| 0/0 |
2026/05/25 04:31 |
flow |
assessment-security |
51m
Results: map[DenialOfService:false Explanation:**Analysis:**
The reported issue is a KMSAN (Kernel Memory Sanitizer) warning about reading
uninitialized memory in `copy_from_kernel_nofault`, which is called by
`prepend_copy` during path resolution (`prepend_path`).
This report is a **false positive** and has **no security impact** for the
following reasons:
1. **Optimistic Lockless Reads:** To optimize performance, `prepend_name` reads
a dentry's name pointer and length locklessly. If a concurrent rename (`d_move`)
occurs, a data race can happen where `prepend_name` fetches a mismatched name
pointer and length (e.g., the pointer to a new, shorter name, but the length of
the old, longer name).
2. **Safe Memory Copy:** `prepend_copy` then uses `copy_from_kernel_nofault` to
copy the name into a temporary buffer. Because of the mismatched length, it may
read past the end of the initialized string into the uninitialized padding of
`dentry->d_shortname`. KMSAN intercepts this read and flags it as an
uninitialized value use.
3. **Sequence Lock Retry Mechanism:** The kernel is fully aware of this race
condition. After the lockless read, `prepend_path` checks the `rename_lock`
sequence counter (`need_seqretry`). If a concurrent rename is detected, the
operation is aborted, the temporary buffer containing the garbage/uninitialized
bytes is discarded, and the path resolution is retried with the lock held.
4. **Fault Handling:** Even if the mismatched length is large enough to cause
`copy_from_kernel_nofault` to read into an unmapped page, the function disables
page faults and gracefully handles the error by returning `-EFAULT`, preventing
any kernel crashes.
**Conclusion:**
The uninitialized bytes are never leaked to userspace, used in any control flow,
or written to persistent storage. They are safely discarded by the sequence lock
retry mechanism. There is no memory corruption, no information leak, and no risk
of Denial of Service (DoS) in production builds where KMSAN is disabled.
While unprivileged users can trigger this warning in a KMSAN-enabled debug
kernel by racing file opens (e.g., via the TOMOYO security module as seen in the
trace) and file renames, it does not represent a vulnerability. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/05/25 04:31 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/05/25 04:31 |
action |
kernel-checkouter |
1m
Results: map[KernelSrc:/app/workdir/cache/src/f39a207a13f89c53fd2e8bbde7e26ff46eff3645]
|
| 3/1 |
2026/05/25 04:32 |
action |
kernel-builder |
27m
Results: map[KernelObj:/app/workdir/cache/build/2f20d6175a15cf012d1b14ef7639efb06f0a1d79]
|
| 4/1 |
2026/05/25 05:00 |
action |
codesearch-prepare |
18m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/05/25 05:18 |
agent |
expert |
3m
Model:
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true 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 greeping
".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 completly 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 tranfers, that's 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 exploitabability 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 Confidetial 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:
=====================================================
BUG: KMSAN: uninit-value in copy_from_kernel_nofault+0x15f/0x570 mm/maccess.c:41
copy_from_kernel_nofault+0x15f/0x570 mm/maccess.c:41
prepend_copy fs/d_path.c:50 [inline]
prepend fs/d_path.c:76 [inline]
prepend_name fs/d_path.c:101 [inline]
__prepend_path fs/d_path.c:133 [inline]
prepend_path+0x64e/0x1090 fs/d_path.c:172
d_absolute_path+0x11b/0x240 fs/d_path.c:234
tomoyo_get_absolute_path security/tomoyo/realpath.c:101 [inline]
tomoyo_realpath_from_path+0x4bd/0x9f0 security/tomoyo/realpath.c:271
tomoyo_get_realpath security/tomoyo/file.c:151 [inline]
tomoyo_check_open_permission+0x1ff/0xc60 security/tomoyo/file.c:776
tomoyo_file_open+0x290/0x310 security/tomoyo/tomoyo.c:334
security_file_open+0x116/0x4d0 security/security.c:2739
do_dentry_open+0x5fe/0x1fc0 fs/open.c:924
vfs_open+0x52/0x5f0 fs/open.c:1079
do_open fs/namei.c:4699 [inline]
path_openat+0x5733/0x64c0 fs/namei.c:4858
do_file_open+0x2aa/0x680 fs/namei.c:4887
do_sys_openat2+0x163/0x370 fs/open.c:1364
do_sys_open fs/open.c:1370 [inline]
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__x64_sys_openat+0x240/0x300 fs/open.c:1381
x64_sys_call+0x2445/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:258
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0x390 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
<Zero or more stacks not recorded to save memory>
Uninit was stored to memory at:
copy_name fs/dcache.c:2928 [inline]
__d_move+0xf72/0x2aa0 fs/dcache.c:2997
d_move+0x71/0xf0 fs/dcache.c:3044
vfs_rename+0x2510/0x2650 fs/namei.c:6069
filename_renameat2+0xb7f/0x1260 fs/namei.c:6172
__do_sys_rename fs/namei.c:6216 [inline]
__se_sys_rename+0xc5/0x5d0 fs/namei.c:6212
__x64_sys_rename+0x78/0xb0 fs/namei.c:6212
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0x390 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Uninit was stored to memory at:
copy_name fs/dcache.c:2928 [inline]
__d_move+0xf72/0x2aa0 fs/dcache.c:2997
d_move+0x71/0xf0 fs/dcache.c:3044
vfs_rename+0x2510/0x2650 fs/namei.c:6069
filename_renameat2+0xb7f/0x1260 fs/namei.c:6172
__do_sys_rename fs/namei.c:6216 [inline]
__se_sys_rename+0xc5/0x5d0 fs/namei.c:6212
__x64_sys_rename+0x78/0xb0 fs/namei.c:6212
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0x390 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Uninit was stored to memory at:
copy_name fs/dcache.c:2928 [inline]
__d_move+0xf72/0x2aa0 fs/dcache.c:2997
d_move+0x71/0xf0 fs/dcache.c:3044
vfs_rename+0x2510/0x2650 fs/namei.c:6069
filename_renameat2+0xb7f/0x1260 fs/namei.c:6172
__do_sys_rename fs/namei.c:6216 [inline]
__se_sys_rename+0xc5/0x5d0 fs/namei.c:6212
__x64_sys_rename+0x78/0xb0 fs/namei.c:6212
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0x390 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Uninit was stored to memory at:
copy_name fs/dcache.c:2928 [inline]
__d_move+0xf72/0x2aa0 fs/dcache.c:2997
d_move+0x71/0xf0 fs/dcache.c:3044
vfs_rename+0x2510/0x2650 fs/namei.c:6069
filename_renameat2+0xb7f/0x1260 fs/namei.c:6172
__do_sys_rename fs/namei.c:6216 [inline]
__se_sys_rename+0xc5/0x5d0 fs/namei.c:6212
__x64_sys_rename+0x78/0xb0 fs/namei.c:6212
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0x390 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Uninit was stored to memory at:
copy_name fs/dcache.c:2928 [inline]
__d_move+0xf72/0x2aa0 fs/dcache.c:2997
d_move+0x71/0xf0 fs/dcache.c:3044
vfs_rename+0x2510/0x2650 fs/namei.c:6069
filename_renameat2+0xb7f/0x1260 fs/namei.c:6172
__do_sys_rename fs/namei.c:6216 [inline]
__se_sys_rename+0xc5/0x5d0 fs/namei.c:6212
__x64_sys_rename+0x78/0xb0 fs/namei.c:6212
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0x390 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Uninit was stored to memory at:
copy_name fs/dcache.c:2928 [inline]
__d_move+0xf72/0x2aa0 fs/dcache.c:2997
d_move+0x71/0xf0 fs/dcache.c:3044
vfs_rename+0x2510/0x2650 fs/namei.c:6069
filename_renameat2+0xb7f/0x1260 fs/namei.c:6172
__do_sys_rename fs/namei.c:6216 [inline]
__se_sys_rename+0xc5/0x5d0 fs/namei.c:6212
__x64_sys_rename+0x78/0xb0 fs/namei.c:6212
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0x390 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Uninit was stored to memory at:
copy_name fs/dcache.c:2928 [inline]
__d_move+0xf72/0x2aa0 fs/dcache.c:2997
d_move+0x71/0xf0 fs/dcache.c:3044
vfs_rename+0x2510/0x2650 fs/namei.c:6069
filename_renameat2+0xb7f/0x1260 fs/namei.c:6172
__do_sys_rename fs/namei.c:6216 [inline]
__se_sys_rename+0xc5/0x5d0 fs/namei.c:6212
__x64_sys_rename+0x78/0xb0 fs/namei.c:6212
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0x390 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Uninit was created at:
slab_post_alloc_hook mm/slub.c:4577 [inline]
slab_alloc_node mm/slub.c:4899 [inline]
kmem_cache_alloc_lru_noprof+0x37a/0x1260 mm/slub.c:4918
__d_alloc+0x55/0xa00 fs/dcache.c:1807
d_alloc+0x57/0x300 fs/dcache.c:1886
lookup_one_qstr_excl+0x19d/0x7b0 fs/namei.c:1801
__start_renaming+0x38e/0x870 fs/namei.c:3890
filename_renameat2+0x735/0x1260 fs/namei.c:6147
__do_sys_rename fs/namei.c:6216 [inline]
__se_sys_rename+0xc5/0x5d0 fs/namei.c:6212
__x64_sys_rename+0x78/0xb0 fs/namei.c:6212
x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0x390 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Bytes 5-7 of 8 are uninitialized
Memory access of size 8 starts at ffff888053c96ab8
CPU: 1 UID: 0 PID: 5951 Comm: udevd Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
=====================================================
Reply: **Analysis:**
The reported issue is a KMSAN (Kernel Memory Sanitizer) warning about reading uninitialized memory in `copy_from_kernel_nofault`, which is called by `prepend_copy` during path resolution (`prepend_path`).
This report is a **false positive** and has **no security impact** for the following reasons:
1. **Optimistic Lockless Reads:** To optimize performance, `prepend_name` reads a dentry's name pointer and length locklessly. If a concurrent rename (`d_move`) occurs, a data race can happen where `prepend_name` fetches a mismatched name pointer and length (e.g., the pointer to a new, shorter name, but the length of the old, longer name).
2. **Safe Memory Copy:** `prepend_copy` then uses `copy_from_kernel_nofault` to copy the name into a temporary buffer. Because of the mismatched length, it may read past the end of the initialized string into the uninitialized padding of `dentry->d_shortname`. KMSAN intercepts this read and flags it as an uninitialized value use.
3. **Sequence Lock Retry Mechanism:** The kernel is fully aware of this race condition. After the lockless read, `prepend_path` checks the `rename_lock` sequence counter (`need_seqretry`). If a concurrent rename is detected, the operation is aborted, the temporary buffer containing the garbage/uninitialized bytes is discarded, and the path resolution is retried with the lock held.
4. **Fault Handling:** Even if the mismatched length is large enough to cause `copy_from_kernel_nofault` to read into an unmapped page, the function disables page faults and gracefully handles the error by returning `-EFAULT`, preventing any kernel crashes.
**Conclusion:**
The uninitialized bytes are never leaked to userspace, used in any control flow, or written to persistent storage. They are safely discarded by the sequence lock retry mechanism. There is no memory corruption, no information leak, and no risk of Denial of Service (DoS) in production builds where KMSAN is disabled.
While unprivileged users can trigger this warning in a KMSAN-enabled debug kernel by racing file opens (e.g., via the TOMOYO security module as seen in the trace) and file renames, it does not represent a vulnerability.
|
| 6/2 |
2026/05/25 05:18 |
llm |
expert |
0m
Model:
Tokens:
input: 9118
output: 39
thoughts: 519
Thoughts: **Analyzing a KMSAN Report: Uninitialized Value in Rename Operation**
Alright, let's break down this KMSAN report. It looks like we have an uninitialized value being read, and it's happening during a rename operation. The report points to `copy_from_kernel_nofault` in `mm/maccess.c` as the site where the uninitialized data is being used. Specifically, it seems like the memory copy happened within the kernel.
The stack trace tells a story. The uninitialized value originated in `mm/slub.c` during memory allocation, specifically within the `kmem_cache_alloc_lru_noprof` function, which then trickles down into `__d_alloc` and `d_alloc` in `fs/dcache.c`. This is where we allocate the dentry, and the uninitialized data gets its start. Then, that uninitialized data gets used during the rename, starting with `lookup_one_qstr_excl` in `fs/namei.c`, and eventually showing up in a `copy_name`, and finally in `vfs_rename` in `fs/namei.c` which leads to the actual call to `copy_from_kernel_nofault`.
Now, the access itself is flagged within `copy_from_kernel_nofault`, and is called from a `prepend_copy` call in `fs/d_path.c:50`. The kernel is copying something using this copy function, and it's reading the uninitialized value during this copy. I'm going to turn my attention to `prepend_copy` in `fs/d_path.c` next to understand how this copy fits into the larger picture of what's happening during the rename operation. I bet we will see that the dentry name may be uninitialized and passed to this function.
|
| 7/2 |
2026/05/25 05:18 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/d_path.c IncludeLines:true Name:prepend_copy]
Results: map[SourceCode: 37: /*
38: * The source of the prepend data can be an optimistic load
39: * of a dentry name and length. And because we don't hold any
40: * locks, the length and the pointer to the name may not be
41: * in sync if a concurrent rename happens, and the kernel
42: * copy might fault as a result.
43: *
44: * The end result will correct itself when we check the
45: * rename sequence count, but we need to be able to handle
46: * the fault gracefully.
47: */
48: static bool prepend_copy(void *dst, const void *src, int len)
49: {
50: if (unlikely(copy_from_kernel_nofault(dst, src, len))) {
51: memset(dst, 'x', len);
52: return false;
53: }
54: return true;
55: }
SourceFile:fs/d_path.c]
|
| 8/2 |
2026/05/25 05:18 |
llm |
expert |
0m
Model:
Tokens:
input: 10374
output: 43
thoughts: 121
Thoughts: **Analyzing `prepend_copy` and Kernel Memory Safety**
Okay, so `prepend_copy`... it's clearly built with concurrency and potential errors from renames in mind, which is good. That `copy_from_kernel_nofault` function is key here. It suggests a focus on robust handling of kernel memory access, even when things get dicey.
Hmm, this uninitialized value seems to be popping up in `dentry->d_name.name`. Makes sense. If the length of the name isn't properly synchronized with the actual content, or if there's just uninitialized padding, that's a classic source for these issues.
Wait a second... KMSAN is specifically flagging an uninit value inside `copy_from_kernel_nofault`? That's not ideal. This needs a closer look. Let's dig into `mm/maccess.c` and see what's actually going on in `copy_from_kernel_nofault`. I need to understand why KMSAN is complaining about this specific function. This could be a deeper problem than initially assumed. Time to put on my debugging hat.
|
| 9/2 |
2026/05/25 05:19 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/maccess.c IncludeLines:true Name:copy_from_kernel_nofault]
Results: map[SourceCode: 29: long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
30: {
31: unsigned long align = 0;
32:
33: if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
34: align = (unsigned long)dst | (unsigned long)src;
35:
36: if (!copy_from_kernel_nofault_allowed(src, size))
37: return -ERANGE;
38:
39: pagefault_disable();
40: if (!(align & 7))
41: copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
42: if (!(align & 3))
43: copy_from_kernel_nofault_loop(dst, src, size, u32, Efault);
44: if (!(align & 1))
45: copy_from_kernel_nofault_loop(dst, src, size, u16, Efault);
46: copy_from_kernel_nofault_loop(dst, src, size, u8, Efault);
47: pagefault_enable();
48: return 0;
49: Efault:
50: pagefault_enable();
51: return -EFAULT;
52: }
SourceFile:mm/maccess.c]
|
| 10/2 |
2026/05/25 05:19 |
llm |
expert |
0m
Model:
Tokens:
input: 11186
output: 45
thoughts: 59
Thoughts: Here's my take on the thought process:
**Diving into the Code: Uninitialized Value Location**
Okay, I'm tracing this down... the uninitialized value is being read at `copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);` which is line 41. Wait a second, hold on, I should know better! `copy_from_kernel_nofault_loop` isn't a function, it's a macro. Right, so I need to expand that and see what's *actually* happening at that point. Gotta dig deeper... macros can be tricky.
|
| 11/2 |
2026/05/25 05:19 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:mm/maccess.c IncludeLines:true Name:copy_from_kernel_nofault_loop]
Results: map[SourceCode: SourceFile:]
|
| 12/2 |
2026/05/25 05:19 |
llm |
expert |
0m
Model:
Tokens:
input: 11452
output: 25
thoughts: 17
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Kernel Memory Copy Analysis**
Okay, so I'm diving into this. My initial instinct is to track down `copy_from_kernel_nofault_loop`. I'm guessing this function, or one closely related, is central to the issue I'm investigating. I need to understand its behavior, especially in relation to the specific problem I'm trying to diagnose. I'm thinking about the implications of the "nofault" aspect. What are the trade-offs there? What failure scenarios are accounted for, and what are the edge cases that might be exposed? My mental checklist is running: potential race conditions, boundary checks, and the underlying architecture it's optimized for β all are key to understand the potential vulnerabilities, efficiencies, or issues I might encounter. I'll need to trace its call sites, understand the contexts where it's used, and the types of data it's copying. This function is likely a pivotal point of access to kernel memory, so getting a solid grasp on it is going to be crucial. I'm anticipating that understanding the loop and memory regions in the associated source code are going to be key, and any relevant architecture and operating systems documentation will also be of use.
|
| 13/2 |
2026/05/25 05:19 |
tool |
grepper |
0m
Args: map[Expression:define copy_from_kernel_nofault_loop]
Results: map[Output:mm/maccess.c=10=bool __weak copy_from_kernel_nofault_allowed(const void *unsafe_src,
--
mm/maccess.c-19- */
mm/maccess.c:20:#define copy_from_kernel_nofault_loop(dst, src, len, type, err_label) \
mm/maccess.c-21- while (len >= sizeof(type)) { \
]
|
| 14/2 |
2026/05/25 05:19 |
llm |
expert |
0m
Model:
Tokens:
input: 11882
output: 21
thoughts: 20
Thoughts: Okay, here's my interpretation of what's going through my mind, delivered in the first person:
**Analyzing mm/maccess.c (Lines 20-28)**
Alright, let's dive into `mm/maccess.c`, specifically lines 20 through 28. My initial thought is probably going to be about context. What's this file *doing*? I need to quickly recall the general purpose of this section of code. Is it related to memory allocation, access control, virtual memory management, or something else entirely? Once I ground myself in the overall functionality, I can look at the specifics. I'll be watching for anything unusual or interesting in this snippet: how the access mechanism is handled, what data structures are in play, if there's any cleverness or obvious inefficiency. I'm going to quickly skim the lines, looking for function calls, variable declarations (and types!), control structures, and any comments that might give some insight into the intent of the code. Is there any obvious optimization, potential for race conditions or security vulnerabilities? Then after that I'll start with a closer read.
|
| 15/2 |
2026/05/25 05:19 |
tool |
read-file |
0m
Args:
Results: map[Contents:// SPDX-License-Identifier: GPL-2.0-only
/*
* Access kernel or user memory without faulting.
*/
#include <linux/export.h>
#include <linux/mm.h>
#include <linux/uaccess.h>
#include <asm/tlb.h>
bool __weak copy_from_kernel_nofault_allowed(const void *unsafe_src,
size_t size)
{
return true;
}
/*
* The below only uses kmsan_check_memory() to ensure uninitialized kernel
* memory isn't leaked.
*/
#define copy_from_kernel_nofault_loop(dst, src, len, type, err_label) \
while (len >= sizeof(type)) { \
__get_kernel_nofault(dst, src, type, err_label); \
kmsan_check_memory(src, sizeof(type)); \
dst += sizeof(type); \
src += sizeof(type); \
len -= sizeof(type); \
}
long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
{
unsigned long align = 0;
if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
align = (unsigned long)dst | (unsigned long)src;
if (!copy_from_kernel_nofault_allowed(src, size))
return -ERANGE;
pagefault_disable();
if (!(align & 7))
copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
if (!(align & 3))
copy_from_kernel_nofault_loop(dst, src, size, u32, Efault);
if (!(align & 1))
copy_from_kernel_nofault_loop(dst, src, size, u16, Efault);
copy_from_kernel_nofault_loop(dst, src, size, u8, Efault);
pagefault_enable();
return 0;
Efault:
pagefault_enable();
return -EFAULT;
}
EXPORT_SYMBOL_GPL(copy_from_kernel_nofault);
#define copy_to_kernel_nofault_loop(dst, src, len, type, err_label) \
while (len >= sizeof(type)) { \
__put_kernel_nofault(dst, src, type, err_label); \
instrument_write(dst, sizeof(type)); \
dst += sizeof(type); \
src += sizeof(type); \
len -= sizeof(type); \
}
long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
{
unsigned long align = 0;
if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
align = (unsigned long)dst | (unsigned long)src;
pagefault_disable();
if (!(align & 7))
copy_to_kernel_nofault_loop(dst, src, size, u64, Efault);
if (!(align & 3))
copy_to_kernel_nofault_loop(dst, src, size, u32, Efault);
if (!(align & 1))
copy_to_kernel_nofault_loop(dst, src, size, u16, Efault);
copy_to_kernel_nofault_loop(dst, src, size, u8, Efault);
pagefault_enable();
return 0;
Efault:
pagefault_enable();
return -EFAULT;
}
long strncpy_from_kernel_nofault(char *dst, const void *unsafe_addr, long count)
{
const void *src = unsafe_addr;
if (unlikely(count <= 0))
return 0;
if (!copy_from_kernel_nofault_allowed(unsafe_addr, count))
return -ERANGE;
pagefault_disable();
do {
__get_kernel_nofault(dst, src, u8, Efault);
dst++;
src++;
} while (dst[-1] && src - unsafe_addr < count);
pagefault_enable();
dst[-1] = '\0';
return src - unsafe_addr;
Efault:
pagefault_enable();
dst[0] = '\0';
return -EFAULT;
}
/**
* copy_from_user_nofault(): safely attempt to read from a user-space location
* @dst: pointer to the buffer that shall take the data
* @src: address to read from. This must be a user address.
* @size: size of the data chunk
*
* Safely read from user address @src to the buffer at @dst. If a kernel fault
* happens, handle that and return -EFAULT.
*/
long copy_from_user_nofault(void *dst, const void __user *src, size_t size)
{
long ret = -EFAULT;
if (!__access_ok(src, size))
return ret;
if (!nmi_uaccess_okay())
return ret;
pagefault_disable();
ret = __copy_from_user_inatomic(dst, src, size);
pagefault_enable();
if (ret)
return -EFAULT;
return 0;
}
EXPORT_SYMBOL_GPL(copy_from_user_nofault);
/**
* copy_to_user_nofault(): safely attempt to write to a user-space location
* @dst: address to write to
* @src: pointer to the data that shall be written
* @size: size of the data chunk
*
* Safely write to address @dst from the buffer at @src. If a kernel fault
* happens, handle that and return -EFAULT.
*/
long copy_to_user_nofault(void __user *dst, const void *src, size_t size)
{
long ret = -EFAULT;
if (access_ok(dst, size)) {
pagefault_disable();
ret = __copy_to_user_inatomic(dst, src, size);
pagefault_enable();
}
if (ret)
return -EFAULT;
return 0;
}
EXPORT_SYMBOL_GPL(copy_to_user_nofault);
/**
* strncpy_from_user_nofault: - Copy a NUL terminated string from unsafe user
* address.
* @dst: Destination address, in kernel space. This buffer must be at
* least @count bytes long.
* @unsafe_addr: Unsafe user address.
* @count: Maximum number of bytes to copy, including the trailing NUL.
*
* Copies a NUL-terminated string from unsafe user address to kernel buffer.
*
* On success, returns the length of the string INCLUDING the trailing NUL.
*
* If access fails, returns -EFAULT (some data may have been copied
* and the trailing NUL added).
*
* If @count is smaller than the length of the string, copies @count-1 bytes,
* sets the last byte of @dst buffer to NUL and returns @count.
*/
long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
long count)
{
long ret;
if (unlikely(count <= 0))
return 0;
pagefault_disable();
ret = strncpy_from_user(dst, unsafe_addr, count);
pagefault_enable();
if (ret >= count) {
ret = count;
dst[ret - 1] = '\0';
} else if (ret >= 0) {
ret++;
}
return ret;
}
/**
* strnlen_user_nofault: - Get the size of a user string INCLUDING final NUL.
* @unsafe_addr: The string to measure.
* @count: Maximum count (including NUL)
*
* Get the size of a NUL-terminated string in user space without pagefault.
*
* Returns the size of the string INCLUDING the terminating NUL.
*
* If the string is too long, returns a number larger than @count. User
* has to check the return value against "> count".
* On exception (or invalid count), returns 0.
*
* Unlike strnlen_user, this can be used from IRQ handler etc. because
* it disables pagefaults.
*/
long strnlen_user_nofault(const void __user *unsafe_addr, long count)
{
int ret;
pagefault_disable();
ret = strnlen_user(unsafe_addr, count);
pagefault_enable();
return ret;
}
void __copy_overflow(int size, unsigned long count)
{
WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
}
EXPORT_SYMBOL(__copy_overflow);
]
|
| 16/2 |
2026/05/25 05:19 |
llm |
expert |
0m
Model:
Tokens:
input: 14358
output: 38
thoughts: 117
Thoughts: **Uninitialized Memory in the Dentry Cache**
Okay, so I see `kmsan_check_memory(src, sizeof(type));` called in `copy_from_kernel_nofault_loop`. That's where we're detecting the uninitialized memory access. I need to trace this back. Right, the uninit value originated at `__d_alloc`, which then led to `kmem_cache_alloc_lru_noprof` and finally `slab_alloc_node`. That makes sense; we're allocating memory from the slab allocator. The critical piece here is *where* that uninitialized memory is being *used*. The key is that the value ended up being stored in `copy_name` at `fs/dcache.c:2928`. Alright, that points me directly to `fs/dcache.c`. Let's take a look at the `copy_name` function; that's where the problem is manifesting. I need to understand how we're writing to this dentry's name field and what the allocation context is.
|
| 17/2 |
2026/05/25 05:19 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:copy_name]
Results: map[SourceCode:2919: static void copy_name(struct dentry *dentry, struct dentry *target)
2920: {
2921: struct external_name *old_name = NULL;
2922: if (unlikely(dname_external(dentry)))
2923: old_name = external_name(dentry);
2924: if (unlikely(dname_external(target))) {
2925: atomic_inc(&external_name(target)->count);
2926: dentry->__d_name = target->__d_name;
2927: } else {
2928: dentry->d_shortname = target->d_shortname;
2929: dentry->__d_name.name = dentry->d_shortname.string;
2930: dentry->__d_name.hash_len = target->__d_name.hash_len;
2931: }
2932: if (old_name && likely(atomic_dec_and_test(&old_name->count)))
2933: kfree_rcu(old_name, head);
2934: }
SourceFile:fs/dcache.c]
|
| 18/2 |
2026/05/25 05:19 |
llm |
expert |
0m
Model:
Tokens:
input: 15074
output: 37
thoughts: 45
Thoughts: Okay, here's my interpretation of that thought process, from an expert's perspective:
**Dentry Shortname Assignment**
Right, so I see a `dentry->d_shortname = target->d_shortname;` operation. Hmm, `d_shortname`. Immediately, my mind flags that it's a union within the `struct dentry`. I need to ensure the correct memory layout and how these two interact. Let's quickly refresh my mental map of the `struct dentry` layout to be certain about its implications. Are there any potential alignment issues or subtle side effects I should be aware of here? Best to double-check that structure definition.
|
| 19/2 |
2026/05/25 05:19 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:fs/dcache.c FieldOffset:<nil> Name:dentry]
Results: map[Fields:[map[Name:d_flags OffsetBits:0 SizeBits:32] map[Name:d_seq OffsetBits:32 SizeBits:32] map[Name:d_hash OffsetBits:64 SizeBits:128] map[Name:d_parent OffsetBits:192 SizeBits:64] map[Name: OffsetBits:256 SizeBits:128] map[Name:d_inode OffsetBits:384 SizeBits:64] map[Name:d_shortname OffsetBits:448 SizeBits:320] map[Name:d_op OffsetBits:768 SizeBits:64] map[Name:d_sb OffsetBits:832 SizeBits:64] map[Name:d_time OffsetBits:896 SizeBits:64] map[Name:d_fsdata OffsetBits:960 SizeBits:64] map[Name:d_lockref OffsetBits:1024 SizeBits:64] map[Name: OffsetBits:1088 SizeBits:128] map[Name:d_sib OffsetBits:1216 SizeBits:128] map[Name:d_children OffsetBits:1344 SizeBits:64] map[Name: OffsetBits:1408 SizeBits:128]]]
|
| 20/2 |
2026/05/25 05:19 |
llm |
expert |
0m
Model:
Tokens:
input: 15645
output: 39
thoughts: 165
Thoughts: **Analyzing Potential Uninitialized Memory Issues**
Okay, so I'm looking at this `d_shortname` field and I see it's 320 bits, or 40 bytes. The code copies it wholesale, like this: `dentry->d_shortname = target->d_shortname;`. Now, a red flag pops up. What if `target->d_shortname` isn't fully initialized? If the actual file name is shorter than 40 bytes, that means uninitialized garbage bytes get copied along with the name.
And then, later on, `prepend_copy` gets called, and *it* reads `dentry->d_name.name`, which points directly to the `dentry->d_shortname.string`, and it reads `len` bytes. Hmm... `prepend_copy` is going to happily chomp up those uninitialized bytes we just copied! This seems like a potential memory disclosure vulnerability. I should probably dig into the `prepend_name` function in `fs/d_path.c` to see how it's being used. This could be exploitable.
|
| 21/2 |
2026/05/25 05:19 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/d_path.c IncludeLines:true Name:prepend_name]
Results: map[SourceCode: 79: /**
80: * prepend_name - prepend a pathname in front of current buffer pointer
81: * @p: prepend buffer which contains buffer pointer and allocated length
82: * @name: name string and length qstr structure
83: *
84: * With RCU path tracing, it may race with d_move(). Use READ_ONCE() to
85: * make sure that either the old or the new name pointer and length are
86: * fetched. However, there may be mismatch between length and pointer.
87: * But since the length cannot be trusted, we need to copy the name very
88: * carefully when doing the prepend_copy(). It also prepends "/" at
89: * the beginning of the name. The sequence number check at the caller will
90: * retry it again when a d_move() does happen. So any garbage in the buffer
91: * due to mismatched pointer and length will be discarded.
92: *
93: * Load acquire is needed to make sure that we see the new name data even
94: * if we might get the length wrong.
95: */
96: static bool prepend_name(struct prepend_buffer *p, const struct qstr *name)
97: {
98: const char *dname = smp_load_acquire(&name->name); /* ^^^ */
99: u32 dlen = READ_ONCE(name->len);
100:
101: return prepend(p, dname, dlen) && prepend_char(p, '/');
102: }
SourceFile:fs/d_path.c]
|
| 22/2 |
2026/05/25 05:19 |
llm |
expert |
0m
Model:
Tokens:
input: 16540
output: 43
thoughts: 894
Thoughts: **Unraveling a Kernel Memory Safety Bug**
Okay, so I'm looking at this `prepend_name` function, and it seems to be reading the name and length information (`name->name` and `name->len`) directly. The problem arises because of a potential data race with `d_move()`. `d_move` is a function that might be renaming a dentry, and it calls `copy_name`. My concern is that `dlen` could reflect the *new* name length, while `dname` still refers to the *old* name, or vice-versa, during the `prepend_name` operation, a window of inconsistency, depending on the sequencing.
Specifically, `d_move` updates `d_shortname`, `__d_name.name`, and `__d_name.hash_len` if renaming a dentry. If `prepend_name` reads `dlen` (part of `hash_len`) and then `dname` (which is the actual `name`), it might encounter a situation where the new length is read with the old name, or the old length with the new name. This introduces a subtle race condition that could lead to memory corruption.
The critical issue is this: If `prepend_name` reads the new, larger length but the old, smaller name, `prepend_copy` will read past the end of the old name.
Now, if `dname` points to `d_shortname`, it is an inline 40-byte array. If the old name was short (say, 2 bytes), the rest of `d_shortname` (38 bytes) could be uninitialized. If `dlen` is read as the new length (e.g., 10 bytes), `prepend_copy` would attempt to read 10 bytes from `dname`, including those potentially uninitialized 8 bytes.
The KMSAN report is saying that uninitialized memory is being read. The report points to `copy_name` being the source of this problem. Looking at the call stack, I see `__d_alloc` allocates the dentry, and then the dentryβs `d_shortname` is uninitialized except for the actual length. The critical piece here is that `copy_name` copies the *entire* `d_shortname` contents from a `target` dentry to the newly allocated dentry. `target` was also allocated by `__d_alloc` and its `d_shortname` also contains uninitialized bytes. Thus, `dentry->d_shortname` now contains uninitialized bytes. Thatβs bad because `prepend_copy` reads from `dentry->d_shortname` (via `dname`).
Hereβs the thing thatβs making me pause: Even if `prepend_copy` reads the correct length `dlen`, shouldn't it be reading the *new* name, which has initialized data? The catch here is that `copy_name` is copying the *entire* contents from `target->d_shortname` to `dentry->d_shortname`. So, when the new name is copied, the uninitialized part of `target` will also be copied. For example, if `target` has a 5-byte name, `target->d_shortname` will have 5 initialized bytes, and 35 uninitialized bytes. `copy_name` copies all 40 bytes, so `dentry->d_shortname` ends up with 5 initialized bytes and 35 uninitialized ones. Afterwards, `dentry->__d_name.hash_len` is updated with the new name's length.
If `prepend_name` reads a consistent pair of `dname` and `dlen`, both reflecting the new name (e.g., `dlen` is 5), `prepend_copy` should only read 5 bytes from `dname`. However, `copy_from_kernel_nofault` is reading in 8-byte chunks if aligned. This is how the uninitialized memory is read. Now, I need to look into how that happens in `copy_from_kernel_nofault`.
|
| 23/2 |
2026/05/25 05:19 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/maccess.c IncludeLines:true Name:copy_from_kernel_nofault]
Results: map[SourceCode: 29: long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
30: {
31: unsigned long align = 0;
32:
33: if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
34: align = (unsigned long)dst | (unsigned long)src;
35:
36: if (!copy_from_kernel_nofault_allowed(src, size))
37: return -ERANGE;
38:
39: pagefault_disable();
40: if (!(align & 7))
41: copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
42: if (!(align & 3))
43: copy_from_kernel_nofault_loop(dst, src, size, u32, Efault);
44: if (!(align & 1))
45: copy_from_kernel_nofault_loop(dst, src, size, u16, Efault);
46: copy_from_kernel_nofault_loop(dst, src, size, u8, Efault);
47: pagefault_enable();
48: return 0;
49: Efault:
50: pagefault_enable();
51: return -EFAULT;
52: }
SourceFile:mm/maccess.c]
|
| 24/2 |
2026/05/25 05:19 |
llm |
expert |
0m
Model:
Tokens:
input: 18780
output: 39
thoughts: 923
Thoughts: **Analyzing a Potential Data Race in `prepend_name`**
Okay, let's break this down. Initially, the code leverages `u64` chunking, which should be fine on x86 due to efficient unaligned access. But, looking at `copy_from_kernel_nofault_loop`, it seems if the length (`len`) is less than 8, the `u64` loop is skipped. The KMSAN report is key here; it highlights an 8-byte access with uninitialized bytes. This is very interesting. The report shows uninitialized bytes 5-7 of an 8 byte access, contradicting the apparent intent.
The critical insight is realizing `dlen` is not always the size of the *new* name, but potentially the *old* name. The KMSAN output is consistent: initialized bytes 0-4, uninitialized 5-7, implies a 5-byte name. But how did we get an 8-byte access? Aha, `dlen` is potentially the *old* name length (8), but `dname` points to the *new* 5-byte name. That's the crux of the race condition.
When `prepend_copy` is called with `size = 8`, it reads 8 bytes from `dname`, which only has 5 initialized bytes. Consequently, bytes 5-7 are uninitialized, as reported by KMSAN. This is happening because of the following code sequence:
```c
const char *dname = smp_load_acquire(&name->name); /* ^^^ */
u32 dlen = READ_ONCE(name->len);
```
If `d_move` is running concurrently, the `name->name` and `name->len` can be updated. This means the `prepend_name` might be reading a stale `dlen` value from the old name, but with the updated name. This results in the uninitialized memory access.
So, the next question is: is this a vulnerability? The uninitialized bytes from `prepend_copy` are copied to `dst` (the prepend buffer). But wait, according to the comment, a sequence number check at the caller *will retry* it again when a d_move occurs, and any garbage will be discarded. This means that a data leak is unlikely. The caller seems to be robust to this particular race, and will retry, discarding any intermediate garbage in the buffer due to the length mismatch. We should verify this assertion.
|
| 25/2 |
2026/05/25 05:20 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/d_path.c IncludeLines:true Name:prepend_path]
Results: map[SourceCode: 140: /**
141: * prepend_path - Prepend path string to a buffer
142: * @path: the dentry/vfsmount to report
143: * @root: root vfsmnt/dentry
144: * @p: prepend buffer which contains buffer pointer and allocated length
145: *
146: * The function will first try to write out the pathname without taking any
147: * lock other than the RCU read lock to make sure that dentries won't go away.
148: * It only checks the sequence number of the global rename_lock as any change
149: * in the dentry's d_seq will be preceded by changes in the rename_lock
150: * sequence number. If the sequence number had been changed, it will restart
151: * the whole pathname back-tracing sequence again by taking the rename_lock.
152: * In this case, there is no need to take the RCU read lock as the recursive
153: * parent pointer references will keep the dentry chain alive as long as no
154: * rename operation is performed.
155: */
156: static int prepend_path(const struct path *path,
157: const struct path *root,
158: struct prepend_buffer *p)
159: {
160: unsigned seq, m_seq = 0;
161: struct prepend_buffer b;
162: int error;
163:
164: rcu_read_lock();
165: restart_mnt:
166: read_seqbegin_or_lock(&mount_lock, &m_seq);
167: seq = 0;
168: rcu_read_lock();
169: restart:
170: b = *p;
171: read_seqbegin_or_lock(&rename_lock, &seq);
172: error = __prepend_path(path->dentry, real_mount(path->mnt), root, &b);
173: if (!(seq & 1))
174: rcu_read_unlock();
175: if (need_seqretry(&rename_lock, seq)) {
176: seq = 1;
177: goto restart;
178: }
179: done_seqretry(&rename_lock, seq);
180:
181: if (!(m_seq & 1))
182: rcu_read_unlock();
183: if (need_seqretry(&mount_lock, m_seq)) {
184: m_seq = 1;
185: goto restart_mnt;
186: }
187: done_seqretry(&mount_lock, m_seq);
188:
189: if (unlikely(error == 3))
190: b = *p;
191:
192: if (b.len == p->len)
193: prepend_char(&b, '/');
194:
195: *p = b;
196: return error;
197: }
SourceFile:fs/d_path.c]
|
| 26/2 |
2026/05/25 05:20 |
llm |
expert |
0m
Model:
Tokens:
input: 21192
output: 38
thoughts: 199
Thoughts: **Analyzing the `prepend_path` Function**
Okay, so I'm diving into this `prepend_path` function. I see it uses `read_seqbegin_or_lock(&rename_lock, &seq)` and `need_seqretry(&rename_lock, seq)`. The key here is this `rename_lock`. If a rename operation happens while this function is running, the sequence number associated with `rename_lock` will change, and `need_seqretry` will rightly return true. That forces a restart of the loop, which makes sense. The buffer `b` is discarded. Good, because `b = *p` is assigned at the `restart:` label. I get it; uninitialized bytes that might have been copied are wiped away on the next attempt.
But wait a minute... what if those uninitialized bytes are copied, but `need_seqretry` returns *false*? Is there a scenario where the `rename_lock`'s sequence number doesn't change, yet `d_move` still updates the `name` and `len`? Hmm, no, I'm pretty sure that can't happen. `d_move` takes a write lock on `rename_lock`. So that's safe. But to be sure, let's just go look at `d_move`.
|
| 27/2 |
2026/05/25 05:20 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:d_move]
Results: map[SourceCode:3032: /*
3033: * d_move - move a dentry
3034: * @dentry: entry to move
3035: * @target: new dentry
3036: *
3037: * Update the dcache to reflect the move of a file name. Negative
3038: * dcache entries should not be moved in this way. See the locking
3039: * requirements for __d_move.
3040: */
3041: void d_move(struct dentry *dentry, struct dentry *target)
3042: {
3043: write_seqlock(&rename_lock);
3044: __d_move(dentry, target, false);
3045: write_sequnlock(&rename_lock);
3046: }
SourceFile:fs/dcache.c]
|
| 28/2 |
2026/05/25 05:20 |
llm |
expert |
0m
Model:
Tokens:
input: 21964
output: 43
thoughts: 1452
Thoughts: **Analyzing a KMSAN False Positive in `prepend_path`**
Okay, so I'm looking at this KMSAN report and it seems like a classic race condition false positive. The issue stems from the interaction between `d_move` and `prepend_path`. Essentially, `d_move` takes the `rename_lock` (with a sequence number) which is used to protect against this kind of issue. If `prepend_path` reads a `name` and `len` that are mismatched due to a concurrent `d_move`, `need_seqretry(&rename_lock, seq)` will detect this change and the process will retry. That means any "garbage" data copied into the buffer will be discarded, rendering it harmless.
The KMSAN, in this case, is correctly hooked into `copy_from_kernel_nofault` and checks for uninitialized memory, which is a good thing to do. The goal is to prevent the leak of uninitialized memory. KMSAN checks are triggered by `kmsan_check_memory` which is called inside of `copy_from_kernel_nofault_loop`. However, `copy_from_kernel_nofault` is *explicitly* designed to read potentially invalid memory without faulting. The caller (`prepend_copy`) is aware that the data might be garbage and is relying on the sequence lock to correctly handle it.
The intent of `copy_from_kernel_nofault` is clearly stated to be prevention of uninitialized memory leaks. But in this case, the *mechanism* preventing the leak is the sequence lock retry, which KMSAN doesn't seem to recognize. This is why it's a false positive, because the report is correct, but the system is handling it.
I needed to double-check that this wasn't an exploitable issue or a denial of service. The fact that the bug report is a KMSAN warning and KMSAN is disabled in production is reassuring. The uninitialized bytes are copied to the buffer, but then promptly discarded. There is no memory corruption, no crash, and no actual leak. The security impact is therefore minimal.
I then considered whether `copy_from_kernel_nofault` could read out-of-bounds memory. This seems unlikely. The function has `pagefault_disable()` and handles faults gracefully with a return of `-EFAULT`. The caller, `prepend_copy`, explicitly checks for the error and handles it: sets the buffer to "x" and returns false. So even unmapped memory won't crash the system.
I needed to figure out whether there was any case where `need_seqretry` might return false, but we're still reading uninitialized memory. The KMSAN report indicates the uninitialized memory comes from `__d_alloc`, the dentry allocation. `__d_alloc` calls `kmem_cache_alloc_lru_noprof` which in turn calls `slab_alloc_node` which calls `slab_post_alloc_hook`. Specifically, it says it happens when we `copy_name` calls `__d_move`, and copies bytes to `dentry->d_shortname`.
This looks like a standard struct copy: `dentry->d_shortname = target->d_shortname;`. But, `d_shortname` is a union (of a string and other stuff) that's 40 bytes long. If the actual string is shorter than 40 bytes, the rest of the union will be uninitialized. Copying the *union* copies this uninitialized memory. KMSAN tracks this uninitialized state through the copy. Then, `prepend_copy` reads from `dentry->d_shortname` using `copy_from_kernel_nofault`, which reads more bytes than the actual string due to the race, hence it reads the uninitialized data. This is a classic source of KMSAN uninit-value reports when copying structs with padding or uninitialized union members.
Could this be triggered without a data race? Suppose the `dlen` and `dname` match, and the length is 5. `prepend_copy` is called with `len = 5`. `copy_from_kernel_nofault` will perform a series of bitwise operations and calls `copy_from_kernel_nofault_loop` with a type like `u64`, `u32`, `u16`, `u8`. `copy_from_kernel_nofault_loop` modifies the local variable `len` which is critical. If `len` is 5, but the loop checks `len >= sizeof(type)` where `type` might be `u64`, then the loop is skipped.
I need to confirm all the details, but it feels like the current code path is fine.
|
| 29/2 |
2026/05/25 05:20 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/maccess.c IncludeLines:true Name:copy_from_kernel_nofault]
Results: map[SourceCode: 29: long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
30: {
31: unsigned long align = 0;
32:
33: if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
34: align = (unsigned long)dst | (unsigned long)src;
35:
36: if (!copy_from_kernel_nofault_allowed(src, size))
37: return -ERANGE;
38:
39: pagefault_disable();
40: if (!(align & 7))
41: copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
42: if (!(align & 3))
43: copy_from_kernel_nofault_loop(dst, src, size, u32, Efault);
44: if (!(align & 1))
45: copy_from_kernel_nofault_loop(dst, src, size, u16, Efault);
46: copy_from_kernel_nofault_loop(dst, src, size, u8, Efault);
47: pagefault_enable();
48: return 0;
49: Efault:
50: pagefault_enable();
51: return -EFAULT;
52: }
SourceFile:mm/maccess.c]
|
| 30/2 |
2026/05/25 05:20 |
llm |
expert |
0m
Model:
Tokens:
input: 24883
output: 45
thoughts: 22
Thoughts: Okay, here's my interpretation of those thoughts, tailored for an expert audience, presented in the first person:
**Analyzing Macro Parameter Usage**
Right, so `size` is definitely being passed as an argument to this macro. Let's re-examine that macro's definition. Need to understand how `size` is being used within the macro expansion itself. Is it a size specifier for allocation, a loop bound, or something else entirely? I need to quickly determine the context of this parameter. A quick review of the macro definition should reveal whether there is something else that is causing a subtle bug. It is crucial to determine if a simple misuse, or a deeper problem is occurring.
|
| 31/2 |
2026/05/25 05:20 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/maccess.c IncludeLines:true Name:copy_from_kernel_nofault_allowed]
Results: map[SourceCode: 10: bool __weak copy_from_kernel_nofault_allowed(const void *unsafe_src,
11: size_t size)
12: {
13: return true;
14: }
SourceFile:mm/maccess.c]
|
| 32/2 |
2026/05/25 05:20 |
llm |
expert |
0m
Model:
Tokens:
input: 25184
output: 25
thoughts: 20
Thoughts: Here's my thought process, as if I were considering this from an expert perspective:
**Investigating `copy_from_kernel_nofault_loop`**
Okay, hold on a second. Something's got me thinking. I need to refresh my memory, or, at least, take a closer look at the `copy_from_kernel_nofault_loop` macro. I know it's a critical piece of the kernel infrastructure, and it's used in memory management. Given my expertise, I suspect there's a specific reason this macro is being flagged and that warrants immediate attention. Is it performance-related? Security? Or perhaps an edge case I'm missing related to a recent change? I need to understand its mechanics, the conditions under which it's invoked, and any potential areas for optimization or vulnerabilities. Time to dig into the source and understand the surrounding context.
|
| 33/2 |
2026/05/25 05:20 |
tool |
grepper |
0m
Args: map[Expression:define copy_from_kernel_nofault_loop]
Results: map[Output:mm/maccess.c=10=bool __weak copy_from_kernel_nofault_allowed(const void *unsafe_src,
--
mm/maccess.c-19- */
mm/maccess.c:20:#define copy_from_kernel_nofault_loop(dst, src, len, type, err_label) \
mm/maccess.c-21- while (len >= sizeof(type)) { \
]
|
| 34/2 |
2026/05/25 05:20 |
llm |
expert |
0m
Model:
Tokens:
input: 25535
output: 21
thoughts: 20
Thoughts: Okay, let's dive into this. Here's my take on those lines from `mm/maccess.c`, assuming I'm an expert and this is how my thought process would unfold:
**Analyzing Memory Access in `mm/maccess.c` - Lines 20-28**
Right, let's zero in on lines 20-28 of `mm/maccess.c`. Specifically, I'm expecting this section to deal with... hmm, let me guess... likely some sort of core memory access primitive or a fundamental check related to address validity, permissions, or perhaps even something related to atomic operations or memory barriers, given the filename. I'm hoping I'll see some inline assembly if it's performance-critical, or at the very least a direct mapping to some platform-specific memory access instructions. It could also be the start of a more complex memory management routine. Given that it's early in the file, it's probably setting up the groundwork for something more elaborate. I'm going to look for any architecture-specific conditional compilation directives, like `#ifdef __x86_64__` or `#ifdef __ARM__`. That'll be a key indicator of its purpose. Also, I'll be watching to see if I recognize any standard library functions that are being used here. Often, they give a clear indication of how the information here fits into the bigger picture. I'm keen to see the data structures being used. Are they dealing with page tables, virtual addresses, physical addresses, or something else entirely? The comments better be good!
|
| 35/2 |
2026/05/25 05:20 |
tool |
read-file |
0m
Args:
Results: map[Contents:// SPDX-License-Identifier: GPL-2.0-only
/*
* Access kernel or user memory without faulting.
*/
#include <linux/export.h>
#include <linux/mm.h>
#include <linux/uaccess.h>
#include <asm/tlb.h>
bool __weak copy_from_kernel_nofault_allowed(const void *unsafe_src,
size_t size)
{
return true;
}
/*
* The below only uses kmsan_check_memory() to ensure uninitialized kernel
* memory isn't leaked.
*/
#define copy_from_kernel_nofault_loop(dst, src, len, type, err_label) \
while (len >= sizeof(type)) { \
__get_kernel_nofault(dst, src, type, err_label); \
kmsan_check_memory(src, sizeof(type)); \
dst += sizeof(type); \
src += sizeof(type); \
len -= sizeof(type); \
}
long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
{
unsigned long align = 0;
if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
align = (unsigned long)dst | (unsigned long)src;
if (!copy_from_kernel_nofault_allowed(src, size))
return -ERANGE;
pagefault_disable();
if (!(align & 7))
copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
if (!(align & 3))
copy_from_kernel_nofault_loop(dst, src, size, u32, Efault);
if (!(align & 1))
copy_from_kernel_nofault_loop(dst, src, size, u16, Efault);
copy_from_kernel_nofault_loop(dst, src, size, u8, Efault);
pagefault_enable();
return 0;
Efault:
pagefault_enable();
return -EFAULT;
}
EXPORT_SYMBOL_GPL(copy_from_kernel_nofault);
#define copy_to_kernel_nofault_loop(dst, src, len, type, err_label) \
while (len >= sizeof(type)) { \
__put_kernel_nofault(dst, src, type, err_label); \
instrument_write(dst, sizeof(type)); \
dst += sizeof(type); \
src += sizeof(type); \
len -= sizeof(type); \
}
long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
{
unsigned long align = 0;
if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
align = (unsigned long)dst | (unsigned long)src;
pagefault_disable();
if (!(align & 7))
copy_to_kernel_nofault_loop(dst, src, size, u64, Efault);
if (!(align & 3))
copy_to_kernel_nofault_loop(dst, src, size, u32, Efault);
if (!(align & 1))
copy_to_kernel_nofault_loop(dst, src, size, u16, Efault);
copy_to_kernel_nofault_loop(dst, src, size, u8, Efault);
pagefault_enable();
return 0;
Efault:
pagefault_enable();
return -EFAULT;
}
long strncpy_from_kernel_nofault(char *dst, const void *unsafe_addr, long count)
{
const void *src = unsafe_addr;
if (unlikely(count <= 0))
return 0;
if (!copy_from_kernel_nofault_allowed(unsafe_addr, count))
return -ERANGE;
pagefault_disable();
do {
__get_kernel_nofault(dst, src, u8, Efault);
dst++;
src++;
} while (dst[-1] && src - unsafe_addr < count);
pagefault_enable();
dst[-1] = '\0';
return src - unsafe_addr;
Efault:
pagefault_enable();
dst[0] = '\0';
return -EFAULT;
}
/**
* copy_from_user_nofault(): safely attempt to read from a user-space location
* @dst: pointer to the buffer that shall take the data
* @src: address to read from. This must be a user address.
* @size: size of the data chunk
*
* Safely read from user address @src to the buffer at @dst. If a kernel fault
* happens, handle that and return -EFAULT.
*/
long copy_from_user_nofault(void *dst, const void __user *src, size_t size)
{
long ret = -EFAULT;
if (!__access_ok(src, size))
return ret;
if (!nmi_uaccess_okay())
return ret;
pagefault_disable();
ret = __copy_from_user_inatomic(dst, src, size);
pagefault_enable();
if (ret)
return -EFAULT;
return 0;
}
EXPORT_SYMBOL_GPL(copy_from_user_nofault);
/**
* copy_to_user_nofault(): safely attempt to write to a user-space location
* @dst: address to write to
* @src: pointer to the data that shall be written
* @size: size of the data chunk
*
* Safely write to address @dst from the buffer at @src. If a kernel fault
* happens, handle that and return -EFAULT.
*/
long copy_to_user_nofault(void __user *dst, const void *src, size_t size)
{
long ret = -EFAULT;
if (access_ok(dst, size)) {
pagefault_disable();
ret = __copy_to_user_inatomic(dst, src, size);
pagefault_enable();
}
if (ret)
return -EFAULT;
return 0;
}
EXPORT_SYMBOL_GPL(copy_to_user_nofault);
/**
* strncpy_from_user_nofault: - Copy a NUL terminated string from unsafe user
* address.
* @dst: Destination address, in kernel space. This buffer must be at
* least @count bytes long.
* @unsafe_addr: Unsafe user address.
* @count: Maximum number of bytes to copy, including the trailing NUL.
*
* Copies a NUL-terminated string from unsafe user address to kernel buffer.
*
* On success, returns the length of the string INCLUDING the trailing NUL.
*
* If access fails, returns -EFAULT (some data may have been copied
* and the trailing NUL added).
*
* If @count is smaller than the length of the string, copies @count-1 bytes,
* sets the last byte of @dst buffer to NUL and returns @count.
*/
long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
long count)
{
long ret;
if (unlikely(count <= 0))
return 0;
pagefault_disable();
ret = strncpy_from_user(dst, unsafe_addr, count);
pagefault_enable();
if (ret >= count) {
ret = count;
dst[ret - 1] = '\0';
} else if (ret >= 0) {
ret++;
}
return ret;
}
/**
* strnlen_user_nofault: - Get the size of a user string INCLUDING final NUL.
* @unsafe_addr: The string to measure.
* @count: Maximum count (including NUL)
*
* Get the size of a NUL-terminated string in user space without pagefault.
*
* Returns the size of the string INCLUDING the terminating NUL.
*
* If the string is too long, returns a number larger than @count. User
* has to check the return value against "> count".
* On exception (or invalid count), returns 0.
*
* Unlike strnlen_user, this can be used from IRQ handler etc. because
* it disables pagefaults.
*/
long strnlen_user_nofault(const void __user *unsafe_addr, long count)
{
int ret;
pagefault_disable();
ret = strnlen_user(unsafe_addr, count);
pagefault_enable();
return ret;
}
void __copy_overflow(int size, unsigned long count)
{
WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
}
EXPORT_SYMBOL(__copy_overflow);
]
|
| 36/2 |
2026/05/25 05:20 |
llm |
expert |
0m
Model:
Tokens:
input: 28109
output: 37
thoughts: 759
Thoughts: **Data Race Analysis and Confirmation**
Okay, let's break this down. First, `size` is passed as `len` to the macro, got it. And that macro does some `len -= sizeof(type)` shenanigans. Since `size` is a local, modifying it should be fine, unless... wait, what if `size` is smaller than the access size? If `size` is 5, then `size -= 8` leaves it negative, so the `size >= 8` check would be bypassed. But the KMSAN report highlights uninitialized bytes at the end of the access. The access is 8 bytes, and bytes 5-7 are uninitialized, implying `size` *has* to be at least 8.
So, if `size` is at least 8, that means `prepend_copy` was called with `len >= 8`. But the initialized string is only 5 bytes long! Bingo. This mismatch is the data race I predicted. `dlen` (passed as `len` to `prepend_copy`) is the length of the *old* name (e.g., 8), and `dname` points to the *new* name (length 5). `prepend_copy` reads 8 bytes from `dname`, and bytes 5-7 are uninitialized. *This* confirms the data race perfectly.
Is there any other potential for such a mismatch? Not really; `dlen` should always represent the correct string length. The only way it's wrong is if there's a race, just as we see. And the sequence lock retry mechanism should handle the garbage that occurs, as we discussed previously.
Alright, but wait... what if those uninitialized bytes are written to the `dst` buffer, and the retry happens? Does the buffer leak? Let's double check `prepend_path`. Okay, `b` is a local copy of `*p`. `__prepend_path` modifies `b`, but if `need_seqretry` is true, we restart and `b = *p` *resets* `b` to its original, safe state. So, the buffer pointer and length are reset. The uninitialized bytes were written, but the pointer/length are updated, so it's either overwritten or those bytes are before the buffer. The uninitialized data should not be leaked, or be part of a future operation.
|
| 37/2 |
2026/05/25 05:21 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/d_path.c IncludeLines:true Name:prepend]
Results: map[SourceCode: 57: static bool prepend(struct prepend_buffer *p, const char *str, int namelen)
58: {
59: // Already overflowed?
60: if (p->len < 0)
61: return false;
62:
63: // Will overflow?
64: if (p->len < namelen) {
65: // Fill as much as possible from the end of the name
66: str += namelen - p->len;
67: p->buf -= p->len;
68: prepend_copy(p->buf, str, p->len);
69: p->len = -1;
70: return false;
71: }
72:
73: // Fits fully
74: p->len -= namelen;
75: p->buf -= namelen;
76: return prepend_copy(p->buf, str, namelen);
77: }
SourceFile:fs/d_path.c]
|
| 38/2 |
2026/05/25 05:21 |
llm |
expert |
0m
Model:
Tokens:
input: 29711
output: 42
thoughts: 1710
Thoughts: **Analysis of a KMSAN False Positive in Pathname Resolution**
Okay, so let's break this down. The KMSAN uninitialized-value report in `copy_from_kernel_nofault` is arising specifically in the context of path resolution, via functions like `d_path`, which eventually call `prepend_path`. I see that the core of the issue stems from an optimistic read of dentry names and lengths in `prepend_name` before a concurrent rename is handled, which in turn feeds `prepend_copy`. The code grabs `name->name` and `name->len` without locks, relying on a sequence counter (`rename_lock`) to handle any concurrent `d_move` operations. This creates a potential data race.
I'm following along, and yes, it's clear now. Because of this data race, it *can* read the pointer to the *new* name, but an *old* length, which may be larger than the name's length. Then `prepend_copy` copies the name, potentially reading beyond the bounds of the newly initialized memory in `d_shortname`, thus triggering KMSAN because we're reading uninitialized bytes.
But here's the crucial point: because the rename is happening concurrently, the sequence counter will have changed. `prepend_path` detects this change via `need_seqretry` and restarts the entire process, effectively discarding the buffer containing the uninitialized data. That's the key: those uninitialized bytes are never actually used, nor are they leaked to userspace.
So, it's a false positive, plain and simple, and it has no real-world implications, whether it's security related or causing some denial of service. The kernel developers even anticipated this and left a comment about the potential for faults, which is a good sign. The `copy_from_kernel_nofault` call is only using `kmsan_check_memory()` to ensure that uninitialized memory isn't leaked, but it's okay here.
Okay, let me go through this and check for other ways this can be triggered. How about if the *shorter* `d_shortname` is copied *without* a concurrent rename? My first thought is: how can the length (`dlen`) be larger than the initialized data in `dname` without a concurrent rename? When the dentry is created, both `dname` and `dlen` are initialized properly. If there is no concurrent rename, then we should read the correct `dlen` and `dname` pointers and then the `prepend_copy` function is called.
Aha! `copy_from_kernel_nofault` copies exactly 5 bytes. It reads 4 bytes with u32, and then 1 byte with u8. So it actually *does not* read past the end of the initialized region. So it *cannot* read those uninitialized bytes if there's no concurrent rename. The only way it reads past 5 bytes is if the length is at least 8. And as I've established, the only way `len` is >= 8 while the initialized part is 5 is due to the data race. That confirms it. So the report is *purely* due to the data race, and the sequence lock handles it.
Wait, let me make sure that `dlen` itself isn't corrupted somehow? No, assuming normal operations, it's just the data race.
Can an attacker use this for a DoS? In a KMSAN-enabled kernel, yes, they *could* trigger the warning, sure. But KMSAN is debugging, not production. In production, `copy_from_kernel_nofault` just copies the bytes, no crash, no warning, so no DoS in a real kernel.
What about a memory leak? No. The retry mechanism discards the buffer. Now, *is* there any way `need_seqretry` could fail? I see that `d_move` uses the sequence lock correctly. `prepend_path` uses the sequence lock properly. It's the `need_seqretry` check which needs to be sound. What are the possibilities here?
It's clear to me now. If the read sequence is before the start, we get an even number. If the read is after the finish, we get an even number. Either way, `need_seqretry` gets a different value, which is detected and retried.
But what if the read happens during the move? We'll get an odd number. And `need_seqretry` correctly detects an odd number and returns true! So we are safe! And we are sure to retry. So there's no way to leak memory.
So, in summary: the KMSAN report is a false positive due to a data race that is
|
| 39/2 |
2026/05/25 05:21 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/seqlock.h IncludeLines:true Name:need_seqretry]
Results: map[SourceCode:1155: /**
1156: * need_seqretry() - validate seqlock_t "locking or lockless" read section
1157: * @lock: Pointer to seqlock_t
1158: * @seq: sequence count, from read_seqbegin_or_lock()
1159: *
1160: * Return: true if a read section retry is required, false otherwise
1161: */
1162: static inline int need_seqretry(seqlock_t *lock, int seq)
1163: __releases_shared(lock) __no_context_analysis
1164: {
1165: return !(seq & 1) && read_seqretry(lock, seq);
1166: }
SourceFile:include/linux/seqlock.h]
|
| 40/2 |
2026/05/25 05:21 |
llm |
expert |
0m
Model:
Tokens:
input: 32721
output: 39
thoughts: 62
Thoughts: Okay, let me break this down.
**Diving into the `need_seqretry` Logic**
Alright, so `need_seqretry` uses this logic: `!(seq & 1) && read_seqretry(lock, seq)`. The key part here is `!(seq & 1)`. That's a bitwise AND with 1, and then a NOT operation. If I understand correctly, the bitwise AND effectively checks if the least significant bit of `seq` is set. The NOT then inverts the result, so it seems like the check is actually ensuring `seq` is *even*.
My immediate question is: *why*? Why would an odd sequence number automatically mean we *don't* need a retry? That seems counterintuitive. I need to understand the relationship between odd sequence numbers and the retry logic. Something in `prepend_path` I'm overlooking probably holds the answer. Let's revisit it to see if it provides clarity on the odd/even sequence significance.
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| 41/2 |
2026/05/25 05:21 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/d_path.c IncludeLines:true Name:prepend_path]
Results: map[SourceCode: 140: /**
141: * prepend_path - Prepend path string to a buffer
142: * @path: the dentry/vfsmount to report
143: * @root: root vfsmnt/dentry
144: * @p: prepend buffer which contains buffer pointer and allocated length
145: *
146: * The function will first try to write out the pathname without taking any
147: * lock other than the RCU read lock to make sure that dentries won't go away.
148: * It only checks the sequence number of the global rename_lock as any change
149: * in the dentry's d_seq will be preceded by changes in the rename_lock
150: * sequence number. If the sequence number had been changed, it will restart
151: * the whole pathname back-tracing sequence again by taking the rename_lock.
152: * In this case, there is no need to take the RCU read lock as the recursive
153: * parent pointer references will keep the dentry chain alive as long as no
154: * rename operation is performed.
155: */
156: static int prepend_path(const struct path *path,
157: const struct path *root,
158: struct prepend_buffer *p)
159: {
160: unsigned seq, m_seq = 0;
161: struct prepend_buffer b;
162: int error;
163:
164: rcu_read_lock();
165: restart_mnt:
166: read_seqbegin_or_lock(&mount_lock, &m_seq);
167: seq = 0;
168: rcu_read_lock();
169: restart:
170: b = *p;
171: read_seqbegin_or_lock(&rename_lock, &seq);
172: error = __prepend_path(path->dentry, real_mount(path->mnt), root, &b);
173: if (!(seq & 1))
174: rcu_read_unlock();
175: if (need_seqretry(&rename_lock, seq)) {
176: seq = 1;
177: goto restart;
178: }
179: done_seqretry(&rename_lock, seq);
180:
181: if (!(m_seq & 1))
182: rcu_read_unlock();
183: if (need_seqretry(&mount_lock, m_seq)) {
184: m_seq = 1;
185: goto restart_mnt;
186: }
187: done_seqretry(&mount_lock, m_seq);
188:
189: if (unlikely(error == 3))
190: b = *p;
191:
192: if (b.len == p->len)
193: prepend_char(&b, '/');
194:
195: *p = b;
196: return error;
197: }
SourceFile:fs/d_path.c]
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| 42/2 |
2026/05/25 05:21 |
llm |
expert |
0m
Model:
Tokens:
input: 33949
output: 46
thoughts: 49
Thoughts: Here's my analysis of the situation:
**Understanding Sequence Retry and Locking**
Okay, so I'm digging into this retry logic, and a key question pops up: if `need_seqretry` returns `false` when the sequence number `seq` is odd, does that automatically mean no retry will occur? It seems like a reasonable assumption, but I need to make sure I'm not missing something subtle.
Ah, but wait a second! That's when `read_seqbegin_or_lock` comes into play. It appears that *if* `seq` is odd, this function is actually responsible for ACQUIRING the lock. So, the lack of a retry from `need_seqretry` when `seq` is odd isn't the whole picture. It's tied to the locking mechanism. The odd sequence number triggers a different path involving the lock. I'll need to remember that the absence of a retry *from that specific function* doesn't mean the operation is lock-free with an odd sequence number. I need to keep in mind the interaction with the locking function to fully understand the behavior.
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| 43/2 |
2026/05/25 05:21 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/seqlock.h IncludeLines:true Name:read_seqbegin_or_lock]
Results: map[SourceCode:1115: /**
1116: * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
1117: * @lock: Pointer to seqlock_t
1118: * @seq : Marker and return parameter. If the passed value is even, the
1119: * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
1120: * If the passed value is odd, the reader will become a *locking* reader
1121: * as in read_seqlock_excl(). In the first call to this function, the
1122: * caller *must* initialize and pass an even value to @seq; this way, a
1123: * lockless read can be optimistically tried first.
1124: *
1125: * read_seqbegin_or_lock is an API designed to optimistically try a normal
1126: * lockless seqlock_t read section first. If an odd counter is found, the
1127: * lockless read trial has failed, and the next read iteration transforms
1128: * itself into a full seqlock_t locking reader.
1129: *
1130: * This is typically used to avoid seqlock_t lockless readers starvation
1131: * (too much retry loops) in the case of a sharp spike in write side
1132: * activity.
1133: *
1134: * Context: if the seqlock_t write section, *or other read sections*, can
1135: * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
1136: * variant of this function instead.
1137: *
1138: * Check Documentation/locking/seqlock.rst for template example code.
1139: *
1140: * Return: the encountered sequence counter value, through the @seq
1141: * parameter, which is overloaded as a return parameter. This returned
1142: * value must be checked with need_seqretry(). If the read section need to
1143: * be retried, this returned value must also be passed as the @seq
1144: * parameter of the next read_seqbegin_or_lock() iteration.
1145: */
1146: static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
1147: __acquires_shared(lock) __no_context_analysis
1148: {
1149: if (!(*seq & 1)) /* Even */
1150: *seq = read_seqbegin(lock);
1151: else /* Odd */
1152: read_seqlock_excl(lock);
1153: }
SourceFile:include/linux/seqlock.h]
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| 44/2 |
2026/05/25 05:21 |
llm |
expert |
0m
Model:
Tokens:
input: 35059
output: 66
thoughts: 1855
Thoughts: **Analysis of the KMSAN False Positive in `d_path`**
Alright, let's break this down. My initial thought was that the `seq` variable gets initialized to 0, which then triggers a call to `read_seqbegin_or_lock`. This function, after calling `read_seqbegin`, checks if it needs to retry. If it does, `seq` is set to 1, and the process jumps to `restart`. The critical part here is when `read_seqbegin_or_lock` is called with `seq = 1`; it acquires an exclusive lock.
This exclusive lock is key. With the lock held, the `__prepend_path` function runs, and, crucially, a concurrent `d_move` cannot run, which allows the consistent read of `name` and `len`. The `need_seqretry` call then returns false, because `seq` is odd, which means `done_seqretry` will release the lock. This means the retry will be performed with the lock held. This implies the KMSAN report is likely a false positive. The data read during the initial, lockless attempt, is discarded, and the buffer is reset during the retry. So, the uninitialized bytes KMSAN is complaining about are never leaked or used in a way that matters.
I've confirmed that the code is deliberately designed to handle concurrent modifications optimistically, reading without a lock initially. It gracefully handles the "garbage" data and retries with a lock if a concurrent modification occurs. KMSAN, in this instance, is flagging a known condition which is designed to be safe and recoverable.
Now, let's fill out the dimensions. Because this is a false positive, I conclude that there's no way to exploit it, so **Exploitable: False**. Similarly, since the code is designed to handle faults and errors gracefully, there will be no DoS, which means **DenialOfService: False**.
The code path is reachable by any process calling `getcwd` or opening a file and reading from `/proc/self/fd/X`, which would trigger `d_path`. This means unprivileged users can trigger the KMSAN warning. Also, this code path can also be triggered from user namespaces, which makes both **Unprivileged: True** and **UserNamespace: True**.
As this is not a virtualization bug, **VMGuestTrigger: False** and **VMHostTrigger: False**. There's no network or other remote triggers, so **NetworkTrigger: False** and **RemoteTrigger: False**. Similarly, there's no peripheral or filesystem triggers, thus **PeripheralTrigger: False** and **FilesystemTrigger: False**.
Let's do a double check. Is it possible that the `dlen` is so large that `prepend_copy` reads into an unmapped page and causes a page fault? Yes, but `copy_from_kernel_nofault` and `prepend_copy` handles these page faults gracefully, and it won't crash the kernel. The code explicitly uses `copy_from_kernel_nofault` which is designed to handle errors and avoid crashes. It will just fill the buffer with 'x' and return false. The sequence lock retry will discard the buffer and retry with the lock held.
So the KMSAN report indicates: `BUG: KMSAN: uninit-value in copy_from_kernel_nofault+0x15f/0x570 mm/maccess.c:41`. The path appears to be related to opening a file when the TOMOYO security module is enabled. I can confirm unprivileged users can open files and rename files, making it easy to trigger this race condition. Hence, Unprivileged remains True.
There are no other aspects to consider.
**Final Response:**
Exploitable: False
DenialOfService: False
Unprivileged: True
UserNamespace: True
VMGuestTrigger: False
VMHostTrigger: False
NetworkTrigger: False
RemoteTrigger: False
PeripheralTrigger: False
FilesystemTrigger: False
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| 45/2 |
2026/05/25 05:22 |
tool |
set-results |
0m
Args: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
|
| 46/2 |
2026/05/25 05:22 |
llm |
expert |
0m
Model:
Tokens:
input: 37904
output: 496
thoughts: 382
Thoughts: **My Analysis of This KMSAN Warning**
Okay, here's the breakdown, as I see it. The KMSAN report flagged an uninitialized-value warning, specifically within `copy_from_kernel_nofault`. This is triggered from `prepend_copy`, during path resolution, specifically within the `prepend_path` function. The core of the issue stems from an optimization in `prepend_name`. To avoid performance bottlenecks, `prepend_name` optimistically grabs the dentry's name pointer and length without any locking. This is where the race condition kicks in.
A concurrent rename operation (`d_move`) can then interfere. Imagine this: `prepend_name` reads a name pointer and length. Now, a rename happens concurrently. Suddenly, we have a mismatch! `prepend_name` might read the new, shorter name pointer, but still retain the old, longer length.
When `prepend_copy` goes to copy that name into a buffer using `copy_from_kernel_nofault`, it ends up reading beyond the end of the initialized string. It hits the uninitialized padding within the `dentry->d_shortname` field, which is what KMSAN detects. Boom β uninitialized value warning.
However, the kernel is actually quite smart about this. The key is in how `prepend_path` handles the potential race. After the optimistic read, `prepend_path` meticulously checks the `rename_lock` sequence counter. If it detects that a concurrent rename occurred (and it almost certainly will), it discards the buffer. This is key. The offending bytes, even if uninitialized, are never passed along to the next layer of the stack. `prepend_path` simply re-attempts the operation with the lock held. This guarantees data consistency and prevents any sort of data corruption.
Further, `copy_from_kernel_nofault` is designed to handle potential page faults, so even if the read extends into unmapped memory, the system won't crash.
The bottom line? This is a textbook example of a false positive. We have a race condition that triggers the KMSAN warning, but that race is handled correctly by the kernel. The uninitialized bytes are always discarded; they are never leaked to userspace, and no corruption can occur. We're looking at a harmless race, not an exploitable vulnerability. It can be triggered in KMSAN builds by unprivileged users racing file opens and renames, but it has no impact on security or production systems (where KMSAN is disabled). No Denial of Service is possible from this. I'm confident in calling this a false positive.
|
| 47/1 |
2026/05/25 05:22 |
action |
format-explanation |
0m
Results: map[Explanation:**Analysis:**
The reported issue is a KMSAN (Kernel Memory Sanitizer) warning about reading
uninitialized memory in `copy_from_kernel_nofault`, which is called by
`prepend_copy` during path resolution (`prepend_path`).
This report is a **false positive** and has **no security impact** for the
following reasons:
1. **Optimistic Lockless Reads:** To optimize performance, `prepend_name` reads
a dentry's name pointer and length locklessly. If a concurrent rename (`d_move`)
occurs, a data race can happen where `prepend_name` fetches a mismatched name
pointer and length (e.g., the pointer to a new, shorter name, but the length of
the old, longer name).
2. **Safe Memory Copy:** `prepend_copy` then uses `copy_from_kernel_nofault` to
copy the name into a temporary buffer. Because of the mismatched length, it may
read past the end of the initialized string into the uninitialized padding of
`dentry->d_shortname`. KMSAN intercepts this read and flags it as an
uninitialized value use.
3. **Sequence Lock Retry Mechanism:** The kernel is fully aware of this race
condition. After the lockless read, `prepend_path` checks the `rename_lock`
sequence counter (`need_seqretry`). If a concurrent rename is detected, the
operation is aborted, the temporary buffer containing the garbage/uninitialized
bytes is discarded, and the path resolution is retried with the lock held.
4. **Fault Handling:** Even if the mismatched length is large enough to cause
`copy_from_kernel_nofault` to read into an unmapped page, the function disables
page faults and gracefully handles the error by returning `-EFAULT`, preventing
any kernel crashes.
**Conclusion:**
The uninitialized bytes are never leaked to userspace, used in any control flow,
or written to persistent storage. They are safely discarded by the sequence lock
retry mechanism. There is no memory corruption, no information leak, and no risk
of Denial of Service (DoS) in production builds where KMSAN is disabled.
While unprivileged users can trigger this warning in a KMSAN-enabled debug
kernel by racing file opens (e.g., via the TOMOYO security module as seen in the
trace) and file renames, it does not represent a vulnerability.]
|