN_MODE_HT)
506: break;
507: if (elen >= sizeof(struct ieee80211_ht_operation))
508: elems->ht_operation = (void *)pos;
509: else
510: elem_parse_failed =
511: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
512: break;
513: case WLAN_EID_VHT_CAPABILITY:
514: if (params->mode < IEEE80211_CONN_MODE_VHT)
515: break;
516: if (elen >= sizeof(struct ieee80211_vht_cap))
517: elems->vht_cap_elem = (void *)pos;
518: else
519: elem_parse_failed =
520: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
521: break;
522: case WLAN_EID_VHT_OPERATION:
523: if (params->mode < IEEE80211_CONN_MODE_VHT)
524: break;
525: if (elen >= sizeof(struct ieee80211_vht_operation)) {
526: elems->vht_operation = (void *)pos;
527: if (calc_crc)
528: crc = crc32_be(crc, pos - 2, elen + 2);
529: break;
530: }
531: elem_parse_failed =
532: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
533: break;
534: case WLAN_EID_OPMODE_NOTIF:
535: if (params->mode < IEEE80211_CONN_MODE_VHT)
536: break;
537: if (elen > 0) {
538: elems->opmode_notif = pos;
539: if (calc_crc)
540: crc = crc32_be(crc, pos - 2, elen + 2);
541: break;
542: }
543: elem_parse_failed =
544: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
545: break;
546: case WLAN_EID_MESH_ID:
547: elems->mesh_id = pos;
548: elems->mesh_id_len = elen;
549: break;
550: case WLAN_EID_MESH_CONFIG:
551: if (elen >= sizeof(struct ieee80211_meshconf_ie))
552: elems->mesh_config = (void *)pos;
553: else
554: elem_parse_failed =
555: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
556: break;
557: case WLAN_EID_PEER_MGMT:
558: elems->peering = pos;
559: elems->peering_len = elen;
560: break;
561: case WLAN_EID_MESH_AWAKE_WINDOW:
562: if (elen >= 2)
563: elems->awake_window = (void *)pos;
564: break;
565: case WLAN_EID_PREQ:
566: elems->preq = pos;
567: elems->preq_len = elen;
568: break;
569: case WLAN_EID_PREP:
570: elems->prep = pos;
571: elems->prep_len = elen;
572: break;
573: case WLAN_EID_PERR:
574: elems->perr = pos;
575: elems->perr_len = elen;
576: break;
577: case WLAN_EID_RANN:
578: if (elen >= sizeof(struct ieee80211_rann_ie))
579: elems->rann = (void *)pos;
580: else
581: elem_parse_failed =
582: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
583: break;
584: case WLAN_EID_CHANNEL_SWITCH:
585: if (elen != sizeof(struct ieee80211_channel_sw_ie)) {
586: elem_parse_failed =
587: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
588: break;
589: }
590: elems->ch_switch_ie = (void *)pos;
591: break;
592: case WLAN_EID_EXT_CHANSWITCH_ANN:
593: if (elen != sizeof(struct ieee80211_ext_chansw_ie)) {
594: elem_parse_failed =
595: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
596: break;
597: }
598: elems->ext_chansw_ie = (void *)pos;
599: break;
600: case WLAN_EID_SECONDARY_CHANNEL_OFFSET:
601: if (params->mode < IEEE80211_CONN_MODE_HT)
602: break;
603: if (elen != sizeof(struct ieee80211_sec_chan_offs_ie)) {
604: elem_parse_failed =
605: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
606: break;
607: }
608: elems->sec_chan_offs = (void *)pos;
609: break;
610: case WLAN_EID_CHAN_SWITCH_PARAM:
611: if (elen <
612: sizeof(*elems->mesh_chansw_params_ie)) {
613: elem_parse_failed =
614: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
615: break;
616: }
617: elems->mesh_chansw_params_ie = (void *)pos;
618: break;
619: case WLAN_EID_WIDE_BW_CHANNEL_SWITCH:
620: if (params->mode < IEEE80211_CONN_MODE_VHT)
621: break;
622:
623: if (params->type != (IEEE80211_FTYPE_MGMT |
624: IEEE80211_STYPE_ACTION)) {
625: elem_parse_failed =
626: IEEE80211_PARSE_ERR_UNEXPECTED_ELEM;
627: break;
628: }
629:
630: if (elen < sizeof(*elems->wide_bw_chansw_ie)) {
631: elem_parse_failed =
632: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
633: break;
634: }
635: elems->wide_bw_chansw_ie = (void *)pos;
636: break;
637: case WLAN_EID_CHANNEL_SWITCH_WRAPPER:
638: if (params->mode < IEEE80211_CONN_MODE_VHT)
639: break;
640: if (params->type == (IEEE80211_FTYPE_MGMT |
641: IEEE80211_STYPE_ACTION)) {
642: elem_parse_failed =
643: IEEE80211_PARSE_ERR_UNEXPECTED_ELEM;
644: break;
645: }
646: /*
647: * This is a bit tricky, but as we only care about
648: * a few elements, parse them out manually.
649: */
650: subelem = cfg80211_find_elem(WLAN_EID_WIDE_BW_CHANNEL_SWITCH,
651: pos, elen);
652: if (subelem) {
653: if (subelem->datalen >= sizeof(*elems->wide_bw_chansw_ie))
654: elems->wide_bw_chansw_ie =
655: (void *)subelem->data;
656: else
657: elem_parse_failed =
658: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
659: }
660:
661: if (params->mode < IEEE80211_CONN_MODE_EHT)
662: break;
663:
664: subelem = cfg80211_find_ext_elem(WLAN_EID_EXT_BANDWIDTH_INDICATION,
665: pos, elen);
666: if (subelem) {
667: const void *edata = subelem->data + 1;
668: u8 edatalen = subelem->datalen - 1;
669:
670: if (ieee80211_bandwidth_indication_size_ok(edata,
671: edatalen))
672: elems->bandwidth_indication = edata;
673: else
674: elem_parse_failed =
675: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
676: }
677:
678: subelem = cfg80211_find_ext_elem(WLAN_EID_TX_POWER_ENVELOPE,
679: pos, elen);
680: if (subelem)
681: ieee80211_parse_tpe(&elems->csa_tpe,
682: subelem->data + 1,
683: subelem->datalen - 1);
684: break;
685: case WLAN_EID_COUNTRY:
686: elems->country_elem = pos;
687: elems->country_elem_len = elen;
688: break;
689: case WLAN_EID_PWR_CONSTRAINT:
690: if (elen != 1) {
691: elem_parse_failed =
692: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
693: break;
694: }
695: elems->pwr_constr_elem = pos;
696: break;
697: case WLAN_EID_CISCO_VENDOR_SPECIFIC:
698: /* Lots of different options exist, but we only care
699: * about the Dynamic Transmit Power Control element.
700: * First check for the Cisco OUI, then for the DTPC
701: * tag (0x00).
702: */
703: if (elen < 4) {
704: elem_parse_failed =
705: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
706: break;
707: }
708:
709: if (pos[0] != 0x00 || pos[1] != 0x40 ||
710: pos[2] != 0x96 || pos[3] != 0x00)
711: break;
712:
713: if (elen != 6) {
714: elem_parse_failed =
715: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
716: break;
717: }
718:
719: if (calc_crc)
720: crc = crc32_be(crc, pos - 2, elen + 2);
721:
722: elems->cisco_dtpc_elem = pos;
723: break;
724: case WLAN_EID_ADDBA_EXT:
725: if (elen < sizeof(struct ieee80211_addba_ext_ie)) {
726: elem_parse_failed =
727: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
728: break;
729: }
730: elems->addba_ext_ie = (void *)pos;
731: break;
732: case WLAN_EID_TIMEOUT_INTERVAL:
733: if (elen >= sizeof(struct ieee80211_timeout_interval_ie))
734: elems->timeout_int = (void *)pos;
735: else
736: elem_parse_failed =
737: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
738: break;
739: case WLAN_EID_BSS_MAX_IDLE_PERIOD:
740: if (elen >= sizeof(*elems->max_idle_period_ie))
741: elems->max_idle_period_ie = (void *)pos;
742: break;
743: case WLAN_EID_RSNX:
744: elems->rsnx = pos;
745: elems->rsnx_len = elen;
746: break;
747: case WLAN_EID_TX_POWER_ENVELOPE:
748: if (params->mode < IEEE80211_CONN_MODE_HE)
749: break;
750: ieee80211_parse_tpe(&elems->tpe, pos, elen);
751: break;
752: case WLAN_EID_EXTENSION:
753: ieee80211_parse_extension_element(calc_crc ?
754: &crc : NULL,
755: elem, elems_parse,
756: params);
757: break;
758: case WLAN_EID_S1G_CAPABILITIES:
759: if (params->mode != IEEE80211_CONN_MODE_S1G)
760: break;
761: if (elen >= sizeof(*elems->s1g_capab))
762: elems->s1g_capab = (void *)pos;
763: else
764: elem_parse_failed =
765: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
766: break;
767: case WLAN_EID_S1G_OPERATION:
768: if (params->mode != IEEE80211_CONN_MODE_S1G)
769: break;
770: if (elen == sizeof(*elems->s1g_oper))
771: elems->s1g_oper = (void *)pos;
772: else
773: elem_parse_failed =
774: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
775: break;
776: case WLAN_EID_S1G_BCN_COMPAT:
777: if (params->mode != IEEE80211_CONN_MODE_S1G)
778: break;
779: if (elen == sizeof(*elems->s1g_bcn_compat))
780: elems->s1g_bcn_compat = (void *)pos;
781: else
782: elem_parse_failed =
783: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
784: break;
785: case WLAN_EID_AID_RESPONSE:
786: if (params->mode != IEEE80211_CONN_MODE_S1G)
787: break;
788: if (elen == sizeof(struct ieee80211_aid_response_ie))
789: elems->aid_resp = (void *)pos;
790: else
791: elem_parse_failed =
792: IEEE80211_PARSE_ERR_BAD_ELEM_SIZE;
793: break;
794: default:
795: break;
796: }
797:
798: if (elem_parse_failed)
799: elems->parse_error |= elem_parse_failed;
800: else
801: __set_bit(id, seen_elems);
802: }
803:
804: if (!for_each_element_completed(elem, params->start, params->len))
805: elems->parse_error |= IEEE80211_PARSE_ERR_INVALID_END;
806:
807: return crc;
808: }
SourceFile:net/mac80211/parse.c]
|
| 40/2 |
2026/05/31 15:51 |
llm |
expert |
0m
Model:
Tokens:
input: 34263
output: 41
thoughts: 67
Thoughts: Here's my thought process, as requested:
**Potential Infinite Loop in Element Parsing**
Okay, I'm looking at this `_ieee802_11_parse_elems_full` function. The key here is that it appears to iterate through some kind of elements. Specifically, I see this:
```c
for_each_element(elem, params->start, params->len) {
```
My brain immediately flags "iterating over elements." Hmm, could this potentially lead to an infinite loop? That's the first question I need to answer. I need to be *sure*. The `for_each_element` macro is the crux of the matter. I need to understand its behavior, how it handles the start pointer, and the length. Any mismatch or improper handling of those inputs could spell disaster. Let me drill down into that macro and ensure I'm getting the intended behavior here.
|
| 41/2 |
2026/05/31 15:51 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:net/mac80211/parse.c Name:for_each_element]
Results: map[SourceCode: SourceFile:]
|
| 42/2 |
2026/05/31 15:51 |
llm |
expert |
0m
Model:
Tokens:
input: 34589
output: 21
thoughts: 12
Thoughts: Here's my take on those thoughts, tailored for an expert audience:
**Analyzing the Search for `for_each_element`**
Okay, so the immediate inclination is to `grep` for `for_each_element`. That's a solid, efficient starting point. It's a fundamental approach for pinpointing any potential uses or definitions of that function or macro within a codebase. The assumption is, of course, that the name is exactly what we are searching for, including the casing. We're expecting to quickly get a list of occurrences, and then from that, we can dig deeper into the context. This is the first level of investigation, the rapid identification of potential locations. We should also consider how to expand our search, for variations of it such as, if it's a macro it will likely have different capitalization, or possible prefixes or suffixes. Furthermore, we may need to refine our search using regular expressions or specific file extensions to narrow down the results to relevant source files or header files depending on our context.
|
| 43/2 |
2026/05/31 15:51 |
tool |
grepper |
0m
Args: map[Expression:#define for_each_element]
Results: map[Output:include/linux/ieee80211.h=2697=struct element {
--
include/linux/ieee80211.h-2703-/* element iteration helpers */
include/linux/ieee80211.h:2704:#define for_each_element(_elem, _data, _datalen) \
include/linux/ieee80211.h-2705- for (_elem = (const struct element *)(_data); \
--
include/linux/ieee80211.h-2711-
include/linux/ieee80211.h:2712:#define for_each_element_id(element, _id, data, datalen) \
include/linux/ieee80211.h-2713- for_each_element(element, data, datalen) \
--
include/linux/ieee80211.h-2715-
include/linux/ieee80211.h:2716:#define for_each_element_extid(element, extid, _data, _datalen) \
include/linux/ieee80211.h-2717- for_each_element(element, _data, _datalen) \
]
|
| 44/2 |
2026/05/31 15:51 |
llm |
expert |
1m
Model:
Tokens:
input: 35174
output: 42
thoughts: 27
Thoughts: Here's my take on those thoughts, assuming I'm an expert:
**Diving into the IEEE 802.11 Header**
Okay, time to crack open the `for_each_element` macro defined in `include/linux/ieee80211.h`. I need to get a clear picture of how this is handled, and the structure of that header. Specifically, I'm trying to see how this macro is implemented with respect to traversing through various 802.11 data structures. I'm already pretty familiar with the wireless stack, but a deeper dive is always worthwhile, especially when looking at the low-level macros. Understanding the control flow and potential complexities is key. Let's see if this macro uses any unusual pointer arithmetic or compiler tricks to achieve its iteration. I'm hoping to quickly grasp how the iterator handles edge cases and avoids potential buffer overflows in the context of frame parsing. Let's get to it.
|
| 45/2 |
2026/05/31 15:52 |
tool |
read-file |
0m
Args: map[File:include/linux/ieee80211.h FirstLine:2704 LineCount:20]
Results: map[Contents:2704: #define for_each_element(_elem, _data, _datalen) \
2705: for (_elem = (const struct element *)(_data); \
2706: (const u8 *)(_data) + (_datalen) - (const u8 *)_elem >= \
2707: (int)sizeof(*_elem) && \
2708: (const u8 *)(_data) + (_datalen) - (const u8 *)_elem >= \
2709: (int)sizeof(*_elem) + _elem->datalen; \
2710: _elem = (const struct element *)(_elem->data + _elem->datalen))
2711:
2712: #define for_each_element_id(element, _id, data, datalen) \
2713: for_each_element(element, data, datalen) \
2714: if (element->id == (_id))
2715:
2716: #define for_each_element_extid(element, extid, _data, _datalen) \
2717: for_each_element(element, _data, _datalen) \
2718: if (element->id == WLAN_EID_EXTENSION && \
2719: element->datalen > 0 && \
2720: element->data[0] == (extid))
2721:
2722: #define for_each_subelement(sub, element) \
2723: for_each_element(sub, (element)->data, (element)->datalen)
]
|
| 46/2 |
2026/05/31 15:52 |
llm |
expert |
0m
Model:
Tokens:
input: 35893
output: 38
thoughts: 25
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Data Type Sanity Check**
Okay, hold on a second. I'm looking at this code and, wait a minute⦠is `_elem->datalen` actually a `u8`? That seems⦠potentially problematic depending on the context. I need to confirm the underlying data structure. Let's quickly review the definition of `struct element` to refresh my memory and verify this. I need to be absolutely certain of the size constraints this introduces, and how that impacts any downstream usage or potential overflow scenarios.
|
| 47/2 |
2026/05/31 15:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/ieee80211.h Name:element]
Results: map[SourceCode:2697: struct element {
2698: u8 id;
2699: u8 datalen;
2700: u8 data[];
2701: } __packed;
SourceFile:include/linux/ieee80211.h]
|
| 48/2 |
2026/05/31 15:52 |
llm |
expert |
0m
Model:
Tokens:
input: 36173
output: 38
thoughts: 1452
Thoughts: **Debugging a Mysterious Blockage**
Okay, let's break this down. First, I was looking at a potential infinite loop involving a `u8 datalen` and an element structure. I quickly concluded that the increment should be at least 2 bytes, thus preventing an infinite loop. The code should terminate. That part seemed straightforward.
Then, I saw CPU 0 was running `_ieee802_11_parse_elems_full` and got the NMI backtrace with `__sanitizer_cov_trace_switch`. Nothing to worry about. But, the real question was: why is a task blocked for 143 seconds?
My immediate thought was that something must be blocked on `q->debugfs_mutex`. The lockdep output confirms multiple tasks trying to acquire this same mutex. If one task has `q->debugfs_mutex` then I expect it to be listed! The problem is, no single task is listed as the holder! This seemed impossible because all locks held should be listed.
Then it hit me: the lockdep output shows `q->debugfs_mutex` as held by multiple tasks *before* they actually block! The output captures the lock as being held by the calling task *before* they block.
But, if a task actually holds the lock, it would also be present in the listing of held locks. I re-examined the output carefully and noticed something odd: `syz.4.21/6202` holds 7 locks, but they are NOT printed. The output is truncated! This meant `syz.4.21/6202` is the likely culprit. This task is holding `q->debugfs_mutex`.
The next step: What is `syz.4.21/6202` doing? No stack trace, but it's holding 7 locks. Is it blocked? If it was blocked for > 143 seconds, then `khungtaskd` *should* have printed its stack trace. And since `khungtaskd` prints only the first hung task it sees (or up to `sysctl_hung_task_warnings`), this hung task isn't the first, which means `syz.4.21/6202` is *not* in the D state!
So, `syz.4.21/6202` is *not* blocked in an uninterruptible state. It could be running or in an interruptible sleep (S) state. However, the NMI backtrace showed that CPU 0 is running `cfg80211_wiphy_work` and CPU 1 is running `khungtaskd`. `syz.4.21/6202` isn't running on a CPU at this moment.
It must be sleeping, but in S state? Why would it hold `q->debugfs_mutex`? I need to look closer at `blk_trace_setup` to understand how this mutex is used and how a task in an interruptible sleep might be holding it, and if it's the root cause.
|
| 49/2 |
2026/05/31 15:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/blktrace.c Name:blk_trace_setup]
Results: map[SourceCode: 762: int blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
763: struct block_device *bdev,
764: char __user *arg)
765: {
766: struct blk_user_trace_setup2 buts2;
767: struct blk_user_trace_setup buts;
768: struct blk_trace *bt;
769: unsigned int memflags;
770: int ret;
771:
772: ret = copy_from_user(&buts, arg, sizeof(buts));
773: if (ret)
774: return -EFAULT;
775:
776: if (!buts.buf_size || !buts.buf_nr)
777: return -EINVAL;
778:
779: buts2 = (struct blk_user_trace_setup2) {
780: .act_mask = buts.act_mask,
781: .buf_size = buts.buf_size,
782: .buf_nr = buts.buf_nr,
783: .start_lba = buts.start_lba,
784: .end_lba = buts.end_lba,
785: .pid = buts.pid,
786: };
787:
788: memflags = blk_debugfs_lock(q);
789: bt = blk_trace_setup_prepare(q, name, dev, buts.buf_size, buts.buf_nr,
790: bdev);
791: if (IS_ERR(bt)) {
792: blk_debugfs_unlock(q, memflags);
793: return PTR_ERR(bt);
794: }
795: blk_trace_setup_finalize(q, name, 1, bt, &buts2);
796: strscpy(buts.name, buts2.name, BLKTRACE_BDEV_SIZE);
797: blk_debugfs_unlock(q, memflags);
798:
799: if (copy_to_user(arg, &buts, sizeof(buts))) {
800: blk_trace_remove(q);
801: return -EFAULT;
802: }
803: return 0;
804: }
SourceFile:kernel/trace/blktrace.c]
|
| 50/2 |
2026/05/31 15:53 |
llm |
expert |
0m
Model:
Tokens:
input: 39056
output: 40
thoughts: 75
Thoughts: Okay, let me think this through.
**Analyzing the Block Trace Setup Function**
Right, so I'm looking at `blk_trace_setup`. I see that on line 788, it acquires `blk_debugfs_lock(q)`. It looks like it holds this lock, which is key to debugfs access, from that point until line 797. That makes sense, keeps things consistent. I need to keep in mind the scope of that lock, especially as it relates to other debugfs operations. Now, inside that critical section, it calls two other functions: `blk_trace_setup_prepare` and `blk_trace_setup_finalize`. Okay, so let's dig into `blk_trace_setup_prepare` first. Thatβs probably where the heavy lifting, the initialization, is done. I need to understand what that prepare function actually does. I'll need to remember to check the finalize function later, since it is likely that the prepare function has an associated "cleanup" or finalizing task.
|
| 51/2 |
2026/05/31 15:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/blktrace.c Name:blk_trace_setup_prepare]
Results: map[SourceCode: 649: /*
650: * Setup everything required to start tracing
651: */
652: static struct blk_trace *blk_trace_setup_prepare(struct request_queue *q,
653: char *name, dev_t dev,
654: u32 buf_size, u32 buf_nr,
655: struct block_device *bdev)
656: {
657: struct blk_trace *bt = NULL;
658: struct dentry *dir = NULL;
659: int ret;
660:
661: lockdep_assert_held(&q->debugfs_mutex);
662:
663: /*
664: * bdev can be NULL, as with scsi-generic, this is a helpful as
665: * we can be.
666: */
667: if (rcu_dereference_protected(q->blk_trace,
668: lockdep_is_held(&q->debugfs_mutex))) {
669: pr_warn("Concurrent blktraces are not allowed on %s\n", name);
670: return ERR_PTR(-EBUSY);
671: }
672:
673: bt = kzalloc_obj(*bt);
674: if (!bt)
675: return ERR_PTR(-ENOMEM);
676:
677: ret = -ENOMEM;
678: bt->sequence = alloc_percpu(unsigned long);
679: if (!bt->sequence)
680: goto err;
681:
682: bt->msg_data = __alloc_percpu(BLK_TN_MAX_MSG, __alignof__(char));
683: if (!bt->msg_data)
684: goto err;
685:
686: /*
687: * When tracing the whole disk reuse the existing debugfs directory
688: * created by the block layer on init. For partitions block devices,
689: * and scsi-generic block devices we create a temporary new debugfs
690: * directory that will be removed once the trace ends.
691: */
692: if (bdev && !bdev_is_partition(bdev))
693: dir = q->debugfs_dir;
694: else
695: bt->dir = dir = debugfs_create_dir(name, blk_debugfs_root);
696:
697: /*
698: * As blktrace relies on debugfs for its interface the debugfs directory
699: * is required, contrary to the usual mantra of not checking for debugfs
700: * files or directories.
701: */
702: if (IS_ERR_OR_NULL(dir)) {
703: pr_warn("debugfs_dir not present for %s so skipping\n", name);
704: ret = -ENOENT;
705: goto err;
706: }
707:
708: bt->dev = dev;
709: INIT_LIST_HEAD(&bt->running_list);
710:
711: ret = -EIO;
712: debugfs_create_file("dropped", 0444, dir, bt, &blk_dropped_fops);
713: debugfs_create_file("msg", 0222, dir, bt, &blk_msg_fops);
714:
715: bt->rchan = relay_open("trace", dir, buf_size, buf_nr,
716: &blk_relay_callbacks, bt);
717: if (!bt->rchan)
718: goto err;
719:
720: blk_trace_setup_lba(bt, bdev);
721:
722: return bt;
723:
724: err:
725: blk_trace_free(q, bt);
726:
727: return ERR_PTR(ret);
728: }
SourceFile:kernel/trace/blktrace.c]
|
| 52/2 |
2026/05/31 15:53 |
llm |
expert |
0m
Model:
Tokens:
input: 40596
output: 32
thoughts: 27
Thoughts: Here's the summary, assuming I'm an expert:
**Tracing a Kernel Function Call**
Okay, so I'm looking at `blk_trace_setup_prepare`, and I remember that function's signature. I thought there was something interesting in that particular function. Ah yes, it calls `relay_open`. Now that's a key detail! `relay_open` is often where the magic of tracepoints is initialized. Given how these things tend to work within the kernel, I need to understand what `relay_open` is doing in this context. Let's dig into `relay_open` directly and see how it fits into the broader tracing picture. I'm guessing this function is central to establishing the infrastructure for the trace data relay. Let's see if my hunch is right.
|
| 53/2 |
2026/05/31 15:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/relay.c Name:relay_open]
Results: map[SourceCode: 458: /**
459: * relay_open - create a new relay channel
460: * @base_filename: base name of files to create
461: * @parent: dentry of parent directory, %NULL for root directory or buffer
462: * @subbuf_size: size of sub-buffers
463: * @n_subbufs: number of sub-buffers
464: * @cb: client callback functions
465: * @private_data: user-defined data
466: *
467: * Returns channel pointer if successful, %NULL otherwise.
468: *
469: * Creates a channel buffer for each cpu using the sizes and
470: * attributes specified. The created channel buffer files
471: * will be named base_filename0...base_filenameN-1. File
472: * permissions will be %S_IRUSR.
473: */
474: struct rchan *relay_open(const char *base_filename,
475: struct dentry *parent,
476: size_t subbuf_size,
477: size_t n_subbufs,
478: const struct rchan_callbacks *cb,
479: void *private_data)
480: {
481: unsigned int i;
482: struct rchan *chan;
483: struct rchan_buf *buf;
484:
485: if (!(subbuf_size && n_subbufs))
486: return NULL;
487: if (subbuf_size > UINT_MAX / n_subbufs)
488: return NULL;
489: if (!cb || !cb->create_buf_file || !cb->remove_buf_file)
490: return NULL;
491:
492: chan = kzalloc_obj(struct rchan);
493: if (!chan)
494: return NULL;
495:
496: chan->buf = alloc_percpu(struct rchan_buf *);
497: if (!chan->buf) {
498: kfree(chan);
499: return NULL;
500: }
501:
502: chan->version = RELAYFS_CHANNEL_VERSION;
503: chan->n_subbufs = n_subbufs;
504: chan->subbuf_size = subbuf_size;
505: chan->alloc_size = PAGE_ALIGN(subbuf_size * n_subbufs);
506: chan->parent = parent;
507: chan->private_data = private_data;
508: if (base_filename) {
509: chan->has_base_filename = 1;
510: strscpy(chan->base_filename, base_filename, NAME_MAX);
511: }
512: chan->cb = cb;
513: kref_init(&chan->kref);
514:
515: mutex_lock(&relay_channels_mutex);
516: for_each_online_cpu(i) {
517: buf = relay_open_buf(chan, i);
518: if (!buf)
519: goto free_bufs;
520: *per_cpu_ptr(chan->buf, i) = buf;
521: }
522: list_add(&chan->list, &relay_channels);
523: mutex_unlock(&relay_channels_mutex);
524:
525: return chan;
526:
527: free_bufs:
528: for_each_possible_cpu(i) {
529: if ((buf = *per_cpu_ptr(chan->buf, i)))
530: relay_close_buf(buf);
531: }
532:
533: kref_put(&chan->kref, relay_destroy_channel);
534: mutex_unlock(&relay_channels_mutex);
535: return NULL;
536: }
SourceFile:kernel/relay.c]
|
| 54/2 |
2026/05/31 15:53 |
llm |
expert |
0m
Model:
Tokens:
input: 42024
output: 34
thoughts: 399
Thoughts: **Analyzing a Potential Memory Allocation Issue**
Okay, so I'm looking at this `relay_open` function and immediately, my brain zeroes in on the memory allocation. It looks like it allocates memory *per CPU*, which is a potential red flag. The crucial line is this:
```c
chan->alloc_size = PAGE_ALIGN(subbuf_size * n_subbufs);
```
I see the reproducer code sets `buf_size` to `0x100100` and `buf_nr` to `0xfff`. Right, so that means `subbuf_size` becomes `0x100100` (that's 1048832 in decimal), and `n_subbufs` becomes `0xfff` (4095).
Now the multiplication: `1048832 * 4095`. I quickly calculate... oh, wait a minute. It's `0x100100 * 0xfff = 0xffffff00`. Nearly a 4GB allocation per CPU!
The code *should* be checking for an integer overflow. Let's see... is `subbuf_size > UINT_MAX / n_subbufs`? Hmm, `UINT_MAX` is `0xffffffff`. So, is `0xffffff00` less than or equal to `0xffffffff`? Yes, of course, the check passes. This is because we are using multiplication before division.
Then, `chan->alloc_size = PAGE_ALIGN(subbuf_size * n_subbufs);` gets executed. The result of the multiplication, `0xffffff00`, is already page aligned. So, `chan->alloc_size` gets set to `0xffffff00`.
Then it calls `relay_open_buf(chan, i)` for each CPU. I need to dig into `relay_open_buf` now, because this is where the real potential for trouble might lie. This whole allocation could be fine, but I suspect this might be exploitable.
|
| 55/2 |
2026/05/31 15:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/relay.c Name:relay_open_buf]
Results: map[SourceCode: 375: /*
376: * relay_open_buf - create a new relay channel buffer
377: *
378: * used by relay_open() and CPU hotplug.
379: */
380: static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
381: {
382: struct rchan_buf *buf;
383: struct dentry *dentry;
384:
385: if (chan->is_global)
386: return *per_cpu_ptr(chan->buf, 0);
387:
388: buf = relay_create_buf(chan);
389: if (!buf)
390: return NULL;
391:
392: if (chan->has_base_filename) {
393: dentry = relay_create_buf_file(chan, buf, cpu);
394: if (!dentry)
395: goto free_buf;
396: relay_set_buf_dentry(buf, dentry);
397: } else {
398: /* Only retrieve global info, nothing more, nothing less */
399: dentry = chan->cb->create_buf_file(NULL, NULL,
400: S_IRUSR, buf,
401: &chan->is_global);
402: if (IS_ERR_OR_NULL(dentry))
403: goto free_buf;
404: }
405:
406: buf->cpu = cpu;
407: __relay_reset(buf, 1);
408:
409: if(chan->is_global) {
410: *per_cpu_ptr(chan->buf, 0) = buf;
411: buf->cpu = 0;
412: }
413:
414: return buf;
415:
416: free_buf:
417: relay_destroy_buf(buf);
418: return NULL;
419: }
SourceFile:kernel/relay.c]
|
| 56/2 |
2026/05/31 15:53 |
llm |
expert |
1m
Model:
Tokens:
input: 43568
output: 34
thoughts: 11
Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience, and delivered as a first-person summary:
**Digging into `relay_create_buf`**
Alright, time to dive in and take a look at `relay_create_buf`. I need to get a clear picture of what's happening under the hood with that function. I'm especially interested in... (pauses to mentally consider) ... well, everything, really, but my initial focus will be on memory allocation and how it interacts with the relay system. I need to understand if there are any potential bottlenecks or areas where we might be losing efficiency. Are we using the right memory pools? Are there any obvious fragmentation issues? I want to quickly evaluate for the typical culprits of performance issues I'm familiar with in this kind of system. It could be a simple oversight, or something more complex. Gotta start with the basics, though, and methodically assess the current implementation. Let's see what the code reveals.
|
| 57/2 |
2026/05/31 15:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/relay.c Name:relay_create_buf]
Results: map[SourceCode: 140: /**
141: * relay_create_buf - allocate and initialize a channel buffer
142: * @chan: the relay channel
143: *
144: * Returns channel buffer if successful, %NULL otherwise.
145: */
146: static struct rchan_buf *relay_create_buf(struct rchan *chan)
147: {
148: struct rchan_buf *buf;
149:
150: if (chan->n_subbufs > KMALLOC_MAX_SIZE / sizeof(size_t))
151: return NULL;
152:
153: buf = kzalloc_obj(struct rchan_buf);
154: if (!buf)
155: return NULL;
156: buf->padding = kmalloc_objs(size_t, chan->n_subbufs);
157: if (!buf->padding)
158: goto free_buf;
159:
160: buf->start = relay_alloc_buf(buf, &chan->alloc_size);
161: if (!buf->start)
162: goto free_buf;
163:
164: buf->chan = chan;
165: kref_get(&buf->chan->kref);
166: return buf;
167:
168: free_buf:
169: kfree(buf->padding);
170: kfree(buf);
171: return NULL;
172: }
SourceFile:kernel/relay.c]
|
| 58/2 |
2026/05/31 15:54 |
llm |
expert |
0m
Model:
Tokens:
input: 44295
output: 34
thoughts: 11
Thoughts: Okay, here's my attempt at summarizing that thought process, keeping in mind I'm speaking to a fellow expert:
**Debugging the Allocation Buffer**
Alright, time to dig in. I'm going to start by getting a handle on `relay_alloc_buf`. This probably means I'm troubleshooting something related to buffer allocation within the relay system. Could be a memory issue, a fragmentation problem, maybe even a performance bottleneck. Gotta start somewhere, and `relay_alloc_buf` seems like the logical first point. Need to see how it's being used, where it's called, what the inputs are, and what the return values represent. Essentially, I'm thinking, "Let's crack open `relay_alloc_buf` and see what's really going on under the hood." This is where I'll get a better handle on the underlying problem.
|
| 59/2 |
2026/05/31 15:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/relay.c Name:relay_alloc_buf]
Results: map[SourceCode: 100: /**
101: * relay_alloc_buf - allocate a channel buffer
102: * @buf: the buffer struct
103: * @size: total size of the buffer
104: *
105: * Returns a pointer to the resulting buffer, %NULL if unsuccessful. The
106: * passed in size will get page aligned, if it isn't already.
107: */
108: static void *relay_alloc_buf(struct rchan_buf *buf, size_t *size)
109: {
110: void *mem;
111: unsigned int i, j, n_pages;
112:
113: *size = PAGE_ALIGN(*size);
114: n_pages = *size >> PAGE_SHIFT;
115:
116: buf->page_array = relay_alloc_page_array(n_pages);
117: if (!buf->page_array)
118: return NULL;
119:
120: for (i = 0; i < n_pages; i++) {
121: buf->page_array[i] = alloc_page(GFP_KERNEL | __GFP_ZERO);
122: if (unlikely(!buf->page_array[i]))
123: goto depopulate;
124: set_page_private(buf->page_array[i], (unsigned long)buf);
125: }
126: mem = vmap(buf->page_array, n_pages, VM_MAP, PAGE_KERNEL);
127: if (!mem)
128: goto depopulate;
129:
130: buf->page_count = n_pages;
131: return mem;
132:
133: depopulate:
134: for (j = 0; j < i; j++)
135: __free_page(buf->page_array[j]);
136: relay_free_page_array(buf->page_array);
137: return NULL;
138: }
SourceFile:kernel/relay.c]
|
| 60/2 |
2026/05/31 15:54 |
llm |
expert |
0m
Model:
Tokens:
input: 45150
output: 36
thoughts: 96
Thoughts: **Analyzing Memory Allocation**
Wait a minute! I'm seeing that `*size` variable is currently set to `0xffffff00`. Okay, let's break this down. If I calculate `n_pages` by right-shifting `*size` by 12 bits ( `0xffffff00 >> 12` ), that translates to `0xfffff`. So, we're looking at a requirement for 1,048,575 pages. Now, `relay_alloc_page_array(n_pages)` is going to allocate an array of that many pointers β a substantial chunk of memory. I need to take a closer look at what `relay_alloc_page_array` actually *does* under the hood. Time to dig into that function's implementation.
|
| 61/2 |
2026/05/31 15:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/relay.c Name:relay_alloc_page_array]
Results: map[SourceCode: 57: /*
58: * allocate an array of pointers of struct page
59: */
60: static struct page **relay_alloc_page_array(unsigned int n_pages)
61: {
62: return kvzalloc_objs(struct page *, n_pages);
63: }
SourceFile:kernel/relay.c]
|
| 62/2 |
2026/05/31 15:54 |
llm |
expert |
0m
Model:
Tokens:
input: 45570
output: 38
thoughts: 881
Thoughts: **Analyzing a Potential Memory Allocation Bottleneck in the Block Trace Setup**
Okay, let's break this down. It looks like `kvzalloc_objs` is being used, so we're dealing with potentially large arrays, allocated with `kvmalloc`. Then, `relay_alloc_buf` gets called, and it's looping `n_pages` times. Inside that loop, it's calling `alloc_page(GFP_KERNEL | __GFP_ZERO)`. Hang on... `alloc_page` *can* sleep, right? And we're talking about looping a potentially HUGE number of times β like, 1,048,575 times, *per CPU*!
If we have a two-CPU system, that's already over two million pages being allocated. At 4KB per page, that's nearly 8 GB of memory right there! And it gets worse: `GFP_KERNEL` allows for sleeping and memory reclaim. So, if we run out of memory, it'll try to reclaim pages, which is a slow process and might even trigger the OOM killer. But even *without* OOM, allocating 8 GB, one page at a time, will take an incredibly long time.
Here's the kicker: `relay_open` is called from `blk_trace_setup_prepare`, which is called from `blk_trace_setup` with `q->debugfs_mutex` *already held*! That means the process is blocked on `q->debugfs_mutex`, allocating pages forever. Other tasks trying to call `blk_trace_setup` will get blocked on this mutex. This explains why `syz.4.21/6202` is holding up `q->debugfs_mutex`, stuck in `relay_alloc_buf`, allocating millions of pages, which is probably triggering the OOM killer or memory reclaim. And other tasks are blocked on the mutex for over 143 seconds, triggering the hung task watchdog.
Now, a crucial point: is the `buf_size` and `buf_nr` validated? Let me see... the code *does* check if either is zero, returning `-EINVAL`. But that's it! These are `u32` variables, so `buf_size * buf_nr` *could* overflow if `buf_size` * `buf_nr` are too large. While it might overflow `size_t` on 32-bit systems, on 64-bit systems this overflow will be much larger. But wait, `relay_open` includes a check:
```c
if (subbuf_size > UINT_MAX / n_subbufs)
return NULL;
```
This restricts the `subbuf_size * n_subbufs` calculation, so the *maximum* size of the buffer is `UINT_MAX` (4GB). But this is 4 GB per CPU! This could lead to a massive total memory allocation. This means on a 2 CPU system, this is 8 GB, and on a 64 CPU system, this would be 256 GB, allocated page by page, using `alloc_page(GFP_KERNEL | __GFP_ZERO)`. This will certainly take forever and introduce serious memory pressure, leading to OOM errors or, at the very least, a bunch of hung tasks.
Let me see, is there any other possible limit that I've missed? Let me check `blk_trace_setup` again...
|
| 63/2 |
2026/05/31 15:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/blktrace.c Name:blk_trace_setup]
Results: map[SourceCode: 762: int blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
763: struct block_device *bdev,
764: char __user *arg)
765: {
766: struct blk_user_trace_setup2 buts2;
767: struct blk_user_trace_setup buts;
768: struct blk_trace *bt;
769: unsigned int memflags;
770: int ret;
771:
772: ret = copy_from_user(&buts, arg, sizeof(buts));
773: if (ret)
774: return -EFAULT;
775:
776: if (!buts.buf_size || !buts.buf_nr)
777: return -EINVAL;
778:
779: buts2 = (struct blk_user_trace_setup2) {
780: .act_mask = buts.act_mask,
781: .buf_size = buts.buf_size,
782: .buf_nr = buts.buf_nr,
783: .start_lba = buts.start_lba,
784: .end_lba = buts.end_lba,
785: .pid = buts.pid,
786: };
787:
788: memflags = blk_debugfs_lock(q);
789: bt = blk_trace_setup_prepare(q, name, dev, buts.buf_size, buts.buf_nr,
790: bdev);
791: if (IS_ERR(bt)) {
792: blk_debugfs_unlock(q, memflags);
793: return PTR_ERR(bt);
794: }
795: blk_trace_setup_finalize(q, name, 1, bt, &buts2);
796: strscpy(buts.name, buts2.name, BLKTRACE_BDEV_SIZE);
797: blk_debugfs_unlock(q, memflags);
798:
799: if (copy_to_user(arg, &buts, sizeof(buts))) {
800: blk_trace_remove(q);
801: return -EFAULT;
802: }
803: return 0;
804: }
SourceFile:kernel/trace/blktrace.c]
|
| 64/2 |
2026/05/31 15:54 |
llm |
expert |
0m
Model:
Tokens:
input: 47960
output: 33
thoughts: 61
Thoughts: **Investigating the `blk_trace_setup` Function**
Okay, so I'm wondering about the permissions for `blk_trace_setup`. Specifically, is it restricted to root users? Let's figure this out systematically. My first instinct is to see *how* it's being used. The reproducer I'm looking at calls `ioctl(fd, BLKTRACESETUP, arg)` on `/dev/loop2`. That's a good starting point. Now, given that's an `ioctl` call, and I need to figure out how this function is used, the logical place to look is `blkdev_ioctl`. That should tell the story of where `blk_trace_setup` fits into the broader IOCTL landscape. I'll focus there.
|
| 65/2 |
2026/05/31 15:55 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:block/ioctl.c Name:blkdev_ioctl]
Results: map[SourceCode: 743: /*
744: * Always keep this in sync with compat_blkdev_ioctl()
745: * to handle all incompatible commands in both functions.
746: *
747: * New commands must be compatible and go into blkdev_common_ioctl
748: */
749: long blkdev_ioctl(struct file *file, unsigned cmd, unsigned long arg)
750: {
751: struct block_device *bdev = I_BDEV(file->f_mapping->host);
752: void __user *argp = (void __user *)arg;
753: blk_mode_t mode = file_to_blk_mode(file);
754: int ret;
755:
756: switch (cmd) {
757: /* These need separate implementations for the data structure */
758: case HDIO_GETGEO:
759: return blkdev_getgeo(bdev, argp);
760: case BLKPG:
761: return blkpg_ioctl(bdev, argp);
762:
763: /* Compat mode returns 32-bit data instead of 'long' */
764: case BLKRAGET:
765: case BLKFRAGET:
766: if (!argp)
767: return -EINVAL;
768: return put_long(argp,
769: (bdev->bd_disk->bdi->ra_pages * PAGE_SIZE) / 512);
770: case BLKGETSIZE:
771: if (bdev_nr_sectors(bdev) > ~0UL)
772: return -EFBIG;
773: return put_ulong(argp, bdev_nr_sectors(bdev));
774:
775: /* The data is compatible, but the command number is different */
776: case BLKBSZGET: /* get block device soft block size (cf. BLKSSZGET) */
777: return put_int(argp, block_size(bdev));
778: case BLKBSZSET:
779: return blkdev_bszset(file, mode, argp);
780: case BLKGETSIZE64:
781: return put_u64(argp, bdev_nr_bytes(bdev));
782:
783: /* Incompatible alignment on i386 */
784: case BLKTRACESETUP:
785: case BLKTRACESETUP2:
786: return blk_trace_ioctl(bdev, cmd, argp);
787: default:
788: break;
789: }
790:
791: ret = blkdev_common_ioctl(bdev, mode, cmd, arg, argp);
792: if (ret != -ENOIOCTLCMD)
793: return ret;
794:
795: if (!bdev->bd_disk->fops->ioctl)
796: return -ENOTTY;
797: return bdev->bd_disk->fops->ioctl(bdev, mode, cmd, arg);
798: }
SourceFile:block/ioctl.c]
|
| 66/2 |
2026/05/31 15:55 |
llm |
expert |
0m
Model:
Tokens:
input: 49160
output: 38
thoughts: 11
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm an expert in this field, presented as a first-person summary:
**Diving into `blk_trace_ioctl`**
Alright, time to dig in. The immediate thought is, "Let's check `blk_trace_ioctl`." This likely means I'm in the middle of diagnosing a block I/O performance issue, possibly looking at tracing tools, and `blk_trace_ioctl` has come to mind as a key function to understand or debug. I could be investigating the tracepoint setup or filtering, or perhaps trying to understand how userspace interacts with the tracing infrastructure in this specific context. Understanding the ioctl interface is crucial for any kind of low-level block device investigation, so this is a logical first step. I could also be considering how to interact with a tracing tool or library that relies on this specific ioctl. Essentially, this function is the gateway, and I need to see what's behind the door.
|
| 67/2 |
2026/05/31 15:55 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/blktrace.c Name:blk_trace_ioctl]
Results: map[SourceCode: 917: /**
918: * blk_trace_ioctl - handle the ioctls associated with tracing
919: * @bdev: the block device
920: * @cmd: the ioctl cmd
921: * @arg: the argument data, if any
922: *
923: **/
924: int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg)
925: {
926: struct request_queue *q = bdev_get_queue(bdev);
927: int ret, start = 0;
928: char b[BDEVNAME_SIZE];
929:
930: switch (cmd) {
931: case BLKTRACESETUP2:
932: snprintf(b, sizeof(b), "%pg", bdev);
933: ret = blk_trace_setup2(q, b, bdev->bd_dev, bdev, arg);
934: break;
935: case BLKTRACESETUP:
936: snprintf(b, sizeof(b), "%pg", bdev);
937: ret = blk_trace_setup(q, b, bdev->bd_dev, bdev, arg);
938: break;
939: #if defined(CONFIG_COMPAT) && defined(CONFIG_X86_64)
940: case BLKTRACESETUP32:
941: snprintf(b, sizeof(b), "%pg", bdev);
942: ret = compat_blk_trace_setup(q, b, bdev->bd_dev, bdev, arg);
943: break;
944: #endif
945: case BLKTRACESTART:
946: start = 1;
947: fallthrough;
948: case BLKTRACESTOP:
949: ret = blk_trace_startstop(q, start);
950: break;
951: case BLKTRACETEARDOWN:
952: ret = blk_trace_remove(q);
953: break;
954: default:
955: ret = -ENOTTY;
956: break;
957: }
958: return ret;
959: }
SourceFile:kernel/trace/blktrace.c]
|
| 68/2 |
2026/05/31 15:55 |
llm |
expert |
0m
Model:
Tokens:
input: 50124
output: 37
thoughts: 719
Thoughts: **Analyzing the Capability Requirements for the Block Trace Vulnerability**
Okay, first things first, is there *any* capability check here? Let's double-check `blk_trace_setup`. Right, let me look at the code snippet again. Aha, no explicit capability check in either `blk_trace_setup` or `blk_trace_ioctl`, which is a classic oversight.
But wait a minute, to even *get* to that point, you need to open the block device in the first place. The reproducer calls `openat` with `/dev/loop2`. `/dev/loop2` is a block device. Now, usually, `/dev/loop2` is owned by `root:disk`, and the permissions are `rw-rw----`. An unprivileged user shouldn't be able to open that!
Ah! But the reproducer *also* uses `O_CREAT`! So what happens if `/dev/loop2` *doesn't* exist? Does it create a *regular* file called `/dev/loop2`? But that's a problem, because `/dev/loop2` *does* exist, and it's a block device. With `O_CREAT`, if the file already exists, it just opens the existing file. So, assuming it's a block device and the user *isn't* root, `openat` *should* fail with `EACCES`.
Unless... the syzkaller environment is running as root, or the user is a member of the `disk` group. Looking at the bug report, "CPU: 1 UID: 0", the UID is zero, so it is running as root.
But, the *crucial* question here is "Accessible From Unprivileged Processes". Can an *unprivileged* user trigger this? If the user can open *any* block device, then yes, they can trigger it.
So, can an unprivileged user open *a* block device? Usually, no. Devices like `/dev/sda` or `/dev/loop0` are restricted. What about something like `/dev/sr0` β a CD-ROM? It might be accessible to a user in the `cdrom` group, or through ACLs managed by udev for a logged-in user through, for example, logind.
If a user *does* have read access to *some* block device (e.g., a CD-ROM or USB stick), then... can they call `BLKTRACESETUP`? I need to check if `BLKTRACESETUP` requires `CAP_SYS_ADMIN`!
|
| 69/2 |
2026/05/31 15:55 |
tool |
grepper |
0m
Error:
Args: map[Expression:capable\(CAP_SYS_ADMIN\) PathPrefix:kernel/trace/blktrace.c]
Results:
|
| 70/2 |
2026/05/31 15:55 |
llm |
expert |
0m
Model:
Tokens:
input: 51458
output: 27
thoughts: 15
Thoughts: Okay, here's my summary, presented as a stream of thought:
**Checking `blkdev_ioctl` for Capability Checks**
Alright, let's get down to brass tacks. I need to quickly ascertain if the `blkdev_ioctl` function, within the context of block device interaction, actually incorporates any checks for capabilities. It's crucial, fundamentally, that such critical system calls, which interface with storage, enforce robust privilege separation. I can't just assume it *does* perform these checks; that would be a perilous assumption, especially given my background in [User can fill in relevant background]. So, I need to verify its behavior in this aspect immediately. It would be an oversight if it did not.
|
| 71/2 |
2026/05/31 15:55 |
tool |
grepper |
0m
Args: map[Expression:capable\( PathPrefix:block/ioctl.c]
Results: map[Output:block/ioctl.c=21=static int blkpg_do_ioctl(struct block_device *bdev,
--
block/ioctl.c-27-
block/ioctl.c:28: if (!capable(CAP_SYS_ADMIN))
block/ioctl.c-29- return -EACCES;
--
block/ioctl.c=319=static bool blkdev_pr_allowed(struct block_device *bdev, blk_mode_t mode,
--
block/ioctl.c-325-
block/ioctl.c:326: if (capable(CAP_SYS_ADMIN))
block/ioctl.c-327- return true;
--
block/ioctl.c=512=static int blkdev_flushbuf(struct block_device *bdev, unsigned cmd,
--
block/ioctl.c-514-{
block/ioctl.c:515: if (!capable(CAP_SYS_ADMIN))
block/ioctl.c-516- return -EACCES;
--
block/ioctl.c=530=static int blkdev_roset(struct block_device *bdev, unsigned cmd,
--
block/ioctl.c-534-
block/ioctl.c:535: if (!capable(CAP_SYS_ADMIN))
block/ioctl.c-536- return -EACCES;
--
block/ioctl.c=618=static int blkdev_bszset(struct file *file, blk_mode_t mode,
--
block/ioctl.c-626-
block/ioctl.c:627: if (!capable(CAP_SYS_ADMIN))
block/ioctl.c-628- return -EACCES;
--
block/ioctl.c=650=static int blkdev_common_ioctl(struct block_device *bdev, blk_mode_t mode,
--
block/ioctl.c-702- case BLKFRASET:
block/ioctl.c:703: if(!capable(CAP_SYS_ADMIN))
block/ioctl.c-704- return -EACCES;
--
block/ioctl.c-707- case BLKRRPART:
block/ioctl.c:708: if (!capable(CAP_SYS_ADMIN))
block/ioctl.c-709- return -EACCES;
]
|
| 72/2 |
2026/05/31 15:55 |
llm |
expert |
0m
Model:
Tokens:
input: 52251
output: 38
thoughts: 136
Thoughts: **Analyzing the Trace Setup Vulnerability**
Okay, hold on a second. I'm revisiting this `BLKTRACESETUP` issue. It looks like `blkdev_ioctl` handles `BLKTRACESETUP` and `BLKTRACESETUP2` *before* it even gets to `blkdev_common_ioctl`. The code snippet clearly shows it:
```c
switch (cmd) {
...
case BLKTRACESETUP:
case BLKTRACESETUP2:
return blk_trace_ioctl(bdev, cmd, argp);
```
That's significant. Because of that sequence, it bypasses the `capable(CAP_SYS_ADMIN)` check, doesn't it? Meaning the user doesn't need root permissions, just the ability to open the device.
Now I need to drill down and confirm. Does `blk_trace_ioctl` itself have any capability checks? Or does `blk_trace_setup` have a check, as it must be called by `blk_trace_ioctl`? Let me re-examine the `blk_trace_ioctl` function again to be sure I'm not missing anything.
|
| 73/2 |
2026/05/31 15:55 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/blktrace.c Name:blk_trace_ioctl]
Results: map[SourceCode: 917: /**
918: * blk_trace_ioctl - handle the ioctls associated with tracing
919: * @bdev: the block device
920: * @cmd: the ioctl cmd
921: * @arg: the argument data, if any
922: *
923: **/
924: int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg)
925: {
926: struct request_queue *q = bdev_get_queue(bdev);
927: int ret, start = 0;
928: char b[BDEVNAME_SIZE];
929:
930: switch (cmd) {
931: case BLKTRACESETUP2:
932: snprintf(b, sizeof(b), "%pg", bdev);
933: ret = blk_trace_setup2(q, b, bdev->bd_dev, bdev, arg);
934: break;
935: case BLKTRACESETUP:
936: snprintf(b, sizeof(b), "%pg", bdev);
937: ret = blk_trace_setup(q, b, bdev->bd_dev, bdev, arg);
938: break;
939: #if defined(CONFIG_COMPAT) && defined(CONFIG_X86_64)
940: case BLKTRACESETUP32:
941: snprintf(b, sizeof(b), "%pg", bdev);
942: ret = compat_blk_trace_setup(q, b, bdev->bd_dev, bdev, arg);
943: break;
944: #endif
945: case BLKTRACESTART:
946: start = 1;
947: fallthrough;
948: case BLKTRACESTOP:
949: ret = blk_trace_startstop(q, start);
950: break;
951: case BLKTRACETEARDOWN:
952: ret = blk_trace_remove(q);
953: break;
954: default:
955: ret = -ENOTTY;
956: break;
957: }
958: return ret;
959: }
SourceFile:kernel/trace/blktrace.c]
|
| 74/2 |
2026/05/31 15:55 |
llm |
expert |
0m
Model:
Tokens:
input: 53385
output: 38
thoughts: 26
Thoughts: Here's my thought process, summarized as you requested:
**Vulnerability Assessment: Blk_trace Subsystem**
Okay, so I'm digging into this `blk_trace` subsystem, and something just pinged in my brain. Hang on... `blk_trace_ioctl`. I was sure I had read that code. Does it actually perform any capability checks? That seems like a really obvious place for a potential vulnerability. I need to go back and double-check, but I don't recall seeing any. If it doesn't, that's a big red flag. A lack of capability checks in an ioctl handler is essentially asking for trouble; it's practically an open door for privilege escalation, right?
Alright, let's step back, and maybe I need to look at another entry point. Now, what about `blk_trace_setup`? Perhaps the vulnerability is somewhere in the setup or initialization phase. I need to determine where the setup occurs and how user space interacts with it. This function looks to be a promising alternative entry point. I want to see how these traces are initialized. Time to go back and analyze the code, carefully.
|
| 75/2 |
2026/05/31 15:55 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/blktrace.c Name:blk_trace_setup]
Results: map[SourceCode: 762: int blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
763: struct block_device *bdev,
764: char __user *arg)
765: {
766: struct blk_user_trace_setup2 buts2;
767: struct blk_user_trace_setup buts;
768: struct blk_trace *bt;
769: unsigned int memflags;
770: int ret;
771:
772: ret = copy_from_user(&buts, arg, sizeof(buts));
773: if (ret)
774: return -EFAULT;
775:
776: if (!buts.buf_size || !buts.buf_nr)
777: return -EINVAL;
778:
779: buts2 = (struct blk_user_trace_setup2) {
780: .act_mask = buts.act_mask,
781: .buf_size = buts.buf_size,
782: .buf_nr = buts.buf_nr,
783: .start_lba = buts.start_lba,
784: .end_lba = buts.end_lba,
785: .pid = buts.pid,
786: };
787:
788: memflags = blk_debugfs_lock(q);
789: bt = blk_trace_setup_prepare(q, name, dev, buts.buf_size, buts.buf_nr,
790: bdev);
791: if (IS_ERR(bt)) {
792: blk_debugfs_unlock(q, memflags);
793: return PTR_ERR(bt);
794: }
795: blk_trace_setup_finalize(q, name, 1, bt, &buts2);
796: strscpy(buts.name, buts2.name, BLKTRACE_BDEV_SIZE);
797: blk_debugfs_unlock(q, memflags);
798:
799: if (copy_to_user(arg, &buts, sizeof(buts))) {
800: blk_trace_remove(q);
801: return -EFAULT;
802: }
803: return 0;
804: }
SourceFile:kernel/trace/blktrace.c]
|
| 76/2 |
2026/05/31 15:55 |
llm |
expert |
0m
Model:
Tokens:
input: 54412
output: 30
thoughts: 256
Thoughts: **My Line of Reasoning**
Okay, here's the situation. First thought, there was no initial capability check, that's interesting. I need to dig deeper. Wait a minute...can an unprivileged user even open a block device in the first place? If they can't open it, then they certainly can't call `ioctl` on it, right? But...what if they get around that by creating a loop device? That could be a loophole. I know `/dev/loop-control` is the key for that.
Let's check if that's even accessible to unprivileged users. Typically, `/dev/loop-control` has permissions of `rw-rw----` with root:disk ownership. Yeah, that's what I thought, unprivileged users can't open that. Okay, what about the actual loop devices, like `/dev/loopX`? Same story, root:disk. Alright, what about something like `/dev/sr0`? Sometimes that's accessible to the logged-in user, but that's going to be a rare case.
Back to the primary concern though, if a user has *read* access to a block device, can they call `BLKTRACESETUP`? I seem to recall that the `blkdev_ioctl` doesn't check the file open mode for `BLKTRACESETUP`. Hmm, now that I think about it, `blkdev_ioctl` is called for *any* `ioctl` operation on a block device. Does the Virtual File System (VFS) layer do the access checks? I should check `vfs_ioctl` or `do_vfs_ioctl` to verify what the file access checks are doing.
|
| 77/2 |
2026/05/31 15:55 |
tool |
grepper |
0m
Error:
Args: map[Expression:BLKTRACESETUP PathPrefix:fs/ioctl.c]
Results:
|
| 78/2 |
2026/05/31 15:55 |
llm |
expert |
0m
Model:
Tokens:
input: 55077
output: 27
thoughts: 306
Thoughts: **Vulnerability Exploration: BLKTRACESETUP and Potential Denial of Service**
Okay, so the VFS layer doesn't seem to care about `BLKTRACESETUP`; it just passes it along to the device's `ioctl` function, which in this case is `blkdev_ioctl`. That means if a user has *read* access to a block device, they can potentially call `BLKTRACESETUP` directly! The scary part is they could then specify large values for `buf_size` and `buf_nr` and allocate a whopping 8 GB of memory, which would lead to a denial-of-service attack. The question now is: can this actually be exploited by unprivileged users?
My mind then jumps to accessibility: I need to figure out if it's possible for a user to actually *have* read access to a block device. In many desktop environments, when you plug in a USB stick, the OS mounts it automatically. My initial thought was that a user might get direct access to the underlying block device, but hold on, the mounting is usually handled by something like udisks, so a user might not have direct access to `/dev/sdX`, but only to the filesystem that's mounted on top of it.
Then, I start to think about CD-ROMs. Ah, `/dev/sr0`! That one's interesting, because it's often readable by the `cdrom` group, and the logged-in user is frequently added to that group, or udev ACLs might grant read access directly. So, a local user might very well have read access to `/dev/sr0`. If they do, they can open it in `O_RDONLY` mode and, boom, call `ioctl(fd, BLKTRACESETUP, ...)`.
Hang on, let's step back for a moment and consider: Does `blk_trace_setup` itself require root privileges? Let's quickly double-check if there's any capability check in `relay_open`, because that's where any permission check would happen.
|
| 79/2 |
2026/05/31 15:55 |
tool |
grepper |
0m
Error:
Args: map[Expression:capable\( PathPrefix:kernel/relay.c]
Results:
|
| 80/2 |
2026/05/31 15:55 |
llm |
expert |
1m
Model:
Tokens:
input: 55872
output: 32
thoughts: 788
Thoughts: **Analyzing a Potential Denial of Service Vulnerability in `debugfs` and Block Tracing**
Okay, so I'm digging into this `debugfs_create_dir` function, and how it's used within `blk_trace_setup_prepare`. The immediate concern is that this directory creation doesn't seem to have any capability checks. It just creates a dentry. So, even an unprivileged user with read access to a block device might be able to trigger something interesting.
Specifically, if `debugfs` isn't even mounted, the dentry is still created within the debugfs tree. This suggests a possible denial-of-service (DoS) vulnerability. The trigger seems to be memory allocation. The question is, how bad is it?
The allocation uses `alloc_page(GFP_KERNEL | __GFP_ZERO)`, which is concerning. It's allocating a potentially large amount of memory and then mapping it with `vmap`. Okay, if this succeeds, we get a valid buffer. If it fails, the allocated pages are freed and NULL is returned. No memory corruption, no use-after-free, no out-of-bounds access. So, it's not a privilege escalation exploit. It's just a DoS through resource exhaustion, potentially leading to hung tasks.
But then, I need to check something important. What about an integer overflow when calculating the allocation size? Let me check: `subbuf_size * n_subbufs`. The code in `relay_open` does a check:
```c
if (subbuf_size > UINT_MAX / n_subbufs)
return NULL;
```
This should prevent the product from exceeding `UINT_MAX` (4GB), so the maximum `alloc_size` should be 4GB.
Then `relay_alloc_buf` does the following:
```c
*size = PAGE_ALIGN(*size);
n_pages = *size >> PAGE_SHIFT;
```
If size = `0xffffffff`, then `PAGE_ALIGN(0xffffffff)` is 4GB! I need to investigate how `PAGE_ALIGN` works.
On 32-bit systems, `PAGE_ALIGN(0xffffffff)` would likely overflow to `0`!
Let's check the macro definitions:
* `#define PAGE_ALIGN(addr) ALIGN(addr, PAGE_SIZE)`
* `#define ALIGN(x, a) __ALIGN_KERNEL((x), (a))`
* `#define __ALIGN_KERNEL(x, a) __ALIGN_KERNEL_MASK(x, (typeof(x))(a) - 1)`
* `#define __ALIGN_KERNEL_MASK(x, mask) (((x) + (mask)) & ~(mask))`
If `x` is `0xffffffff` (which is `UINT_MAX`), and `mask` is `0xfff`, then `x + mask` = `0xffffffff + 0xfff`. Because of 32-bit overflow, this results in `0xffe` . Then `& ~mask` = `0xffe & ~0xfff` = `0`. So `PAGE_ALIGN(0xffffffff)` is `0` on 32-bit.
If *size becomes 0, then `n_pages` will also be 0. So then, `relay_alloc_page_array(0)` returns `ZERO_SIZE_PTR`. Then the loop allocating pages does not execute. Then `vmap(buf->page_array, 0, VM_MAP, PAGE_KERNEL)` is called. I need to check `vmap` with `count=0`.
|
| 81/2 |
2026/05/31 15:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/vmalloc.c Name:vmap]
Results: map[SourceCode:3508: /**
3509: * vmap - map an array of pages into virtually contiguous space
3510: * @pages: array of page pointers
3511: * @count: number of pages to map
3512: * @flags: vm_area->flags
3513: * @prot: page protection for the mapping
3514: *
3515: * Maps @count pages from @pages into contiguous kernel virtual space.
3516: * If @flags contains %VM_MAP_PUT_PAGES the ownership of the pages array itself
3517: * (which must be kmalloc or vmalloc memory) and one reference per pages in it
3518: * are transferred from the caller to vmap(), and will be freed / dropped when
3519: * vfree() is called on the return value.
3520: *
3521: * Return: the address of the area or %NULL on failure
3522: */
3523: void *vmap(struct page **pages, unsigned int count,
3524: unsigned long flags, pgprot_t prot)
3525: {
3526: struct vm_struct *area;
3527: unsigned long addr;
3528: unsigned long size; /* In bytes */
3529:
3530: might_sleep();
3531:
3532: if (WARN_ON_ONCE(flags & VM_FLUSH_RESET_PERMS))
3533: return NULL;
3534:
3535: /*
3536: * Your top guard is someone else's bottom guard. Not having a top
3537: * guard compromises someone else's mappings too.
3538: */
3539: if (WARN_ON_ONCE(flags & VM_NO_GUARD))
3540: flags &= ~VM_NO_GUARD;
3541:
3542: if (count > totalram_pages())
3543: return NULL;
3544:
3545: size = (unsigned long)count << PAGE_SHIFT;
3546: area = get_vm_area_caller(size, flags, __builtin_return_address(0));
3547: if (!area)
3548: return NULL;
3549:
3550: addr = (unsigned long)area->addr;
3551: if (vmap_pages_range(addr, addr + size, pgprot_nx(prot),
3552: pages, PAGE_SHIFT) < 0) {
3553: vunmap(area->addr);
3554: return NULL;
3555: }
3556:
3557: if (flags & VM_MAP_PUT_PAGES) {
3558: area->pages = pages;
3559: area->nr_pages = count;
3560: }
3561: return area->addr;
3562: }
SourceFile:mm/vmalloc.c]
|
| 82/2 |
2026/05/31 15:56 |
llm |
expert |
0m
Model:
Tokens:
input: 58323
output: 37
thoughts: 56
Thoughts: **Analyzing Memory Allocation Logic**
Okay, so I'm looking at this condition: `count == 0`. My immediate thought is, when that's true, the `size` should also be set to 0. That's pretty straightforward, a clear boundary case. Now, the next thing that pops into my head is what `get_vm_area_caller(0, ...)` is going to do in this scenario. Given that the count is zero, I'd expect it to return either a `NULL` pointer, indicating no area is available (or a specific error condition), or, potentially, a valid pointer to a virtual memory area with a size of zero. It depends on the internal logic of that function, how it handles the zero-sized request. That's the key question here. To understand what's actually happening, I need to dig deeper into the implementation of `get_vm_area_caller`. I'm going to check that function to verify how it behaves with that zero count parameter, looking for error handling or unexpected results, and verify which of the two cases above it returns. That's the next logical step.
|
| 83/2 |
2026/05/31 15:57 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/vmalloc.c Name:get_vm_area_caller]
Results: map[SourceCode:3274: struct vm_struct *get_vm_area_caller(unsigned long size, unsigned long flags,
3275: const void *caller)
3276: {
3277: return __get_vm_area_node(size, 1, PAGE_SHIFT, flags,
3278: VMALLOC_START, VMALLOC_END,
3279: NUMA_NO_NODE, GFP_KERNEL, caller);
3280: }
SourceFile:mm/vmalloc.c]
|
| 84/2 |
2026/05/31 15:57 |
llm |
expert |
0m
Model:
Tokens:
input: 58810
output: 38
thoughts: 14
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm an expert in this area and the context is relevant:
**Diving into Virtual Memory Management**
Alright, I see a need to investigate something related to virtual memory areas. The function call `__get_vm_area_node` immediately catches my attention. This suggests we're likely dealing with some aspect of the kernel's virtual memory management, possibly involving the allocation, retrieval, or manipulation of `vm_area_struct` structures. I need to understand what's happening within this specific function. Is there an issue with node selection? Is it failing to find the expected VMA? The context will be key here - what triggered this investigation? Let me start by reminding myself of the basic layout of this structure; it's probably been a while. Then I will go deeper.
|
| 85/2 |
2026/05/31 15:57 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/vmalloc.c Name:__get_vm_area_node]
Results: map[SourceCode:3197: struct vm_struct *__get_vm_area_node(unsigned long size,
3198: unsigned long align, unsigned long shift, unsigned long flags,
3199: unsigned long start, unsigned long end, int node,
3200: gfp_t gfp_mask, const void *caller)
3201: {
3202: struct vmap_area *va;
3203: struct vm_struct *area;
3204: unsigned long requested_size = size;
3205:
3206: BUG_ON(in_nmi() || in_hardirq());
3207: size = ALIGN(size, 1ul << shift);
3208: if (unlikely(!size))
3209: return NULL;
3210:
3211: if (flags & VM_IOREMAP)
3212: align = 1ul << clamp_t(int, get_count_order_long(size),
3213: PAGE_SHIFT, IOREMAP_MAX_ORDER);
3214:
3215: area = kzalloc_node(sizeof(*area), gfp_mask & GFP_RECLAIM_MASK, node);
3216: if (unlikely(!area))
3217: return NULL;
3218:
3219: if (!(flags & VM_NO_GUARD))
3220: size += PAGE_SIZE;
3221:
3222: area->flags = flags;
3223: area->caller = caller;
3224: area->requested_size = requested_size;
3225:
3226: va = alloc_vmap_area(size, align, start, end, node, gfp_mask, 0, area);
3227: if (IS_ERR(va)) {
3228: kfree(area);
3229: return NULL;
3230: }
3231:
3232: /*
3233: * Mark pages for non-VM_ALLOC mappings as accessible. Do it now as a
3234: * best-effort approach, as they can be mapped outside of vmalloc code.
3235: * For VM_ALLOC mappings, the pages are marked as accessible after
3236: * getting mapped in __vmalloc_node_range().
3237: * With hardware tag-based KASAN, marking is skipped for
3238: * non-VM_ALLOC mappings, see __kasan_unpoison_vmalloc().
3239: */
3240: if (!(flags & VM_ALLOC))
3241: area->addr = kasan_unpoison_vmalloc(area->addr, requested_size,
3242: KASAN_VMALLOC_PROT_NORMAL);
3243:
3244: return area;
3245: }
SourceFile:mm/vmalloc.c]
|
| 86/2 |
2026/05/31 15:57 |
llm |
expert |
0m
Model:
Tokens:
input: 59839
output: 28
thoughts: 377
Thoughts: **Analyzing a Potential Denial of Service Vulnerability**
Okay, so I'm tracing this `blk_trace_setup` call and something's not right. If `size` is zero, everything cascades to returning `NULL` and eventually `-ENOMEM`. That makes sense. It's the expected error path. But what happens if `PAGE_ALIGN` doesn't give us a zero result? Let's say `subbuf_size * n_subbufs` results in `0xfffff000`. `PAGE_ALIGN` leaves it as is. Then `n_pages` becomes `0xfffff`, which is a substantial number. I'm talking about allocating over a million pages, which is around 4GB per CPU. Two CPUs? 8GB. 64 CPUs? That's 256GB!
This is a clear denial-of-service vulnerability. We're talking about an enormous memory allocation triggered by what seems like a standard setup. And, to make things worse, this whole allocation happens under the protection of `q->debugfs_mutex`, which creates a lock around the operation. This is blocking other tasks that are trying to do `blk_trace_setup`, `blk_trace_remove`, or even `blk_debugfs_lock` which is called from other places that are not related to `blk_trace`. That mutex is a critical piece here. I need to figure out where else that mutex is being used to fully understand the impact. I need to follow the mutex usage to see where else it's being acquired.
|
| 87/2 |
2026/05/31 15:57 |
tool |
grepper |
0m
Args: map[Expression:blk_debugfs_lock PathPrefix:block/]
Results: map[Output:block/blk-mq-debugfs.c=689=void blk_mq_debugfs_register_hctxs(struct request_queue *q)
--
block/blk-mq-debugfs.c-694-
block/blk-mq-debugfs.c:695: memflags = blk_debugfs_lock(q);
block/blk-mq-debugfs.c-696- queue_for_each_hw_ctx(q, hctx, i)
--
block/blk-mq-sched.c=390=void blk_mq_sched_reg_debugfs(struct request_queue *q)
--
block/blk-mq-sched.c-395-
block/blk-mq-sched.c:396: memflags = blk_debugfs_lock(q);
block/blk-mq-sched.c-397- blk_mq_debugfs_register_sched(q);
--
block/blk-mq-sched.c=403=void blk_mq_sched_unreg_debugfs(struct request_queue *q)
--
block/blk-mq-sched.c-407-
block/blk-mq-sched.c:408: blk_debugfs_lock_nomemsave(q);
block/blk-mq-sched.c-409- queue_for_each_hw_ctx(q, hctx, i)
--
block/blk-sysfs.c=930=static void blk_debugfs_remove(struct gendisk *disk)
--
block/blk-sysfs.c-933-
block/blk-sysfs.c:934: blk_debugfs_lock_nomemsave(q);
block/blk-sysfs.c-935- blk_trace_shutdown(q);
--
block/blk-sysfs.c=947=int blk_register_queue(struct gendisk *disk)
--
block/blk-sysfs.c-963-
block/blk-sysfs.c:964: memflags = blk_debugfs_lock(q);
block/blk-sysfs.c-965- q->debugfs_dir = debugfs_create_dir(disk->disk_name, blk_debugfs_root);
--
block/blk-wbt.c=775=void wbt_init_enable_default(struct gendisk *disk)
--
block/blk-wbt.c-793-
block/blk-wbt.c:794: memflags = blk_debugfs_lock(q);
block/blk-wbt.c-795- blk_mq_debugfs_register_rq_qos(q);
--
block/blk-wbt.c=975=int wbt_set_lat(struct gendisk *disk, s64 val)
--
block/blk-wbt.c-1020-
block/blk-wbt.c:1021: memflags = blk_debugfs_lock(q);
block/blk-wbt.c-1022- blk_mq_debugfs_register_rq_qos(q);
--
block/blk.h=745=static inline void blk_unfreeze_release_lock(struct request_queue *q)
--
block/blk.h-755- */
block/blk.h:756:static inline void blk_debugfs_lock_nomemsave(struct request_queue *q)
block/blk.h-757-{
--
block/blk.h=761=static inline void blk_debugfs_unlock_nomemrestore(struct request_queue *q)
--
block/blk.h-765-
block/blk.h:766:static inline unsigned int __must_check blk_debugfs_lock(struct request_queue *q)
block/blk.h-767-{
--
block/blk.h-769-
block/blk.h:770: blk_debugfs_lock_nomemsave(q);
block/blk.h-771- return memflags;
]
|
| 88/2 |
2026/05/31 15:57 |
llm |
expert |
0m
Model:
Tokens:
input: 61611
output: 33
thoughts: 1006
Thoughts: **Analyzing the Vulnerability's Accessibility**
Okay, so I'm digging into this `blk_debugfs_lock` issue, trying to figure out if an unprivileged process can exploit it. It seems the core problem is a potential Denial of Service caused by excessive memory allocation triggered during block trace setup. My initial thought is: can an unprivileged user even *get* to this `blk_trace_setup` function?
The trigger seems to be when registering queues or schedulers, potentially getting blocked if `q->debugfs_mutex` is held by a stalled process in `blk_trace_setup`. The crucial question then is, how does this process get triggered?
First, I consider if an unprivileged user can read a block device. If so, they could potentially trigger the memory allocation. However, what if the user doesn't have read access to a block device? Can they create a loop device themselves? No, since unprivileged users cannot use `/dev/loop-control`. But then the reproducer is attempting to `O_CREAT` `/dev/loop2` so that's something else to consider.
Next, I think about user namespaces. If a user is in a user namespace, can they create a block device? Nope, `mknod` is restricted to the `init_user_ns`. They can mount filesystems in their namespace, but not create block devices.
The reproducer shows opening `/dev/loop2`, but in syzkaller, the test runs as root, so this is possible. If a non-root user *can't* open a block device, this bug is not exploitable. Are there any block devices readable by a normal user by default? Usually, `/dev/sr0` (CD-ROM) is readable by the logged-in user. USB sticks might get mounted by the desktop environment, but the block device itself is *usually* NOT world-readable, meaning not by a standard user. The prompt explicitly says the user has no special capabilities or access to restricted device nodes.
So the user *must* be able to open a block device node to trigger this vulnerability. Given that an unprivileged user *generally* cannot, this bug is inaccessible to the user UNLESS they are granted access to a block device such as `/dev/sr0`.
Given the scenario outlined, the prompt specifies that the user does *not* have access to devices restricted to root, the user may have access to `/dev/sr0`. Is `/dev/sr0` restricted to root? Usually root:cdrom. If the user is in the `cdrom` group, they can access it. But, if we're assuming *no* special groups, they can't open a block device. So it's NOT accessible.
I've also thought about user namespaces and if they help. If a user creates a user namespace, they still can't open block devices on the host. They can mount a loopback filesystem if they have a loop device, but they can't create one. `/dev/loop-control` is not accessible in a user namespace because device nodes are not accessible unless bind-mounted, and they retain original permissions.
So, user namespaces are not a factor here.
Finally, I checked if `BLKTRACESETUP` can be called on a partition. Yes, `bdev_is_partition(bdev)` is checked in `blk_trace_setup_prepare`, and if it's a partition, a debugfs directory is created. Can a user open a partition? That's the same permissions as the whole disk.
Ultimately, I'm trying to find other ways to trigger `blk_trace_setup`. I started looking at `sysfs`, to see if there is another way. Indeed, tracing can be set up via `sysfs`. So I'm now looking into `block/blk-sysfs.c`.
|
| 89/2 |
2026/05/31 15:57 |
tool |
grepper |
0m
Error:
Args: map[Expression:blk_trace_setup PathPrefix:block/blk-sysfs.c]
Results:
|
| 90/2 |
2026/05/31 15:57 |
llm |
expert |
1m
Model:
Tokens:
input: 63491
output: 31
thoughts: 20
Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience:
**Diving into blktrace's Sysfs Interface**
Right, let's get down to brass tacks. I need to refresh my memory on how `blktrace` exposes its functionality through the sysfs interface. Specifically, I'm thinking I should start digging into `kernel/trace/blktrace.c`. That's the obvious place to find the kernel-side implementation details for the sysfs attributes related to block tracing. I want to see how the settings are handled, how the tracing data is controlled, and how the various parameters like device filtering and event masking are plumbed through to the tracing subsystem. I'm hoping it's well-structured and documented, but knowing kernel code, I'm prepared for a bit of a hunt! This should be a good starting point to understand the low-level interactions and potentially uncover some subtle behaviors or optimization opportunities.
|
| 91/2 |
2026/05/31 15:58 |
tool |
grepper |
0m
Args: map[Expression:sysfs PathPrefix:kernel/trace/blktrace.c]
Results: map[Output:kernel/trace/blktrace.c=310=static void __blk_add_trace(struct blk_trace *bt, sector_t sector, int bytes,
--
kernel/trace/blktrace.c-396- *
kernel/trace/blktrace.c:397: * For sysfs-enabled tracing path (enabled via
kernel/trace/blktrace.c-398- * /sys/block/DEV/trace/enable), blk_trace_setup_queue()
--
kernel/trace/blktrace.c-437- * Fall through to default handles version==0 case
kernel/trace/blktrace.c:438: * (from sysfs path), ensuring we always use correct
kernel/trace/blktrace.c-439- * v2 recording function to match the v2 buffer
--
kernel/trace/blktrace.c=909=EXPORT_SYMBOL_GPL(blk_trace_startstop);
--
kernel/trace/blktrace.c-911-/*
kernel/trace/blktrace.c:912: * When reading or writing the blktrace sysfs files, the references to the
kernel/trace/blktrace.c:913: * opened sysfs or device files should prevent the underlying block device
kernel/trace/blktrace.c-914- * from being removed. So no further delete protection is really needed.
--
kernel/trace/blktrace.c=1900=static int blk_trace_setup_queue(struct request_queue *q,
--
kernel/trace/blktrace.c-1928-/*
kernel/trace/blktrace.c:1929: * sysfs interface to enable and configure tracing
kernel/trace/blktrace.c-1930- */
kernel/trace/blktrace.c-1931-
kernel/trace/blktrace.c:1932:static ssize_t sysfs_blk_trace_attr_show(struct device *dev,
kernel/trace/blktrace.c-1933- struct device_attribute *attr,
kernel/trace/blktrace.c-1934- char *buf);
kernel/trace/blktrace.c:1935:static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
kernel/trace/blktrace.c-1936- struct device_attribute *attr,
--
kernel/trace/blktrace.c-1939- DEVICE_ATTR(_name, S_IRUGO | S_IWUSR, \
kernel/trace/blktrace.c:1940: sysfs_blk_trace_attr_show, \
kernel/trace/blktrace.c:1941: sysfs_blk_trace_attr_store)
kernel/trace/blktrace.c-1942-
--
kernel/trace/blktrace.c=2021=static ssize_t blk_trace_mask2str(char *buf, int mask)
--
kernel/trace/blktrace.c-2036-
kernel/trace/blktrace.c:2037:static ssize_t sysfs_blk_trace_attr_show(struct device *dev,
kernel/trace/blktrace.c-2038- struct device_attribute *attr,
--
kernel/trace/blktrace.c-2070-
kernel/trace/blktrace.c:2071:static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
kernel/trace/blktrace.c-2072- struct device_attribute *attr,
]
|
| 92/2 |
2026/05/31 15:58 |
llm |
expert |
0m
Model:
Tokens:
input: 64663
output: 43
thoughts: 16
Thoughts: Okay, let's dive into this. Here's my thought process on checking `sysfs_blk_trace_attr_store`, tailored for an expert like yourself:
**Diving into `sysfs_blk_trace_attr_store`**
Right, `sysfs_blk_trace_attr_store`. Always good to refresh the mental map of where this lives in the block tracing landscape. I'm immediately thinking about its role in the tracing infrastructure exposed through sysfs. It's the *store* function, so it's the write handler for some attribute, likely related to configuring block trace behavior. My mind instantly jumps to the various attributes we typically tweak: enabling/disabling tracing, setting filter expressions, perhaps configuring the output buffer.
I need to quickly recall the typical usage patterns here. Is it called directly, or is it invoked through a higher-level framework? What are the key data structures it likely interacts with? I'm picturing `blk_trace` structs and associated control flags. Memory barriers⦠are they appropriately placed here for concurrency safety? We *know* that's a frequent point of concern in any block I/O related code.
The questions I'm asking myself:
* What kinds of input validation are there? Any obvious security vulnerabilities?
* How does it handle errors? Are error conditions properly propagated, or are we potentially losing error information silently?
* What resources are acquired and released? Locks, memory allocations, etc. Potential for deadlocks or resource leaks are always worth a quick look.
* Is it atomic with respect to other similar control operations? We might be updating some settings which could break atomicity or introduce inconsistent trace states.
* What's the overall architecture? Is this a monolithic function, or are we dealing with a chain of calls with a clear set of responsibilities?
* The overall goal of this function is to store a trace attribute into a `sysfs` file. What could happen if that fails? Are we checking return codes?
Ultimately, my goal here is to quickly evaluate whether this is solid, maintainable code. I want to spot any potential red flags that could lead to performance bottlenecks, concurrency issues, or security concerns down the road. It's a quick triage for a critical piece of the block tracing subsystem. And finally, can I clearly explain this code to a newer engineer? If not, then a deeper dive might be required.
|
| 93/2 |
2026/05/31 15:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/blktrace.c Name:sysfs_blk_trace_attr_store]
Results: map[SourceCode:2071: static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
2072: struct device_attribute *attr,
2073: const char *buf, size_t count)
2074: {
2075: struct block_device *bdev = dev_to_bdev(dev);
2076: struct request_queue *q = bdev_get_queue(bdev);
2077: struct blk_trace *bt;
2078: unsigned int memflags;
2079: u64 value;
2080: ssize_t ret = -EINVAL;
2081:
2082: if (count == 0)
2083: goto out;
2084:
2085: if (attr == &dev_attr_act_mask) {
2086: if (kstrtoull(buf, 0, &value)) {
2087: /* Assume it is a list of trace category names */
2088: ret = blk_trace_str2mask(buf);
2089: if (ret < 0)
2090: goto out;
2091: value = ret;
2092: }
2093: } else {
2094: if (kstrtoull(buf, 0, &value))
2095: goto out;
2096: }
2097:
2098: memflags = blk_debugfs_lock(q);
2099:
2100: bt = rcu_dereference_protected(q->blk_trace,
2101: lockdep_is_held(&q->debugfs_mutex));
2102: if (attr == &dev_attr_enable) {
2103: if (!!value == !!bt) {
2104: ret = 0;
2105: goto out_unlock_bdev;
2106: }
2107: if (value)
2108: ret = blk_trace_setup_queue(q, bdev);
2109: else
2110: ret = blk_trace_remove_queue(q);
2111: goto out_unlock_bdev;
2112: }
2113:
2114: ret = 0;
2115: if (bt == NULL) {
2116: ret = blk_trace_setup_queue(q, bdev);
2117: bt = rcu_dereference_protected(q->blk_trace,
2118: lockdep_is_held(&q->debugfs_mutex));
2119: }
2120:
2121: if (ret == 0) {
2122: if (attr == &dev_attr_act_mask)
2123: bt->act_mask = value;
2124: else if (attr == &dev_attr_pid)
2125: bt->pid = value;
2126: else if (attr == &dev_attr_start_lba)
2127: bt->start_lba = value;
2128: else if (attr == &dev_attr_end_lba)
2129: bt->end_lba = value;
2130: }
2131:
2132: out_unlock_bdev:
2133: blk_debugfs_unlock(q, memflags);
2134: out:
2135: return ret ? ret : count;
2136: }
SourceFile:kernel/trace/blktrace.c]
|
| 94/2 |
2026/05/31 15:58 |
llm |
expert |
0m
Model:
Tokens:
input: 66239
output: 27
thoughts: 1472
Thoughts: **Analysis of the blk_trace_setup Vulnerability**
Okay, so I've been digging into this `sysfs_blk_trace_attr_store` function and how it relates to `/sys/block/DEV/trace/enable` and similar interfaces. The key observation is that these sysfs attributes are created with `S_IWUSR`, meaning only root can write to them. This immediately limits the scope of unprivileged users triggering anything here.
I then considered `blk_trace_setup_queue` and whether *that* function has any knobs that can be turned by an unprivileged user. Turns out, it doesn't allow explicit setting of `buf_size` or `buf_nr`. However, the `BLKTRACESETUP` ioctl *does* allow specifying these parameters. And, importantly, there are no capability checks on that ioctl, which raises a flag. The catch, of course, is that the ioctl requires an open file descriptor to the block device.
Since an unprivileged user can't *open* a block device like `/dev/loop2`, this seems like a dead end. The reproducer leverages `O_CREAT` but if the user is unprivileged, they get an `EACCES` error when trying to open `/dev/loop2`. So, the bug isn't accessible to unprivileged processes.
My next line of thought went to user namespaces. Are there any tricks there? No. Device nodes are not accessible in user namespaces *unless* they are bind-mounted. But even then, they retain their original permissions (root:disk). It's a non-issue from within user namespaces.
Alright, so the core of the issue is that it *can* allocate an enormous amount of memory β up to 8GB β potentially leading to a denial of service. The mutex on `q->debugfs_mutex` can block other tasks for a long time causing hangs and OOM situations.
However, since there is no exploit here, it cannot be leveraged.
I decided to double-check every possible angle. `blkdev_ioctl` is called through `def_blk_fops`. So we *must* be dealing with a block device. Can an unprivileged user *create* one? No. `mknod` requires `CAP_MKNOD`. Can an unprivileged user mount a filesystem with a block device node? Still no. Unprivileged mounts are `nodev` by default, anyway.
This is where things got interesting. If a user *somehow* managed to get a block device node created on a FUSE filesystem, with permissive permissions (0777), *and* the filesystem was *not* mounted with `nodev`, then yes, it *might* be openable. But unprivileged users can only mount in user namespaces, and the kernel always forces `nodev` for user namespace mounts! I even went and verified it in `fs/namespace.c`. `MNT_NODEV` is enforced.
What about `/dev/loop-control`? If someone had access to it they could create loop devices. Still, they couldn't open the resulting device. Devices like `/dev/binder` are character devices and not block devices.
I have to rule out an unprivileged user triggering this.
I made sure to cover the other ways `blk_trace_setup` might get called: `compat_blk_trace_setup` and `blk_trace_setup2`, both called through `blkdev_ioctl`, which means they have the same restrictions.
Here's my assessment:
* **Exploitable:** No. Just a memory allocation DoS.
* **Denial of Service:** Yes. Absolutely.
* **Accessible from Unprivileged Processes:** No.
* **Accessible from User Namespaces:** No.
* **VM Guest Trigger:** No, it's a host kernel bug.
* **Malicious Filesystem Trigger:** Unlikely due to the `nodev` mount option enforced by default. Root action would be required.
But, can a malicious filesystem be the trigger? If root mounts it without `nodev`. Desktop environments (e.g. udisks) almost always mount removable media with `nodev, nosuid, noexec`, but if root manually mounts without `nodev` the node is open.
I went back to review the memory allocation limits again. The `relay_open` function has a check: `if (subbuf_size > UINT_MAX / n_subbufs)`. This prevents alloc_size exceeding `UINT_MAX`, which is 4 GB. The problem is `UINT_MAX` per CPU. If a 64-core machine is running, that's up to 256 GB.
|
| 95/2 |
2026/05/31 15:59 |
tool |
git-log |
10m
Error:
Args: map[CodeRegexp:blk_trace_setup Count:10]
Results:
|
| 96/2 |
2026/05/31 16:09 |
llm |
expert |
0m
Model:
Tokens:
input: 68798
output: 30
thoughts: 17
Thoughts: Here's my take on the thought process, as requested:
**Diving into the Blktrace History**
Okay, so I need to understand something related to `blktrace`, probably a recent change or perhaps a bug fix that's influencing the behavior I'm seeing. My first instinct, given the context, is to go straight to the source. The best place to start is the history of the core implementation, so let's check `kernel/trace/blktrace.c`. I need to review the commit history; hopefully, I can narrow down a specific timeframe or keyword to see what relevant changes have been implemented recently. I'm hoping to quickly identify if a patch here might be related to my current investigation. Knowing the commit history will provide insight into how the code has evolved and pinpoint any regressions or modifications to a specific feature I'm testing.
|
| 97/2 |
2026/05/31 16:09 |
tool |
git-log |
0m
Args: map[Count:10 PathPrefix:kernel/trace/blktrace.c]
Results: map[Output:da46b5dfef48 blktrace: fix __this_cpu_read/write in preemptible context
bf4afc53b77a Convert 'alloc_obj' family to use the new default GFP_KERNEL argument
69050f8d6d07 treewide: Replace kmalloc with kmalloc_obj for non-scalar types
dfe48ea17973 blk-mq: use NOIO context to prevent deadlock during debugfs creation
0c2580a80946 blktrace: Make init_blk_tracer() asynchronous
ee4784a83fb2 block: don't use strcpy to copy blockdev name
bbec8e28cac5 tracing: Allow tracer to add more than 32 options
bc49af56eea8 blktrace: add support for REQ_OP_WRITE_ZEROES tracing
e48886b9d668 blktrace: for ftrace use correct trace format ver
4a0940bdcac2 blktrace: use debug print to report dropped events
]
|
| 98/2 |
2026/05/31 16:09 |
llm |
expert |
0m
Model:
Tokens:
input: 69291
output: 24
thoughts: 78
Thoughts: Here's my analysis of the provided information, as if these were my internal thoughts:
**Dissecting a Potential Blk-Mq Debugfs Deadlock Fix**
Okay, first thing's first. This `dfe48ea17973` commit, the one addressing the `blk-mq` and debugfs interaction, it's piquing my interest. Specifically, the mention of `blk_debugfs_lock_nomemsave` screams "potential deadlock mitigation". My initial hunch is that we were seeing a situation where creating debugfs entries within the `blk-mq` context could lead to locking problems, especially given the kernel's complexities. I need to understand *why* this lock was introduced. What specific scenario necessitated this fix? How might it have interacted with memory saving?
Now, let me cross-reference that with `da46b5dfef48`. The mention of `blktrace` and the `__this_cpu_read/write` functions within a preemptible context gives me another potential angle for the original issue. If a debug function was accidentally preempting during a crucial operation using those functions, it could also contribute to those deadlocks, and that commit might have been part of the solution.
I should investigate whether the commits are connected. Were they addressing the same root cause from different angles? Maybe the second one was introduced to allow for the first. Understanding the interplay of these commits is key to grasping the original problem and the effectiveness of the solution. Ultimately, I need to know the specific circumstances that necessitated the addition of a `NOIO` context to prevent the deadlock during debugfs creation within `blk-mq`. The details are crucial.
|
| 99/2 |
2026/05/31 16:09 |
tool |
git-show |
0m
Args:
Results: map[Output:commit da46b5dfef48658d03347cda21532bcdbb521e67
Author: Chaitanya Kulkarni <kch@nvidia.com>
Date: Sun Mar 1 16:22:07 2026 -0800
blktrace: fix __this_cpu_read/write in preemptible context
tracing_record_cmdline() internally uses __this_cpu_read() and
__this_cpu_write() on the per-CPU variable trace_cmdline_save, and
trace_save_cmdline() explicitly asserts preemption is disabled via
lockdep_assert_preemption_disabled(). These operations are only safe
when preemption is off, as they were designed to be called from the
scheduler context (probe_wakeup_sched_switch() / probe_wakeup()).
__blk_add_trace() was calling tracing_record_cmdline(current) early in
the blk_tracer path, before ring buffer reservation, from process
context where preemption is fully enabled. This triggers the following
using blktests/blktrace/002:
blktrace/002 (blktrace ftrace corruption with sysfs trace) [failed]
runtime 0.367s ... 0.437s
something found in dmesg:
[ 81.211018] run blktests blktrace/002 at 2026-02-25 22:24:33
[ 81.239580] null_blk: disk nullb1 created
[ 81.357294] BUG: using __this_cpu_read() in preemptible [00000000] code: dd/2516
[ 81.362842] caller is tracing_record_cmdline+0x10/0x40
[ 81.362872] CPU: 16 UID: 0 PID: 2516 Comm: dd Tainted: G N 7.0.0-rc1lblk+ #84 PREEMPT(full)
[ 81.362877] Tainted: [N]=TEST
[ 81.362878] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
[ 81.362881] Call Trace:
[ 81.362884] <TASK>
[ 81.362886] dump_stack_lvl+0x8d/0xb0
...
(See '/mnt/sda/blktests/results/nodev/blktrace/002.dmesg' for the entire message)
[ 81.211018] run blktests blktrace/002 at 2026-02-25 22:24:33
[ 81.239580] null_blk: disk nullb1 created
[ 81.357294] BUG: using __this_cpu_read() in preemptible [00000000] code: dd/2516
[ 81.362842] caller is tracing_record_cmdline+0x10/0x40
[ 81.362872] CPU: 16 UID: 0 PID: 2516 Comm: dd Tainted: G N 7.0.0-rc1lblk+ #84 PREEMPT(full)
[ 81.362877] Tainted: [N]=TEST
[ 81.362878] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
[ 81.362881] Call Trace:
[ 81.362884] <TASK>
[ 81.362886] dump_stack_lvl+0x8d/0xb0
[ 81.362895] check_preemption_disabled+0xce/0xe0
[ 81.362902] tracing_record_cmdline+0x10/0x40
[ 81.362923] __blk_add_trace+0x307/0x5d0
[ 81.362934] ? lock_acquire+0xe0/0x300
[ 81.362940] ? iov_iter_extract_pages+0x101/0xa30
[ 81.362959] blk_add_trace_bio+0x106/0x1e0
[ 81.362968] submit_bio_noacct_nocheck+0x24b/0x3a0
[ 81.362979] ? lockdep_init_map_type+0x58/0x260
[ 81.362988] submit_bio_wait+0x56/0x90
[ 81.363009] __blkdev_direct_IO_simple+0x16c/0x250
[ 81.363026] ? __pfx_submit_bio_wait_endio+0x10/0x10
[ 81.363038] ? rcu_read_lock_any_held+0x73/0xa0
[ 81.363051] blkdev_read_iter+0xc1/0x140
[ 81.363059] vfs_read+0x20b/0x330
[ 81.363083] ksys_read+0x67/0xe0
[ 81.363090] do_syscall_64+0xbf/0xf00
[ 81.363102] entry_SYSCALL_64_after_hwframe+0x76/0x7e
[ 81.363106] RIP: 0033:0x7f281906029d
[ 81.363111] Code: 31 c0 e9 c6 fe ff ff 50 48 8d 3d 66 63 0a 00 e8 59 ff 01 00 66 0f 1f 84 00 00 00 00 00 80 3d 41 33 0e 00 00 74 17 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 5b c3 66 2e 0f 1f 84 00 00 00 00 00 48 83 ec
[ 81.363113] RSP: 002b:00007ffca127dd48 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[ 81.363120] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f281906029d
[ 81.363122] RDX: 0000000000001000 RSI: 0000559f8bfae000 RDI: 0000000000000000
[ 81.363123] RBP: 0000000000001000 R08: 0000002863a10a81 R09: 00007f281915f000
[ 81.363124] R10: 00007f2818f77b60 R11: 0000000000000246 R12: 0000559f8bfae000
[ 81.363126] R13: 0000000000000000 R14: 0000000000000000 R15: 000000000000000a
[ 81.363142] </TASK>
The same BUG fires from blk_add_trace_plug(), blk_add_trace_unplug(),
and blk_add_trace_rq() paths as well.
The purpose of tracing_record_cmdline() is to cache the task->comm for
a given PID so that the trace can later resolve it. It is only
meaningful when a trace event is actually being recorded. Ring buffer
reservation via ring_buffer_lock_reserve() disables preemption, and
preemption remains disabled until the event is committed :-
__blk_add_trace()
__trace_buffer_lock_reserve()
__trace_buffer_lock_reserve()
ring_buffer_lock_reserve()
preempt_disable_notrace(); <---
With this fix blktests for blktrace pass:
blktests (master) # ./check blktrace
blktrace/001 (blktrace zone management command tracing) [passed]
runtime 3.650s ... 3.647s
blktrace/002 (blktrace ftrace corruption with sysfs trace) [passed]
runtime 0.411s ... 0.384s
Fixes: 7ffbd48d5cab ("tracing: Cache comms only after an event occurred")
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index e6988929ead2..18a9b84cc768 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -383,8 +383,6 @@ static void __blk_add_trace(struct blk_trace *bt, sector_t sector, int bytes,
cpu = raw_smp_processor_id();
if (blk_tracer) {
- tracing_record_cmdline(current);
-
buffer = blk_tr->array_buffer.buffer;
trace_ctx = tracing_gen_ctx_flags(0);
switch (bt->version) {
@@ -419,6 +417,7 @@ static void __blk_add_trace(struct blk_trace *bt, sector_t sector, int bytes,
if (!event)
return;
+ tracing_record_cmdline(current);
switch (bt->version) {
case 1:
record_blktrace_event(ring_buffer_event_data(event),
]
|
| 100/2 |
2026/05/31 16:09 |
llm |
expert |
0m
Model:
Tokens:
input: 72848
output: 66
thoughts: 1745
Thoughts: **Assessment of the blktrace DoS Vulnerability**
Okay, here's what I've figured out. This is a denial-of-service vulnerability related to `blk_trace_setup`. It looks like the core issue is that `blk_trace_setup`, when called with excessively large buffer sizes (`buf_size` and `buf_nr`), can trigger a massive memory allocation within `relay_open`. Specifically, it's the `GFP_KERNEL | __GFP_ZERO` allocation in `relay_open` that's the culprit. This can lead to exhaustion of system memory, potentially up to 4GB per CPU, and cause tasks to hang. It looks like the allocation is done in `relay_open` via the `relay_alloc_buf` function.
Now, the important part. Can this be exploited by a non-root user? The short answer is no. `blk_trace_setup` is accessible through block device nodes. However, opening these block device nodes, like `/dev/loop2`, is restricted to either root or users belonging to the `disk` group. Since the prompt specifies a typical unprivileged user *without* any special access or capabilities, they won't be able to open block devices. The reproducer calls `openat` on `/dev/loop2` to set up this test, but unprivileged users will receive an `EACCES` error, as they do not have sufficient permissions to perform this action.
I considered the `disk` group, but that's still considered a privileged access scenario. The prompt specifies "NOT have any special capabilities or access to device nodes restricted to root," so that user scenario is excluded.
I also considered if this vulnerability can be triggered through user namespaces, but block device access is typically restricted there as well. The system calls being used are not accessible from user namespaces.
I also considered other ways `blk_trace_setup` might be triggered. Is it exported? No. Is it called elsewhere? No. It appears to be exclusively accessed through `blk_trace_ioctl` and the `compat_blk_trace_setup` function, which are themselves reached through `blkdev_ioctl`. So, again, it's tied to block device nodes.
I verified that `blk_trace_setup` cannot be called on a mounted filesystem that uses `ioctl` via a loop device. It is called directly from the block device node. Filesystems don't pass `BLKTRACESETUP` to the underlying block device.
Is it a Denial of Service? Yes. Is it exploitable? No. Can it be triggered from unprivileged users or user namespaces? No. Not from a VM Guest or Host, or via network, peripherals, or malicious filesystems, either.
I then did a deeper dive, looking for potential integer overflows that could lead to out-of-bounds writes. The check in `relay_open` seems to prevent an overflow in the `subbuf_size * n_subbufs` calculation, but I needed to double check the multiplication, and what would happen with an `alloc_size` of 0. I found that if an overflow resulted in a zero-sized allocation in `relay_alloc_buf`, `vmap` would return `NULL`. This would prevent any out-of-bounds access.
So, in summary, on 32-bit systems an overflow leading to an allocation size of zero is handled gracefully, causing the system to return an error. On 64-bit, the large allocation size will simply result in a denial of service, but does not lead to any memory corruption.
Therefore, the issue is purely a denial of service that requires either root or `disk` group access. It is not accessible to unprivileged users.
|
| 101/2 |
2026/05/31 16:09 |
tool |
set-results |
0m
Args: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 102/2 |
2026/05/31 16:09 |
llm |
expert |
1m
Model:
Tokens:
input: 75516
output: 700
thoughts: 399
Thoughts: **My Analysis of the Block Trace Hung Task**
Okay, the results are in, and now it's time to craft the final response. My reasoning needs to clearly articulate the problem, so here's how I'm going to structure it:
First, this hung task is unequivocally traced back to `blk_trace_setup`. This function is invoked when a user utilizes the `BLKTRACESETUP` ioctl on a block device. Pretty straightforward so far.
The core of the issue lies within the ioctl's parameters: `buf_size` and `buf_nr`. These values, as the astute user knows, dictate the allocation strategy within `relay_open`. The function allocates `buf_size * buf_nr` bytes per CPU. The only check in place prevents the product from exceeding `UINT_MAX` β a somewhat naive safeguard.
Now, here's where things get interesting, and potentially problematic. On a multi-core system, this seemingly simple calculation balloons into a substantial memory allocation. I'm talking potentially gigabytes, even hundreds of gigabytes, depending on the number of CPUs and the specified parameters. A 2-CPU system could request 8GB and a 64-CPU machine could request 256GB.
This massive allocation is done page-by-page, using `alloc_page(GFP_KERNEL | __GFP_ZERO)`, all while holding `q->debugfs_mutex`. The act of the allocation consumes time and resources, creating significant memory pressure. The watchdog triggers hung task warnings as other tasks attempt to acquire `q->debugfs_mutex`, effectively stalling or even deadlocking processes.
In essence, this is a denial-of-service vulnerability, based on both resource exhaustion and the potential for deadlock. We're talking about a significant performance hit, though it is not a privileged escalation vulnerability. There is no memory corruption, and a failed allocation gracefully handles the error. If `PAGE_ALIGN` overflows on a 32-bit system, it results in a size of 0. This is caught when `vmap` returns `NULL`.
A final, and crucial, point: this bug isn't a free-for-all. It's not accessible to unprivileged users. It requires opening a block device node, such as `/dev/loop2`. This operation is restricted to the root user or users belonging to the `disk` group. User namespaces provide no workaround. Further, the bug is not triggered remotely, via network, or by a malicious filesystem mount.
|
| 103/1 |
2026/05/31 16:11 |
action |
format-explanation |
0m
Results: map[Explanation:**Bug Analysis:**
The bug report indicates a hung task in `blk_trace_setup`, specifically blocked
while trying to acquire `q->debugfs_mutex`.
Looking at the reproducer and the kernel source code, the issue is triggered by
calling the `BLKTRACESETUP` ioctl on a block device (in this case,
`/dev/loop2`). This ioctl allows the user to configure block tracing and specify
the size (`buf_size`) and number (`buf_nr`) of the relay buffers to be
allocated.
In `blk_trace_setup_prepare`, the kernel calls `relay_open` to allocate these
buffers. The only validation performed on the size is ensuring that `buf_size *
buf_nr` does not exceed `UINT_MAX` (4 GB). If a user provides values that
maximize this product (e.g., `buf_size = 0x100100` and `buf_nr = 0xfff` as seen
in the reproducer), `relay_open` will attempt to allocate nearly 4 GB of memory
**per CPU**. On a standard 2-CPU system, this is 8 GB; on a 64-CPU server, this
would be 256 GB.
The memory is allocated page-by-page in a loop using `alloc_page(GFP_KERNEL |
__GFP_ZERO)`. Allocating millions of pages takes a significant amount of time
and induces massive memory pressure (potentially triggering the OOM killer).
Crucially, this allocation happens while the task is holding the
`q->debugfs_mutex`. Any other tasks attempting to perform operations on the
block device that require this mutex (like other tracing setups, or
registering/unregistering queues) will be blocked indefinitely, eventually
triggering the hung task watchdog.
**Security Impact Assessment:**
* **Exploitable:** No. This is a pure resource exhaustion issue. There is no
memory corruption, use-after-free, or out-of-bounds access. If the allocation
eventually fails, the kernel gracefully frees the already allocated pages and
returns an error. Even on 32-bit systems where `PAGE_ALIGN` could theoretically
overflow to 0, the underlying `vmap` implementation safely rejects 0-sized
allocations and returns `NULL`.
* **Denial Of Service:** Yes. A user can easily exhaust system memory and cause
deadlocks by holding the `debugfs_mutex` for extended periods, leading to system
instability and hung tasks.
* **Accessible From Unprivileged Processes:** No. Triggering this bug requires
calling an ioctl on a block device node. By default, block device nodes (like
`/dev/loop2`, `/dev/sda`) are owned by `root:disk` with `rw-rw----` permissions.
A typical unprivileged user does not have the permissions required to open these
device nodes.
* **Accessible From User Namespaces:** No. User namespaces do not grant access
to block device nodes on the host. Even if a user namespace is created, device
nodes retain their original permissions and cannot be opened or recreated by
unprivileged users.
* **Other Triggers:** The bug cannot be triggered via network traffic, remote
connections, peripherals, or malicious filesystems. It strictly requires a local
process with sufficient privileges to open a block device node and issue the
specific ioctl.]
|