| 0/0 |
2026/08/02 00:06 |
flow |
assessment-security |
47m
Results: map[DenialOfService:true Explanation:Based on the analysis of the bug report and the kernel source code, here is the
assessment of the vulnerability:
### Bug Mechanism
The bug is a classic spinlock deadlock caused by taking a lock with interrupts
enabled, which is also acquired by an interrupt handler on the same CPU.
1. When a task is returning to user space, `__exit_to_user_mode_loop()` checks
for pending work. If rescheduling is needed (`TIF_NEED_RESCHED`), it enables
interrupts via `local_irq_enable()` and calls `rseq_grant_slice_extension()`.
2. `rseq_grant_slice_extension()` checks if the user-space process requested an
`rseq` time slice extension. If requested, it grants the extension and calls
`hrtimer_rearm_deferred_tif()`.
3. This eventually calls `__hrtimer_rearm_deferred()`, which acquires the local
CPU's `hrtimer_bases.lock` using `guard(raw_spinlock)`. Crucially,
`raw_spinlock` disables preemption but **leaves interrupts enabled**.
4. The `lockdep_assert_irqs_disabled()` warning in
`hrtimer_test_and_clear_rearm_deferred_tif()` correctly flags this precondition
violation.
5. If a timer interrupt fires on the same CPU during the small window while
`hrtimer_bases.lock` is held, the interrupt handler `hrtimer_interrupt()` will
execute and attempt to acquire the exact same lock using
`raw_spin_lock_irqsave()`.
6. The interrupt handler will spin forever waiting for the lock to be released,
but the lock holder was interrupted and cannot make progress, resulting in a
deadlock. The syzkaller report confirms this by showing `Tainted:
[L]=SOFTLOCKUP`.
### Security Impact
* **Exploitable (False):** The bug is a pure deadlock. It does not cause any
memory corruption (like use-after-free or out-of-bounds access), information
leaks, or control-flow hijacking. Therefore, it cannot be exploited for
privilege escalation.
* **Denial of Service (True):** The deadlock causes a CPU soft lockup. If
`panic_on_softlockup` is set, the system will crash; otherwise, the CPU will be
permanently stuck, degrading system performance and eventually leading to a full
system hang.
* **Accessible From Unprivileged Processes (True):** The `rseq` (Restartable
Sequences) system call and its time slice extension feature are available to
unprivileged user-space processes. A malicious unprivileged user can repeatedly
request slice extensions and force preemptions to reliably hit the race window
and trigger the deadlock.
* **Accessible From User Namespaces (True):** Since it requires no privileges,
it is trivially accessible from within user namespaces.
* **Other Triggers (False):** The bug is located in the core kernel entry/exit
and timer code. It is not reachable via network packets, malicious filesystems,
peripheral devices, or from a VM guest (a guest can only crash itself, not the
host). Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/08/02 00:06 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/08/02 00:06 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/c45b8d2d54442f1c99f033bbcbb42035c84d8267]
|
| 3/1 |
2026/08/02 00:06 |
action |
kernel-builder |
23m
Results: map[KernelObj:/app/workdir/cache/build/2e0132c2ee7682899020e46689b22af10d3b3cf3]
|
| 4/1 |
2026/08/02 00:29 |
action |
codesearch-prepare |
19m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/08/02 00:48 |
agent |
expert |
4m
Model:
Results: map[DenialOfService:true 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 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, elevated privileges, or an information leak.
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.
Information leaks are exploitable on their own and should be classified as such. A bug that copies kernel
memory contents to userspace (e.g. an out-of-bounds read whose result is returned to the caller, or
uninitialized stack/heap bytes written to a user buffer) is exploitable: it can reveal kernel pointer
values and defeat KASLR, expose sensitive data such as cryptographic keys or other processes' memory, and
serves as a necessary building block in most modern kernel privilege-escalation exploit chains. Do not classify
an information leak as non-exploitable solely because it does not directly cause a memory write or control-flow
hijack; the leak itself is the exploit primitive.
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:
------------[ cut here ]------------
__lockdep_enabled && this_cpu_read(hardirqs_enabled)
WARNING: ./include/linux/hrtimer_rearm.h:17 at hrtimer_test_and_clear_rearm_deferred_tif include/linux/hrtimer_rearm.h:17 [inline], CPU#0: syz.2.3762/30761
WARNING: ./include/linux/hrtimer_rearm.h:17 at hrtimer_rearm_deferred_tif include/linux/hrtimer_rearm.h:52 [inline], CPU#0: syz.2.3762/30761
WARNING: ./include/linux/hrtimer_rearm.h:17 at rseq_grant_slice_extension include/linux/rseq_entry.h:236 [inline], CPU#0: syz.2.3762/30761
WARNING: ./include/linux/hrtimer_rearm.h:17 at __exit_to_user_mode_loop kernel/entry/common.c:54 [inline], CPU#0: syz.2.3762/30761
WARNING: ./include/linux/hrtimer_rearm.h:17 at exit_to_user_mode_loop kernel/entry/common.c:101 [inline], CPU#0: syz.2.3762/30761
WARNING: ./include/linux/hrtimer_rearm.h:17 at __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline], CPU#0: syz.2.3762/30761
WARNING: ./include/linux/hrtimer_rearm.h:17 at irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:244 [inline], CPU#0: syz.2.3762/30761
WARNING: ./include/linux/hrtimer_rearm.h:17 at irqentry_exit_to_user_mode include/linux/irq-entry-common.h:315 [inline], CPU#0: syz.2.3762/30761
WARNING: ./include/linux/hrtimer_rearm.h:17 at irqentry_exit+0x718/0xa00 kernel/entry/common.c:165, CPU#0: syz.2.3762/30761
Modules linked in:
CPU: 0 UID: 0 PID: 30761 Comm: syz.2.3762 Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
RIP: 0010:hrtimer_test_and_clear_rearm_deferred_tif include/linux/hrtimer_rearm.h:17 [inline]
RIP: 0010:hrtimer_rearm_deferred_tif include/linux/hrtimer_rearm.h:52 [inline]
RIP: 0010:rseq_grant_slice_extension include/linux/rseq_entry.h:236 [inline]
RIP: 0010:__exit_to_user_mode_loop kernel/entry/common.c:54 [inline]
RIP: 0010:exit_to_user_mode_loop kernel/entry/common.c:101 [inline]
RIP: 0010:__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
RIP: 0010:irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:244 [inline]
RIP: 0010:irqentry_exit_to_user_mode include/linux/irq-entry-common.h:315 [inline]
RIP: 0010:irqentry_exit+0x718/0xa00 kernel/entry/common.c:165
Code: 00 00 00 80 e8 c9 ee 58 f6 fb 83 3d 95 d1 69 05 00 74 1a 65 8b 05 d8 2a ab 08 85 c0 75 0f 65 8b 05 29 26 ab 08 85 c0 74 04 90 <0f> 0b 90 41 f7 c5 00 10 00 00 0f 84 74 fc ff ff f0 41 80 64 24 01
RSP: 0000:ffffc9000414ff00 EFLAGS: 00010202
RAX: 0000000000000001 RBX: ffffc9000414ff48 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff8e1a8f2e RDI: ffffffff8c400a80
RBP: ffffc9000414ff38 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff888022f39f00
R13: 0000000000000010 R14: 000001efbbd43bba R15: 0000000000000000
FS: 00007f3e64dd56c0(0000) GS:ffff888123de1000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2ddddff8 CR3: 0000000034bd6000 CR4: 00000000003526f0
Call Trace:
<TASK>
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:674
RIP: 0033:0x7f3e66b9e019
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f3e64dd50e8 EFLAGS: 00000246
RAX: 0000000000000000 RBX: 00007f3e66e26098 RCX: 00007f3e66b9e019
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 00007f3e66e26098
RBP: 00007f3e66e26090 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3e66e26128 R14: 00007ffe410c4bf0 R15: 00007ffe410c4cd8
</TASK>
Reply: Based on the analysis of the bug report and the kernel source code, here is the assessment of the vulnerability:
### Bug Mechanism
The bug is a classic spinlock deadlock caused by taking a lock with interrupts enabled, which is also acquired by an interrupt handler on the same CPU.
1. When a task is returning to user space, `__exit_to_user_mode_loop()` checks for pending work. If rescheduling is needed (`TIF_NEED_RESCHED`), it enables interrupts via `local_irq_enable()` and calls `rseq_grant_slice_extension()`.
2. `rseq_grant_slice_extension()` checks if the user-space process requested an `rseq` time slice extension. If requested, it grants the extension and calls `hrtimer_rearm_deferred_tif()`.
3. This eventually calls `__hrtimer_rearm_deferred()`, which acquires the local CPU's `hrtimer_bases.lock` using `guard(raw_spinlock)`. Crucially, `raw_spinlock` disables preemption but **leaves interrupts enabled**.
4. The `lockdep_assert_irqs_disabled()` warning in `hrtimer_test_and_clear_rearm_deferred_tif()` correctly flags this precondition violation.
5. If a timer interrupt fires on the same CPU during the small window while `hrtimer_bases.lock` is held, the interrupt handler `hrtimer_interrupt()` will execute and attempt to acquire the exact same lock using `raw_spin_lock_irqsave()`.
6. The interrupt handler will spin forever waiting for the lock to be released, but the lock holder was interrupted and cannot make progress, resulting in a deadlock. The syzkaller report confirms this by showing `Tainted: [L]=SOFTLOCKUP`.
### Security Impact
* **Exploitable (False):** The bug is a pure deadlock. It does not cause any memory corruption (like use-after-free or out-of-bounds access), information leaks, or control-flow hijacking. Therefore, it cannot be exploited for privilege escalation.
* **Denial of Service (True):** The deadlock causes a CPU soft lockup. If `panic_on_softlockup` is set, the system will crash; otherwise, the CPU will be permanently stuck, degrading system performance and eventually leading to a full system hang.
* **Accessible From Unprivileged Processes (True):** The `rseq` (Restartable Sequences) system call and its time slice extension feature are available to unprivileged user-space processes. A malicious unprivileged user can repeatedly request slice extensions and force preemptions to reliably hit the race window and trigger the deadlock.
* **Accessible From User Namespaces (True):** Since it requires no privileges, it is trivially accessible from within user namespaces.
* **Other Triggers (False):** The bug is located in the core kernel entry/exit and timer code. It is not reachable via network packets, malicious filesystems, peripheral devices, or from a VM guest (a guest can only crash itself, not the host).
|
| 6/2 |
2026/08/02 00:48 |
llm |
expert |
0m
Model:
Tokens:
input: 8004
output: 38
thoughts: 395
Thoughts: **Analyzing a Potential Race Condition in the Kernel's High-Resolution Timer Handling**
Okay, so I've hit a warning in `hrtimer_test_and_clear_rearm_deferred_tif` and I need to figure out what's going on. The call trace shows it's originating from `include/linux/hrtimer_rearm.h:17`, which is being called in a chain through `rseq_grant_slice_extension`, eventually hitting `exit_to_user_mode`. The warning condition is `__lockdep_enabled && this_cpu_read(hardirqs_enabled)`. This is interesting; it's suggesting a potential issue with lockdep and hard interrupt handling.
Let's look at the source code for the problematic function:
```c
static inline bool hrtimer_test_and_clear_rearm_deferred_tif(void)
{
WARN_ON_ONCE(!irqs_disabled());
...
}
```
The key line is `WARN_ON_ONCE(!irqs_disabled())`. This function is warning if interrupts are *not* disabled. The context is `rseq_grant_slice_extension` which is used in the `rseq` mechanism. This is a mechanism to speed up access to shared variables in a multi-threaded scenario. It is possible that this function, called from a system call, is attempting to modify something related to hrtimer, and that the interrupts should have been disabled. The check `__lockdep_enabled && this_cpu_read(hardirqs_enabled)` strongly suggests that we *might* be encountering a race condition or a lock order issue if the lockdep is enabled. Let me dig deeper into how the hrtimer rearming interacts with `rseq` and the interrupt context here, and see if this warning is a real issue that needs fixing.
|
| 7/2 |
2026/08/02 00:48 |
tool |
read-file |
0m
Args: map[File:include/linux/hrtimer_rearm.h FirstLine:1 LineCount:100]
Results: map[Contents: 1: // SPDX-License-Identifier: GPL-2.0
2: #ifndef _LINUX_HRTIMER_REARM_H
3: #define _LINUX_HRTIMER_REARM_H
4:
5: #ifdef CONFIG_HRTIMER_REARM_DEFERRED
6: #include <linux/thread_info.h>
7:
8: void __hrtimer_rearm_deferred(void);
9:
10: /*
11: * This is purely CPU local, so check the TIF bit first to avoid the overhead of
12: * the atomic test_and_clear_bit() operation for the common case where the bit
13: * is not set.
14: */
15: static __always_inline bool hrtimer_test_and_clear_rearm_deferred_tif(unsigned long tif_work)
16: {
17: lockdep_assert_irqs_disabled();
18:
19: if (unlikely(tif_work & _TIF_HRTIMER_REARM)) {
20: clear_thread_flag(TIF_HRTIMER_REARM);
21: return true;
22: }
23: return false;
24: }
25:
26: #define TIF_REARM_MASK (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY | _TIF_HRTIMER_REARM)
27:
28: /* Invoked from the exit to user before invoking exit_to_user_mode_loop() */
29: static __always_inline bool
30: hrtimer_rearm_deferred_user_irq(unsigned long *tif_work, const unsigned long tif_mask)
31: {
32: /* Help the compiler to optimize the function out for syscall returns */
33: if (!(tif_mask & _TIF_HRTIMER_REARM))
34: return false;
35: /*
36: * Rearm the timer if none of the resched flags is set before going into
37: * the loop which re-enables interrupts.
38: */
39: if (unlikely((*tif_work & TIF_REARM_MASK) == _TIF_HRTIMER_REARM)) {
40: clear_thread_flag(TIF_HRTIMER_REARM);
41: __hrtimer_rearm_deferred();
42: /* Don't go into the loop if HRTIMER_REARM was the only flag */
43: *tif_work &= ~TIF_HRTIMER_REARM;
44: return !*tif_work;
45: }
46: return false;
47: }
48:
49: /* Invoked from the time slice extension decision function */
50: static __always_inline void hrtimer_rearm_deferred_tif(unsigned long tif_work)
51: {
52: if (hrtimer_test_and_clear_rearm_deferred_tif(tif_work))
53: __hrtimer_rearm_deferred();
54: }
55:
56: /*
57: * This is to be called on all irqentry_exit() paths that will enable
58: * interrupts.
59: */
60: static __always_inline void hrtimer_rearm_deferred(void)
61: {
62: hrtimer_rearm_deferred_tif(read_thread_flags());
63: }
64:
65: /*
66: * Invoked from the scheduler on entry to __schedule() so it can defer
67: * rearming after the load balancing callbacks which might change hrtick.
68: */
69: static __always_inline bool hrtimer_test_and_clear_rearm_deferred(void)
70: {
71: return hrtimer_test_and_clear_rearm_deferred_tif(read_thread_flags());
72: }
73:
74: #else /* CONFIG_HRTIMER_REARM_DEFERRED */
75: static __always_inline void __hrtimer_rearm_deferred(void) { }
76: static __always_inline void hrtimer_rearm_deferred(void) { }
77: static __always_inline void hrtimer_rearm_deferred_tif(unsigned long tif_work) { }
78: static __always_inline bool
79: hrtimer_rearm_deferred_user_irq(unsigned long *tif_work, const unsigned long tif_mask) { return false; }
80: static __always_inline bool hrtimer_test_and_clear_rearm_deferred(void) { return false; }
81: #endif /* !CONFIG_HRTIMER_REARM_DEFERRED */
82:
83: #endif
]
|
| 8/2 |
2026/08/02 00:48 |
llm |
expert |
0m
Model:
Tokens:
input: 10173
output: 39
thoughts: 85
Thoughts: **Analyzing a Potential Race Condition in the Kernel's High-Resolution Timer Handling**
Okay, so the kernel's throwing a `lockdep_assert_irqs_disabled()` warning, specifically at `include/linux/hrtimer_rearm.h:17`. This means I'm looking at a situation where the code *expects* interrupts to be disabled, but they're *actually* enabled. That's a classic recipe for a race condition, so I need to dig deeper. I'm going to follow the breadcrumbs from here.
Let's see if something is going on with `rseq_grant_slice_extension`, given the context. I'll need to check the code at `include/linux/rseq_entry.h:236` to see what is happening.
|
| 9/2 |
2026/08/02 00:48 |
tool |
read-file |
0m
Args: map[File:include/linux/rseq_entry.h FirstLine:200 LineCount:100]
Results: map[Contents: 200: * #1 is not any different from a regular remote reschedule as it
201: * sets the previously not set bit and then raises the IPI which
202: * folds it into the preempt counter
203: *
204: * #2 is obviously incorrect from a scheduler POV, but it's not
205: * differently incorrect than the code below clearing the
206: * reschedule request with the safety net of the timer.
207: *
208: * The important part is that the clearing is protected against the
209: * scheduler IPI and also against any other interrupt which might
210: * end up waking up a task and setting the bits in the middle of
211: * the operation:
212: *
213: * clear_tsk()
214: * ---> Interrupt
215: * wakeup_on_this_cpu()
216: * set_tsk()
217: * set_preempt()
218: * clear_preempt()
219: *
220: * which would be inconsistent state.
221: */
222: scoped_guard(irq) {
223: clear_tsk_need_resched(curr);
224: clear_preempt_need_resched();
225: }
226: return true;
227:
228: efault:
229: force_sig(SIGSEGV);
230: return false;
231: }
232:
233: static __always_inline bool rseq_grant_slice_extension(unsigned long ti_work, unsigned long mask)
234: {
235: if (unlikely(__rseq_grant_slice_extension(ti_work & mask))) {
236: hrtimer_rearm_deferred_tif(ti_work);
237: return true;
238: }
239: return false;
240: }
241:
242: #else /* CONFIG_RSEQ_SLICE_EXTENSION */
243: static __always_inline bool rseq_slice_extension_enabled(void) { return false; }
244: static __always_inline bool rseq_arm_slice_extension_timer(void) { return false; }
245: static __always_inline void rseq_slice_clear_grant(struct task_struct *t) { }
246: static __always_inline bool rseq_grant_slice_extension(unsigned long ti_work, unsigned long mask) { return false; }
247: #define rseq_slice_clear_user(rseq, efault) do { } while (0)
248: #endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
249:
250: bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr);
251:
252: static __always_inline void rseq_note_user_irq_entry(void)
253: {
254: if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY))
255: current->rseq.event.user_irq = true;
256: }
257:
258: /*
259: * Check whether there is a valid critical section and whether the
260: * instruction pointer in @regs is inside the critical section.
261: *
262: * - If the critical section is invalid, terminate the task.
263: *
264: * - If valid and the instruction pointer is inside, set it to the abort IP.
265: *
266: * - If valid and the instruction pointer is outside, clear the critical
267: * section address.
268: *
269: * Returns true, if the section was valid and either fixup or clear was
270: * done, false otherwise.
271: *
272: * In the failure case task::rseq_event::fatal is set when a invalid
273: * section was found. It's clear when the failure was an unresolved page
274: * fault.
275: *
276: * If inlined into the exit to user path with interrupts disabled, the
277: * caller has to protect against page faults with pagefault_disable().
278: *
279: * In preemptible task context this would be counterproductive as the page
280: * faults could not be fully resolved. As a consequence unresolved page
281: * faults in task context are fatal too.
282: */
283:
284: #ifdef RSEQ_BUILD_SLOW_PATH
285: /*
286: * The debug version is put out of line, but kept here so the code stays
287: * together.
288: *
289: * @csaddr has already been checked by the caller to be in user space
290: */
291: bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs,
292: unsigned long csaddr)
293: {
294: struct rseq_cs __user *ucs = (struct rseq_cs __user *)(unsigned long)csaddr;
295: u64 start_ip, abort_ip, offset, cs_end, head, tasksize = TASK_SIZE;
296: unsigned long ip = instruction_pointer(regs);
297: u64 __user *uc_head = (u64 __user *) ucs;
298: u32 usig, __user *uc_sig;
299:
]
|
| 10/2 |
2026/08/02 00:48 |
llm |
expert |
0m
Model:
Tokens:
input: 12091
output: 34
thoughts: 93
Thoughts: Okay, here's my interpretation of this chain of events, focusing on the potential implications I see as a kernel expert:
**Tracing a Potential Synchronization Issue in the Timer System**
Right, let me see... so `rseq_grant_slice_extension` is using `hrtimer_rearm_deferred_tif`. That's interesting, as that function itself eventually calls `hrtimer_test_and_clear_rearm_deferred_tif`. Hold on a minute⦠I see `lockdep_assert_irqs_disabled()` in there. This is a red flag. This assert is designed to verify that interrupts are disabled. It screams race condition risk and potential deadlocks. This typically implies that the timer rearming is happening in a context where interrupt handling needs to be strictly controlled, which isn't always the case, in general.
Now, I'm going to shift gears. I need to understand the wider context. This is all happening as the system transitions back to user mode, so let me get my bearings by examining `__exit_to_user_mode_loop` in `kernel/entry/common.c`. This is where the transition from kernel to user space takes place. Understanding the interaction between the timer system and user mode entry/exit is crucial here. I'm wondering if there's an interaction between the rearming of the timer and the transition back to user space that could lead to a race condition. I need to dive deeper to check where this function is called.
|
| 11/2 |
2026/08/02 00:48 |
tool |
read-file |
0m
Args: map[File:kernel/entry/common.c FirstLine:1 LineCount:100]
Results: map[Contents: 1: // SPDX-License-Identifier: GPL-2.0
2:
3: #include <linux/futex.h>
4: #include <linux/highmem.h>
5: #include <linux/irq-entry-common.h>
6: #include <linux/jump_label.h>
7: #include <linux/kmsan.h>
8: #include <linux/livepatch.h>
9: #include <linux/resume_user_mode.h>
10: #include <linux/tick.h>
11:
12: /* Workaround to allow gradual conversion of architecture code */
13: void __weak arch_do_signal_or_restart(struct pt_regs *regs) { }
14:
15: #ifdef CONFIG_HAVE_GENERIC_TIF_BITS
16: #define EXIT_TO_USER_MODE_WORK_LOOP (EXIT_TO_USER_MODE_WORK & ~_TIF_RSEQ)
17: #else
18: #define EXIT_TO_USER_MODE_WORK_LOOP (EXIT_TO_USER_MODE_WORK)
19: #endif
20:
21: /* TIF bits, which prevent a time slice extension. */
22: #ifdef CONFIG_PREEMPT_RT
23: /*
24: * Since rseq slice ext has a direct correlation to the worst case
25: * scheduling latency (schedule is delayed after all), only have it affect
26: * LAZY reschedules on PREEMPT_RT for now.
27: *
28: * However, since this delay is only applicable to userspace, a value
29: * for rseq_slice_extension_nsec that is strictly less than the worst case
30: * kernel space preempt_disable() region, should mean the scheduling latency
31: * is not affected, even for !LAZY.
32: *
33: * However, since this value depends on the hardware at hand, it cannot be
34: * pre-determined in any sensible way. Hence punt on this problem for now.
35: */
36: # define TIF_SLICE_EXT_SCHED (_TIF_NEED_RESCHED_LAZY)
37: #else
38: # define TIF_SLICE_EXT_SCHED (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)
39: #endif
40: #define TIF_SLICE_EXT_DENY (EXIT_TO_USER_MODE_WORK & ~TIF_SLICE_EXT_SCHED)
41:
42: static __always_inline unsigned long __exit_to_user_mode_loop(struct pt_regs *regs,
43: unsigned long ti_work)
44: {
45: /*
46: * Before returning to user space ensure that all pending work
47: * items have been completed.
48: */
49: while (ti_work & EXIT_TO_USER_MODE_WORK_LOOP) {
50:
51: local_irq_enable();
52:
53: if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)) {
54: if (!rseq_grant_slice_extension(ti_work, TIF_SLICE_EXT_DENY))
55: schedule();
56: }
57:
58: if (ti_work & _TIF_UPROBE)
59: uprobe_notify_resume(regs);
60:
61: if (ti_work & _TIF_PATCH_PENDING)
62: klp_update_patch_state(current);
63:
64: if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) {
65: futex_fixup_robust_unlock(regs);
66: arch_do_signal_or_restart(regs);
67: }
68:
69: if (ti_work & _TIF_NOTIFY_RESUME)
70: resume_user_mode_work(regs);
71:
72: /* Architecture specific TIF work */
73: arch_exit_to_user_mode_work(regs, ti_work);
74:
75: /*
76: * Disable interrupts and reevaluate the work flags as they
77: * might have changed while interrupts and preemption was
78: * enabled above.
79: */
80: local_irq_disable();
81:
82: /* Check if any of the above work has queued a deferred wakeup */
83: tick_nohz_user_enter_prepare();
84:
85: ti_work = read_thread_flags();
86: }
87:
88: /* Return the latest work state for arch_exit_to_user_mode() */
89: return ti_work;
90: }
91:
92: /**
93: * exit_to_user_mode_loop - do any pending work before leaving to user space
94: * @regs: Pointer to pt_regs on entry stack
95: * @ti_work: TIF work flags as read by the caller
96: */
97: __always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
98: unsigned long ti_work)
99: {
100: for (;;) {
]
|
| 12/2 |
2026/08/02 00:48 |
llm |
expert |
0m
Model:
Tokens:
input: 14066
output: 39
thoughts: 245
Thoughts: **Analyzing the Interrupt Context in `__exit_to_user_mode_loop`**
Okay, so I'm looking at this `__exit_to_user_mode_loop` function and immediately a potential issue jumps out. We've got `local_irq_enable()` right before a call to `rseq_grant_slice_extension()`. Now, that's interesting. My mind's automatically jumping through the call stack: `rseq_grant_slice_extension()` leads to `hrtimer_rearm_deferred_tif()`, which in turn calls `hrtimer_test_and_clear_rearm_deferred_tif()`. And that last function... Ah, yes, it contains a `lockdep_assert_irqs_disabled()`.
So, the design is suggesting that `rseq_grant_slice_extension()` *expects* interrupts to be disabled, or at the very least, expects a specific interrupt state. Yet, we're explicitly enabling them right before the call. That's a red flag. This requires further investigation to ensure we're not introducing a subtle race condition or violating some locking assumptions. Let me pull up `include/linux/rseq_entry.h` and confirm the interrupt context expectations of `rseq_grant_slice_extension` directly.
|
| 13/2 |
2026/08/02 00:48 |
tool |
read-file |
0m
Args: map[File:include/linux/rseq_entry.h FirstLine:150 LineCount:100]
Results: map[Contents: 150: *
151: * 1) Any TIF bit which needs to do extra work aside of
152: * rescheduling prevents a grant.
153: *
154: * 2) A previous rescheduling request resulted in a slice
155: * extension grant.
156: */
157: if (unlikely(work_pending || state.granted)) {
158: /* Clear user control unconditionally. No point for checking */
159: unsafe_put_user(0U, &rseq->slice_ctrl.all, efault);
160: rseq_slice_clear_grant(curr);
161: return false;
162: }
163:
164: unsafe_get_user(usr_ctrl.all, &rseq->slice_ctrl.all, efault);
165: if (likely(!(usr_ctrl.request)))
166: return false;
167:
168: /* Grant the slice extention */
169: usr_ctrl.request = 0;
170: usr_ctrl.granted = 1;
171: unsafe_put_user(usr_ctrl.all, &rseq->slice_ctrl.all, efault);
172: }
173:
174: rseq_stat_inc(rseq_stats.s_granted);
175:
176: curr->rseq.slice.state.granted = true;
177: /* Store expiry time for arming the timer on the way out */
178: curr->rseq.slice.expires = data_race(rseq_slice_ext_nsecs) + ktime_get_mono_fast_ns();
179: /*
180: * This is racy against a remote CPU setting TIF_NEED_RESCHED in
181: * several ways:
182: *
183: * 1)
184: * CPU0 CPU1
185: * clear_tsk()
186: * set_tsk()
187: * clear_preempt()
188: * Raise scheduler IPI on CPU0
189: * --> IPI
190: * fold_need_resched() -> Folds correctly
191: * 2)
192: * CPU0 CPU1
193: * set_tsk()
194: * clear_tsk()
195: * clear_preempt()
196: * Raise scheduler IPI on CPU0
197: * --> IPI
198: * fold_need_resched() <- NOOP as TIF_NEED_RESCHED is false
199: *
200: * #1 is not any different from a regular remote reschedule as it
201: * sets the previously not set bit and then raises the IPI which
202: * folds it into the preempt counter
203: *
204: * #2 is obviously incorrect from a scheduler POV, but it's not
205: * differently incorrect than the code below clearing the
206: * reschedule request with the safety net of the timer.
207: *
208: * The important part is that the clearing is protected against the
209: * scheduler IPI and also against any other interrupt which might
210: * end up waking up a task and setting the bits in the middle of
211: * the operation:
212: *
213: * clear_tsk()
214: * ---> Interrupt
215: * wakeup_on_this_cpu()
216: * set_tsk()
217: * set_preempt()
218: * clear_preempt()
219: *
220: * which would be inconsistent state.
221: */
222: scoped_guard(irq) {
223: clear_tsk_need_resched(curr);
224: clear_preempt_need_resched();
225: }
226: return true;
227:
228: efault:
229: force_sig(SIGSEGV);
230: return false;
231: }
232:
233: static __always_inline bool rseq_grant_slice_extension(unsigned long ti_work, unsigned long mask)
234: {
235: if (unlikely(__rseq_grant_slice_extension(ti_work & mask))) {
236: hrtimer_rearm_deferred_tif(ti_work);
237: return true;
238: }
239: return false;
240: }
241:
242: #else /* CONFIG_RSEQ_SLICE_EXTENSION */
243: static __always_inline bool rseq_slice_extension_enabled(void) { return false; }
244: static __always_inline bool rseq_arm_slice_extension_timer(void) { return false; }
245: static __always_inline void rseq_slice_clear_grant(struct task_struct *t) { }
246: static __always_inline bool rseq_grant_slice_extension(unsigned long ti_work, unsigned long mask) { return false; }
247: #define rseq_slice_clear_user(rseq, efault) do { } while (0)
248: #endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
249:
]
|
| 14/2 |
2026/08/02 00:48 |
llm |
expert |
0m
Model:
Tokens:
input: 16207
output: 38
thoughts: 231
Thoughts: **Debugging a Potential Interrupt Context Issue**
Okay, so I'm diving into this `__exit_to_user_mode_loop` function and immediately I see this `while` loop that's key. It's working with `ti_work` and checking for the `EXIT_TO_USER_MODE_WORK_LOOP` flag. Inside the loop, interrupts are enabled. Then, the code checks for rescheduling flags, and, if present, it calls `rseq_grant_slice_extension`. My spidey senses are tingling because `rseq_grant_slice_extension` is the one that calls `hrtimer_rearm_deferred_tif`. And `hrtimer_rearm_deferred_tif` leads us to `hrtimer_test_and_clear_rearm_deferred_tif`, which has a `lockdep_assert_irqs_disabled()` call. That's a red flag! *Specifically*, `rseq_grant_slice_extension` is called with interrupts _enabled_! That's a potential context issue, and I need to verify what is supposed to happen here. Now I'm going to check the `hrtimer_rearm_deferred_tif` definition in `include/linux/hrtimer_rearm.h`.
|
| 15/2 |
2026/08/02 00:48 |
tool |
read-file |
0m
Args: map[File:include/linux/hrtimer_rearm.h FirstLine:40 LineCount:30]
Results: map[Contents: 40: clear_thread_flag(TIF_HRTIMER_REARM);
41: __hrtimer_rearm_deferred();
42: /* Don't go into the loop if HRTIMER_REARM was the only flag */
43: *tif_work &= ~TIF_HRTIMER_REARM;
44: return !*tif_work;
45: }
46: return false;
47: }
48:
49: /* Invoked from the time slice extension decision function */
50: static __always_inline void hrtimer_rearm_deferred_tif(unsigned long tif_work)
51: {
52: if (hrtimer_test_and_clear_rearm_deferred_tif(tif_work))
53: __hrtimer_rearm_deferred();
54: }
55:
56: /*
57: * This is to be called on all irqentry_exit() paths that will enable
58: * interrupts.
59: */
60: static __always_inline void hrtimer_rearm_deferred(void)
61: {
62: hrtimer_rearm_deferred_tif(read_thread_flags());
63: }
64:
65: /*
66: * Invoked from the scheduler on entry to __schedule() so it can defer
67: * rearming after the load balancing callbacks which might change hrtick.
68: */
69: static __always_inline bool hrtimer_test_and_clear_rearm_deferred(void)
]
|
| 16/2 |
2026/08/02 00:48 |
llm |
expert |
0m
Model:
Tokens:
input: 17192
output: 39
thoughts: 172
Thoughts: **Analyzing the RSEQ Grant Slice Extension**
Okay, let me think this through. The code's using `__rseq_grant_slice_extension`. Specifically, I see this:
```c
scoped_guard(irq) {
clear_tsk_need_resched(curr);
clear_preempt_need_resched();
}
return true;
```
It's returning `true` here, and then the calling function, `rseq_grant_slice_extension`, calls `hrtimer_rearm_deferred_tif(ti_work)`. My immediate thought is, hang on... that `scoped_guard(irq)` uses a scoped guard mechanism. That sounds like it *disables* interrupts, and then *re-enables* them when the scope ends! So the important part is, when `hrtimer_rearm_deferred_tif` is called, interrupts are back *on*! I need to double-check that this is really what's happening. Just to be completely sure, let me go look at the header file for this `__rseq_grant_slice_extension`, in `include/linux/rseq_entry.h` and confirm how this interaction is handled.
|
| 17/2 |
2026/08/02 00:49 |
tool |
read-file |
0m
Args: map[File:include/linux/rseq_entry.h FirstLine:100 LineCount:100]
Results: map[Contents: 100:
101: if (likely(!current->rseq.slice.state.granted))
102: return false;
103:
104: return __rseq_arm_slice_extension_timer();
105: }
106:
107: static __always_inline void rseq_slice_clear_grant(struct task_struct *t)
108: {
109: if (IS_ENABLED(CONFIG_RSEQ_STATS) && t->rseq.slice.state.granted)
110: rseq_stat_inc(rseq_stats.s_revoked);
111: t->rseq.slice.state.granted = false;
112: }
113:
114: /*
115: * Open coded, so it can be invoked within a user access region.
116: *
117: * This clears the user space state of the time slice extensions field only when
118: * the task has registered the optimized RSEQ_ABI V2. Some legacy registrations,
119: * e.g. TCMalloc, have conflicting non-ABI fields in struct RSEQ, which would be
120: * overwritten by an unconditional write.
121: */
122: #define rseq_slice_clear_user(rseq, efault) \
123: do { \
124: if (rseq_slice_extension_enabled()) \
125: unsafe_put_user(0U, &rseq->slice_ctrl.all, efault); \
126: } while (0)
127:
128: static __always_inline bool __rseq_grant_slice_extension(bool work_pending)
129: {
130: struct task_struct *curr = current;
131: struct rseq_slice_ctrl usr_ctrl;
132: union rseq_slice_state state;
133: struct rseq __user *rseq;
134:
135: if (!rseq_slice_extension_enabled())
136: return false;
137:
138: /* If not enabled or not a return from interrupt, nothing to do. */
139: state = curr->rseq.slice.state;
140: state.enabled &= curr->rseq.event.user_irq;
141: if (likely(!state.state))
142: return false;
143:
144: rseq = curr->rseq.usrptr;
145: scoped_user_rw_access(rseq, efault) {
146:
147: /*
148: * Quick check conditions where a grant is not possible or
149: * needs to be revoked.
150: *
151: * 1) Any TIF bit which needs to do extra work aside of
152: * rescheduling prevents a grant.
153: *
154: * 2) A previous rescheduling request resulted in a slice
155: * extension grant.
156: */
157: if (unlikely(work_pending || state.granted)) {
158: /* Clear user control unconditionally. No point for checking */
159: unsafe_put_user(0U, &rseq->slice_ctrl.all, efault);
160: rseq_slice_clear_grant(curr);
161: return false;
162: }
163:
164: unsafe_get_user(usr_ctrl.all, &rseq->slice_ctrl.all, efault);
165: if (likely(!(usr_ctrl.request)))
166: return false;
167:
168: /* Grant the slice extention */
169: usr_ctrl.request = 0;
170: usr_ctrl.granted = 1;
171: unsafe_put_user(usr_ctrl.all, &rseq->slice_ctrl.all, efault);
172: }
173:
174: rseq_stat_inc(rseq_stats.s_granted);
175:
176: curr->rseq.slice.state.granted = true;
177: /* Store expiry time for arming the timer on the way out */
178: curr->rseq.slice.expires = data_race(rseq_slice_ext_nsecs) + ktime_get_mono_fast_ns();
179: /*
180: * This is racy against a remote CPU setting TIF_NEED_RESCHED in
181: * several ways:
182: *
183: * 1)
184: * CPU0 CPU1
185: * clear_tsk()
186: * set_tsk()
187: * clear_preempt()
188: * Raise scheduler IPI on CPU0
189: * --> IPI
190: * fold_need_resched() -> Folds correctly
191: * 2)
192: * CPU0 CPU1
193: * set_tsk()
194: * clear_tsk()
195: * clear_preempt()
196: * Raise scheduler IPI on CPU0
197: * --> IPI
198: * fold_need_resched() <- NOOP as TIF_NEED_RESCHED is false
199: *
]
|
| 18/2 |
2026/08/02 00:49 |
llm |
expert |
0m
Model:
Tokens:
input: 19239
output: 36
thoughts: 469
Thoughts: **Kernel Bug: Interrupts Enabled Where They Shouldn't Be**
Okay, let's break this down. I see that `rseq_grant_slice_extension` is defined like this:
```c
static __always_inline bool rseq_grant_slice_extension(unsigned long ti_work, unsigned long mask)
{
if (unlikely(__rseq_grant_slice_extension(ti_work & mask))) {
hrtimer_rearm_deferred_tif(ti_work);
return true;
}
return false;
}
```
Now, `hrtimer_rearm_deferred_tif` itself calls `hrtimer_test_and_clear_rearm_deferred_tif`, which, critically, includes `lockdep_assert_irqs_disabled()`. That's a red flag. We're assuming the interrupt context.
But then, consider how `rseq_grant_slice_extension` is *called* within `__exit_to_user_mode_loop`:
```c
while (ti_work & EXIT_TO_USER_MODE_WORK_LOOP) {
local_irq_enable();
if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)) {
if (!rseq_grant_slice_extension(ti_work, TIF_SLICE_EXT_DENY))
schedule();
}
```
See the `local_irq_enable()`? That means that when `rseq_grant_slice_extension` is called, interrupts are ENABLED. I'm starting to see a problem here.
Okay, let's step back a bit. I need to look closer. Okay, I see that `__rseq_grant_slice_extension` uses `scoped_guard(irq)`. So, that disables interrupts, and then re-enables them when it exits.
So what we have is: Interrupts are enabled -> `rseq_grant_slice_extension` is called, it disables interrupts, it does something, it re-enables interrupts -> `hrtimer_rearm_deferred_tif` is called.
The problem? `hrtimer_rearm_deferred_tif` calls something that *asserts* interrupts are disabled, but `rseq_grant_slice_extension` is called *with* interrupts enabled and leaves them enabled. That's a direct violation! We are calling a function that expects interrupts to be disabled when they are, in fact, enabled.
So, this is a clear bug in the kernel. `hrtimer_rearm_deferred_tif` expects interrupts to be disabled, but it is called with interrupts enabled.
The immediate consequence? We're likely going to get a lockdep warning. Although, if lockdep is disabled, `lockdep_assert_irqs_disabled()` is a no-op! So, without lockdep, this could go unnoticed. Not ideal.
Is it exploitable? Itβs probably a lockdep warning at best, but the potential for some subtle race conditions depending on what happens in `hrtimer_rearm_deferred_tif` shouldn't be overlooked. This needs further analysis.
|
| 19/2 |
2026/08/02 00:49 |
tool |
read-file |
0m
Args: map[File:include/linux/lockdep.h FirstLine:600 LineCount:50]
Results: map[Contents: 600: (preempt_count() != 0 || \
601: !this_cpu_read(hardirqs_enabled))); \
602: } while (0)
603:
604: #define lockdep_assert_preemption_disabled() \
605: do { \
606: WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT) && \
607: __lockdep_enabled && \
608: (preempt_count() == 0 && \
609: this_cpu_read(hardirqs_enabled))); \
610: } while (0)
611:
612: /*
613: * Acceptable for protecting per-CPU resources accessed from BH.
614: * Much like in_softirq() - semantics are ambiguous, use carefully.
615: */
616: #define lockdep_assert_in_softirq() \
617: do { \
618: WARN_ON_ONCE(__lockdep_enabled && \
619: (!in_softirq() || in_hardirq() || in_nmi())); \
620: } while (0)
621:
622: extern void lockdep_assert_in_softirq_func(void);
623:
624: #else
625: # define might_lock(lock) do { } while (0)
626: # define might_lock_read(lock) do { } while (0)
627: # define might_lock_nested(lock, subclass) do { } while (0)
628:
629: # define lockdep_assert_irqs_enabled() do { } while (0)
630: # define lockdep_assert_irqs_disabled() do { } while (0)
631: # define lockdep_assert_in_irq() do { } while (0)
632: # define lockdep_assert_no_hardirq() do { } while (0)
633:
634: # define lockdep_assert_preemption_enabled() do { } while (0)
635: # define lockdep_assert_preemption_disabled() do { } while (0)
636: # define lockdep_assert_in_softirq() do { } while (0)
637: # define lockdep_assert_in_softirq_func() do { } while (0)
638: #endif
639:
640: #ifdef CONFIG_PROVE_RAW_LOCK_NESTING
641:
642: # define lockdep_assert_RT_in_threaded_ctx() do { \
643: WARN_ONCE(debug_locks && !current->lockdep_recursion && \
644: lockdep_hardirq_context() && \
645: !(current->hardirq_threaded || current->irq_config), \
646: "Not in threaded context on PREEMPT_RT as expected\n"); \
647: } while (0)
648:
649: #else
]
|
| 20/2 |
2026/08/02 00:49 |
llm |
expert |
0m
Model:
Tokens:
input: 21369
output: 36
thoughts: 44
Thoughts: Okay, here's my take on those thoughts, framed as an expert's internal monologue:
**Checking `lockdep_assert_irqs_disabled()` Behavior**
Alright, so `lockdep_assert_irqs_disabled()` is a no-op when lockdep is off. That's a pretty standard optimization, minimizing overhead when the debugging features are disabled. But, hold on a second... Let me just verify that assumption. It's always a good practice to double-check these things, especially in low-level code. Let's dig into the definition of `lockdep_assert_irqs_disabled()` and make absolutely certain that the no-op behavior is implemented as expected. Gotta trust but verify! It's likely a macro or conditional, but a quick check never hurts, especially when you are trying to find where the bottleneck is.
|
| 21/2 |
2026/08/02 00:49 |
tool |
read-file |
0m
Args: map[File:include/linux/lockdep.h FirstLine:570 LineCount:50]
Results: map[Contents: 570: DECLARE_PER_CPU(int, hardirq_context);
571: DECLARE_PER_CPU(unsigned int, lockdep_recursion);
572:
573: #define __lockdep_enabled (debug_locks && !this_cpu_read(lockdep_recursion))
574:
575: #define lockdep_assert_irqs_enabled() \
576: do { \
577: WARN_ON_ONCE(__lockdep_enabled && !this_cpu_read(hardirqs_enabled)); \
578: } while (0)
579:
580: #define lockdep_assert_irqs_disabled() \
581: do { \
582: WARN_ON_ONCE(__lockdep_enabled && this_cpu_read(hardirqs_enabled)); \
583: } while (0)
584:
585: #define lockdep_assert_in_irq() \
586: do { \
587: WARN_ON_ONCE(__lockdep_enabled && !this_cpu_read(hardirq_context)); \
588: } while (0)
589:
590: #define lockdep_assert_no_hardirq() \
591: do { \
592: WARN_ON_ONCE(__lockdep_enabled && (this_cpu_read(hardirq_context) || \
593: !this_cpu_read(hardirqs_enabled))); \
594: } while (0)
595:
596: #define lockdep_assert_preemption_enabled() \
597: do { \
598: WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT) && \
599: __lockdep_enabled && \
600: (preempt_count() != 0 || \
601: !this_cpu_read(hardirqs_enabled))); \
602: } while (0)
603:
604: #define lockdep_assert_preemption_disabled() \
605: do { \
606: WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT) && \
607: __lockdep_enabled && \
608: (preempt_count() == 0 && \
609: this_cpu_read(hardirqs_enabled))); \
610: } while (0)
611:
612: /*
613: * Acceptable for protecting per-CPU resources accessed from BH.
614: * Much like in_softirq() - semantics are ambiguous, use carefully.
615: */
616: #define lockdep_assert_in_softirq() \
617: do { \
618: WARN_ON_ONCE(__lockdep_enabled && \
619: (!in_softirq() || in_hardirq() || in_nmi())); \
]
|
| 22/2 |
2026/08/02 00:49 |
llm |
expert |
0m
Model:
Tokens:
input: 22517
output: 37
thoughts: 459
Thoughts: **Analyzing a Potential Race Condition in hrtimer_test_and_clear_rearm_deferred_tif**
Okay, so I'm looking at this `lockdep_assert_irqs_disabled()` call, which is a `WARN_ON_ONCE` guarded by `__lockdep_enabled`. Meaning in a production build without lockdep, it won't actually trigger. My initial thought is, is there even a real bug here if interrupts *are* enabled? Let's dig in.
The code in question is `hrtimer_test_and_clear_rearm_deferred_tif`:
```c
static __always_inline bool hrtimer_test_and_clear_rearm_deferred_tif(unsigned long tif_work)
{
lockdep_assert_irqs_disabled();
if (unlikely(tif_work & _TIF_HRTIMER_REARM)) {
clear_thread_flag(TIF_HRTIMER_REARM);
return true;
}
return false;
}
```
My concern is, with interrupts enabled, could there be a race? Specifically, if an interrupt arrives between the `tif_work` check and `clear_thread_flag(TIF_HRTIMER_REARM)`, would it cause a problem? Initially, I thought `tif_work` was being read from a global thread flag, opening the door to a race. But wait, it's passed as an argument. Okay, that's better. But, `clear_thread_flag` clears the thread flag and an interrupt might set the flag.
So the potential issue is, if an interrupt comes in *after* the `tif_work` check and sets the `TIF_HRTIMER_REARM` flag, then `clear_thread_flag` could clear a flag that was just set in the interrupt handler. But, `clear_thread_flag` uses atomic bitwise operations, like `clear_bit`, doesn't it? Right. And on x86, those are atomic, so it *should* be fine.
Now, let's trace this further. `hrtimer_test_and_clear_rearm_deferred_tif` is called from `hrtimer_rearm_deferred_tif`:
```c
static __always_inline void hrtimer_rearm_deferred_tif(unsigned long tif_work)
{
if (hrtimer_test_and_clear_rearm_deferred_tif(tif_work))
__hrtimer_rearm_deferred();
}
```
So if the test passes (meaning the flag was set) `__hrtimer_rearm_deferred()` is called. I need to check what that function does to understand the bigger picture, and see if I missed any part of the problem.
|
| 23/2 |
2026/08/02 00:49 |
tool |
read-file |
0m
Args: map[File:kernel/time/hrtimer.c FirstLine:1800 LineCount:50]
Results: map[Contents:1800: * @exclude: timer to exclude
1801: *
1802: * Returns the next expiry time over all timers except for the @exclude one or
1803: * KTIME_MAX if none of them is pending.
1804: */
1805: ktime_t hrtimer_next_event_without(const struct hrtimer *exclude)
1806: {
1807: struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
1808: ktime_t expires = KTIME_MAX;
1809: unsigned int active;
1810:
1811: guard(raw_spinlock_irqsave)(&cpu_base->lock);
1812: if (!hrtimer_hres_active(cpu_base))
1813: return expires;
1814:
1815: active = cpu_base->active_bases & HRTIMER_ACTIVE_SOFT;
1816: if (active && !cpu_base->softirq_activated)
1817: expires = hrtimer_bases_next_event_without(cpu_base, exclude, active, KTIME_MAX);
1818:
1819: active = cpu_base->active_bases & HRTIMER_ACTIVE_HARD;
1820: if (!active)
1821: return expires;
1822: return hrtimer_bases_next_event_without(cpu_base, exclude, active, expires);
1823: }
1824: #endif
1825:
1826: static inline int hrtimer_clockid_to_base(clockid_t clock_id)
1827: {
1828: switch (clock_id) {
1829: case CLOCK_MONOTONIC:
1830: return HRTIMER_BASE_MONOTONIC;
1831: case CLOCK_REALTIME:
1832: return HRTIMER_BASE_REALTIME;
1833: case CLOCK_BOOTTIME:
1834: return HRTIMER_BASE_BOOTTIME;
1835: case CLOCK_TAI:
1836: return HRTIMER_BASE_TAI;
1837: default:
1838: WARN(1, "Invalid clockid %d. Using MONOTONIC\n", clock_id);
1839: return HRTIMER_BASE_MONOTONIC;
1840: }
1841: }
1842:
1843: static ktime_t __hrtimer_cb_get_time(clockid_t clock_id)
1844: {
1845: switch (clock_id) {
1846: case CLOCK_MONOTONIC:
1847: return ktime_get();
1848: case CLOCK_REALTIME:
1849: return ktime_get_real();
]
|
| 24/2 |
2026/08/02 00:49 |
llm |
expert |
1m
Model:
Tokens:
input: 24434
output: 40
thoughts: 14
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Hunting Down `__hrtimer_rearm_deferred`**
Okay, so I need to find the definition or relevant usage of `__hrtimer_rearm_deferred`. Immediately, my brain starts running through the usual suspects: I'm probably looking at some core timer code in the kernel, likely dealing with high-resolution timers, since we have the `hrtimer` prefix. The "rearm" part suggests something about rescheduling a timer after it's fired or perhaps after it's been initialized but not yet active. And "deferred" probably means there's some kind of delay or specific context involved in the rearming process β maybe because of interrupt context or locking concerns.
I know the `hrtimer` code is a bit complex, especially the scheduling aspects so I'm thinking I'll need to follow the function calls from any likely candidates. I'll need to check the headers for definitions and potential prototypes. Then, I might need to run a `cscope` or a `grep` through the kernel source to find where it's called and how it's used. Based on the name, it's likely a kernel-internal function, so I'll also be looking at the `include/linux/hrtimer.h` and the surrounding source files in the timers subsystem. I'll need to determine where it's defined and how it's used within the context of the timer's logic, to get a clear picture of what's going on. I'll have to see if it's used for any specific timer flags or if it is part of a deferral mechanism.
|
| 25/2 |
2026/08/02 00:50 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/hrtimer.c Name:__hrtimer_rearm_deferred]
Results: map[SourceCode:2145: void __hrtimer_rearm_deferred(void)
2146: {
2147: struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
2148: ktime_t expires_next;
2149:
2150: if (!cpu_base->deferred_rearm)
2151: return;
2152:
2153: guard(raw_spinlock)(&cpu_base->lock);
2154: if (cpu_base->deferred_needs_update) {
2155: hrtimer_update_base(cpu_base);
2156: expires_next = hrtimer_update_next_event(cpu_base);
2157: } else {
2158: /* No timer added/removed. Use the cached value */
2159: expires_next = cpu_base->deferred_expires_next;
2160: }
2161: hrtimer_rearm(cpu_base, expires_next, true);
2162: }
SourceFile:kernel/time/hrtimer.c]
|
| 26/2 |
2026/08/02 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 25162
output: 36
thoughts: 115
Thoughts: **Oh Crap, Deadlock Alert!**
Okay, so I'm looking at `__hrtimer_rearm_deferred`, and it's using `guard(raw_spinlock)(&cpu_base->lock)`. Hold on a second... *`raw_spinlock` doesn't disable interrupts!* That's a huge problem. If this function is called with interrupts enabled β which is totally possible β and *then* an interrupt hits and tries to grab `cpu_base->lock` (say, from within a timer interrupt), we've got a classic deadlock scenario brewing. This is exactly the kind of bug that can bring the entire system down, I need to check something...
Okay, I need to confirm that `cpu_base->lock` can actually be taken in an interrupt context. A quick search... Yep, there it is! `hrtimer_interrupt` *definitely* takes `cpu_base->lock`! We have a genuine, potential deadlock in the codebase. This needs immediate attention.
|
| 27/2 |
2026/08/02 00:50 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/hrtimer.c Name:hrtimer_interrupt]
Results: map[SourceCode:2181: /*
2182: * High resolution timer interrupt
2183: * Called with interrupts disabled
2184: */
2185: void hrtimer_interrupt(struct clock_event_device *dev)
2186: {
2187: struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
2188: ktime_t expires_next, now, entry_time, delta;
2189: unsigned long flags;
2190: int retries = 0;
2191:
2192: BUG_ON(!cpu_base->hres_active);
2193: cpu_base->nr_events++;
2194: dev->next_event = KTIME_MAX;
2195: dev->next_event_forced = 0;
2196:
2197: raw_spin_lock_irqsave(&cpu_base->lock, flags);
2198: entry_time = now = hrtimer_update_base(cpu_base);
2199: retry:
2200: cpu_base->deferred_rearm = true;
2201: /*
2202: * Set expires_next to KTIME_MAX, which prevents that remote CPUs queue
2203: * timers while __hrtimer_run_queues() is expiring the clock bases.
2204: * Timers which are re/enqueued on the local CPU are not affected by
2205: * this.
2206: */
2207: cpu_base->expires_next = KTIME_MAX;
2208:
2209: if (!ktime_before(now, cpu_base->softirq_expires_next)) {
2210: cpu_base->softirq_expires_next = KTIME_MAX;
2211: cpu_base->softirq_activated = true;
2212: raise_timer_softirq(HRTIMER_SOFTIRQ);
2213: }
2214:
2215: __hrtimer_run_queues(cpu_base, now, flags, HRTIMER_ACTIVE_HARD);
2216:
2217: /*
2218: * The next timer was already expired due to:
2219: * - tracing
2220: * - long lasting callbacks
2221: * - being scheduled away when running in a VM
2222: *
2223: * We need to prevent that we loop forever in the hrtiner interrupt
2224: * routine. We give it 3 attempts to avoid overreacting on some
2225: * spurious event.
2226: */
2227: now = hrtimer_update_base(cpu_base);
2228: expires_next = hrtimer_update_next_event(cpu_base);
2229: cpu_base->hang_detected = false;
2230: if (expires_next < now) {
2231: if (++retries < 3)
2232: goto retry;
2233:
2234: delta = ktime_sub(now, entry_time);
2235: cpu_base->max_hang_time = max_t(unsigned int, cpu_base->max_hang_time, delta);
2236: cpu_base->nr_hangs++;
2237: cpu_base->hang_detected = true;
2238: }
2239:
2240: hrtimer_interrupt_rearm(cpu_base, expires_next);
2241: raw_spin_unlock_irqrestore(&cpu_base->lock, flags);
2242: }
SourceFile:kernel/time/hrtimer.c]
|
| 28/2 |
2026/08/02 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 26550
output: 35
thoughts: 188
Thoughts: **Deadlock Deep Dive: A Real-Time Kernel Conundrum**
Okay, so `hrtimer_interrupt` grabs `cpu_base->lock` with `raw_spin_lock_irqsave`. Right. And if `__hrtimer_rearm_deferred` gets invoked with interrupts *enabled*, it goes for that same `cpu_base->lock` using `guard(raw_spinlock)(&cpu_base->lock)`. Bingo! This is where it gets interesting...
If a timer interrupt kicks off while `__hrtimer_rearm_deferred` is already holding `cpu_base->lock`, boom β the interrupt handler tries to snag the lock andβ¦ deadlock! Classic. I've seen this kind of thing before; it's practically a rite of passage in real-time kernel debugging.
But wait. Something doesn't feel quite right here. `__hrtimer_rearm_deferred` uses `guard(raw_spinlock)(&cpu_base->lock)`. *Is* `raw_spinlock` disabling interrupts? I need to verify this; Iβm pretty sure it doesn't. My gut says `raw_spinlock` is just `raw_spin_lock`, which wouldnβt block interrupts. This is critical. Time to confirm the code and ensure my assumptions are sound.
|
| 29/2 |
2026/08/02 00:50 |
tool |
read-file |
0m
Args: map[File:include/linux/spinlock.h FirstLine:400 LineCount:50]
Results: map[Contents: 400: __releases(lock) __no_context_analysis
401: {
402: raw_spin_unlock_irq(&lock->rlock);
403: }
404:
405: static __always_inline void spin_unlock_irqrestore(spinlock_t *lock, unsigned long flags)
406: __releases(lock) __no_context_analysis
407: {
408: raw_spin_unlock_irqrestore(&lock->rlock, flags);
409: }
410:
411: static __always_inline int spin_trylock_bh(spinlock_t *lock)
412: __cond_acquires(true, lock) __no_context_analysis
413: {
414: return raw_spin_trylock_bh(&lock->rlock);
415: }
416:
417: static __always_inline int spin_trylock_irq(spinlock_t *lock)
418: __cond_acquires(true, lock) __no_context_analysis
419: {
420: return raw_spin_trylock_irq(&lock->rlock);
421: }
422:
423: static __always_inline bool _spin_trylock_irqsave(spinlock_t *lock, unsigned long *flags)
424: __cond_acquires(true, lock) __no_context_analysis
425: {
426: return raw_spin_trylock_irqsave(spinlock_check(lock), *flags);
427: }
428: #define spin_trylock_irqsave(lock, flags) _spin_trylock_irqsave(lock, &(flags))
429:
430: /**
431: * spin_is_locked() - Check whether a spinlock is locked.
432: * @lock: Pointer to the spinlock.
433: *
434: * This function is NOT required to provide any memory ordering
435: * guarantees; it could be used for debugging purposes or, when
436: * additional synchronization is needed, accompanied with other
437: * constructs (memory barriers) enforcing the synchronization.
438: *
439: * Returns: 1 if @lock is locked, 0 otherwise.
440: *
441: * Note that the function only tells you that the spinlock is
442: * seen to be locked, not that it is locked on your CPU.
443: *
444: * Further, on CONFIG_SMP=n builds with CONFIG_DEBUG_SPINLOCK=n,
445: * the return value is always 0 (see include/linux/spinlock_up.h).
446: * Therefore you should not rely heavily on the return value.
447: */
448: static __always_inline int spin_is_locked(spinlock_t *lock)
449: {
]
|
| 30/2 |
2026/08/02 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 27886
output: 25
thoughts: 13
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Analyzing the `guard(raw_spinlock)` Definition**
Okay, so I'm diving into this code snippet and the first thing that jumps out is that `guard(raw_spinlock)` call. Before I go further down the rabbit hole, I need to make sure I have a solid understanding of how that `guard` abstraction interacts with the underlying `raw_spinlock`. The whole point of these abstractions is to manage complexity, so I need to be sure the abstraction is behaving as expected.
I need to quickly refresh my memory on the specifics of how the `guard` is defined in relation to `raw_spinlock`. What are the relevant data structures, lock/unlock semantics, and any potential side effects? Is it a scoped lock, a RAII-style wrapper, or something else entirely? I need to know if the `raw_spinlock` is being initialized, destroyed, and whether proper exception safety is guaranteed within the `guard`'s scope.
A quick check of the definition should clarify if this is a wrapper that adds features or if it is just a convenient syntax. If it adds features, I'll need to remember those features when I try to reason about the logic. Once I can verify the exact behavior of `guard`, I'll be in a much better position to trace the control flow and understand the intent here. I'm hoping I don't have to trace down into assembly, but you never know.
|
| 31/2 |
2026/08/02 00:50 |
tool |
grepper |
0m
Args: map[Expression:DEFINE_LOCK_GUARD_1\(raw_spinlock]
Results: map[Output:include/linux/spinlock.h=533=void free_bucket_spinlocks(spinlock_t *locks);
include/linux/spinlock.h-534-
include/linux/spinlock.h:535:DEFINE_LOCK_GUARD_1(raw_spinlock, raw_spinlock_t,
include/linux/spinlock.h-536- raw_spin_lock(_T->lock),
--
include/linux/spinlock.h=542=DECLARE_LOCK_GUARD_1_ATTRS(raw_spinlock_try, __acquires(_T), __releases(*(raw_spinlock_t **)_T))
--
include/linux/spinlock.h-544-
include/linux/spinlock.h:545:DEFINE_LOCK_GUARD_1(raw_spinlock_nested, raw_spinlock_t,
include/linux/spinlock.h-546- raw_spin_lock_nested(_T->lock, SINGLE_DEPTH_NESTING),
--
include/linux/spinlock.h=548=DECLARE_LOCK_GUARD_1_ATTRS(raw_spinlock_nested, __acquires(_T), __releases(*(raw_spinlock_t **)_T))
--
include/linux/spinlock.h-550-
include/linux/spinlock.h:551:DEFINE_LOCK_GUARD_1(raw_spinlock_irq, raw_spinlock_t,
include/linux/spinlock.h-552- raw_spin_lock_irq(_T->lock),
--
include/linux/spinlock.h=558=DECLARE_LOCK_GUARD_1_ATTRS(raw_spinlock_irq_try, __acquires(_T), __releases(*(raw_spinlock_t **)_T))
--
include/linux/spinlock.h-560-
include/linux/spinlock.h:561:DEFINE_LOCK_GUARD_1(raw_spinlock_bh, raw_spinlock_t,
include/linux/spinlock.h-562- raw_spin_lock_bh(_T->lock),
--
include/linux/spinlock.h=568=DECLARE_LOCK_GUARD_1_ATTRS(raw_spinlock_bh_try, __acquires(_T), __releases(*(raw_spinlock_t **)_T))
--
include/linux/spinlock.h-570-
include/linux/spinlock.h:571:DEFINE_LOCK_GUARD_1(raw_spinlock_irqsave, raw_spinlock_t,
include/linux/spinlock.h-572- raw_spin_lock_irqsave(_T->lock, _T->flags),
--
include/linux/spinlock.h=580=DECLARE_LOCK_GUARD_1_ATTRS(raw_spinlock_irqsave_try, __acquires(_T), __releases(*(raw_spinlock_t **)_T))
--
include/linux/spinlock.h-582-
include/linux/spinlock.h:583:DEFINE_LOCK_GUARD_1(raw_spinlock_init, raw_spinlock_t, raw_spin_lock_init(_T->lock), /* */)
include/linux/spinlock.h-584-DECLARE_LOCK_GUARD_1_ATTRS(raw_spinlock_init, __acquires(_T), __releases(*(raw_spinlock_t **)_T))
]
|
| 32/2 |
2026/08/02 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 29035
output: 38
thoughts: 357
Thoughts: **Analyzing the Soft Lockup Bug**
Okay, so let's break this down. My initial observation is correct: `guard(raw_spinlock)` utilizes `raw_spin_lock`, which *doesn't* disable interrupts. That's a crucial starting point. Now, if `__hrtimer_rearm_deferred` gets called with interrupts still enabled, it's going to grab `cpu_base->lock` without any interrupt masking. And the bug report mentions a "SOFTLOCKUP," which is a soft lockup or deadlock. Perfect match!
So, the scenario is clear: a timer interrupt hits the same CPU while itβs holding `cpu_base->lock`. That `hrtimer_interrupt` routine then tries to get the same lock using `raw_spin_lock_irqsave`, spinning forever. Deadlock. Bang on.
Now, the root cause is `__exit_to_user_mode_loop` calling `rseq_grant_slice_extension` with interrupts enabled. `rseq_grant_slice_extension` in turn calls `hrtimer_rearm_deferred_tif`, which leads to `__hrtimer_rearm_deferred`, and that's the culprit that takes `cpu_base->lock` without disabling interrupts. Timer interrupt fires, deadlock. Got it.
Is this exploitable? Well, it's a soft lockup. So, Denial of Service (DoS), definitely. Memory corruption? No, just a deadlock. And the big question is, can an unprivileged user trigger this? Yes, because `rseq_grant_slice_extension` is involved, which can be triggered by `rseq` system calls or by requesting a slice extension. It's time to dig into how this `rseq_grant_slice_extension` gets activated to understand exactly how easy it is to exploit.
|
| 33/2 |
2026/08/02 00:50 |
tool |
read-file |
0m
Args: map[File:include/linux/rseq_entry.h FirstLine:120 LineCount:50]
Results: map[Contents: 120: * overwritten by an unconditional write.
121: */
122: #define rseq_slice_clear_user(rseq, efault) \
123: do { \
124: if (rseq_slice_extension_enabled()) \
125: unsafe_put_user(0U, &rseq->slice_ctrl.all, efault); \
126: } while (0)
127:
128: static __always_inline bool __rseq_grant_slice_extension(bool work_pending)
129: {
130: struct task_struct *curr = current;
131: struct rseq_slice_ctrl usr_ctrl;
132: union rseq_slice_state state;
133: struct rseq __user *rseq;
134:
135: if (!rseq_slice_extension_enabled())
136: return false;
137:
138: /* If not enabled or not a return from interrupt, nothing to do. */
139: state = curr->rseq.slice.state;
140: state.enabled &= curr->rseq.event.user_irq;
141: if (likely(!state.state))
142: return false;
143:
144: rseq = curr->rseq.usrptr;
145: scoped_user_rw_access(rseq, efault) {
146:
147: /*
148: * Quick check conditions where a grant is not possible or
149: * needs to be revoked.
150: *
151: * 1) Any TIF bit which needs to do extra work aside of
152: * rescheduling prevents a grant.
153: *
154: * 2) A previous rescheduling request resulted in a slice
155: * extension grant.
156: */
157: if (unlikely(work_pending || state.granted)) {
158: /* Clear user control unconditionally. No point for checking */
159: unsafe_put_user(0U, &rseq->slice_ctrl.all, efault);
160: rseq_slice_clear_grant(curr);
161: return false;
162: }
163:
164: unsafe_get_user(usr_ctrl.all, &rseq->slice_ctrl.all, efault);
165: if (likely(!(usr_ctrl.request)))
166: return false;
167:
168: /* Grant the slice extention */
169: usr_ctrl.request = 0;
]
|
| 34/2 |
2026/08/02 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 30593
output: 38
thoughts: 954
Thoughts: **Analysis of a Potential Kernel Deadlock**
Okay, so I've got a potential deadlock situation here. Let's break down the conditions. It looks like a user process can trigger this by:
1. Registering an `rseq` area.
2. Potentially enabling slice extension if the system allows it (`rseq_slice_extension_enabled()`).
3. Setting `usr_ctrl.request = 1`.
4. Then, crucially, getting interrupted β likely a timer interrupt setting `TIF_NEED_RESCHED` or `TIF_NEED_RESCHED_LAZY`.
5. Returning to user mode triggers `__exit_to_user_mode_loop`, which sees the `TIF_NEED_RESCHED` flag.
6. This leads to `rseq_grant_slice_extension` being called.
7. `__rseq_grant_slice_extension` then grants the extension, clears `TIF_NEED_RESCHED`, and returns.
8. Now, we hit `hrtimer_rearm_deferred_tif`, which calls `__hrtimer_rearm_deferred`.
9. This is where things get interesting, `__hrtimer_rearm_deferred` acquires `cpu_base->lock` with interrupts *enabled*.
The heart of the issue: how long is that lock held? Looking at the code: it's a *very* short window. Basically, it checks if `cpu_base->deferred_needs_update`, updates the timer base, gets the next expiration time, and calls `hrtimer_rearm`. If no updates are needed, it uses a cached value. This sequence happens under the lock.
To trigger the deadlock, the timer interrupt *must* fire during this incredibly narrow window. Can an unprivileged user make this happen reliably? Maybe not first time, but a user could loop this process and eventually the interrupt *will* hit the window, resulting in a soft lockup. So, this looks like a Denial of Service (DoS) vulnerability. An unprivileged user could trigger this.
Now, the important question: is it exploitable for anything more than a DoS? Privilege escalation, memory corruption?
Based on what I see, no. It's a deadlock. The CPU will spin in `raw_spin_lock_irqsave` in `hrtimer_interrupt`.
Therefore:
* Exploitable: False
* DenialOfService: True
* Unprivileged: True
* UserNamespace: True (because it is triggered by an unprivileged user process)
* VMGuestTrigger: False (this is a host kernel issue, not something triggered in the host from a VM guest)
I need to re-read that definition again. "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." Okay, it is about triggering the host from the guest. Not that.
Let's quickly double-check that this isn't a memory corruption issue under the hood. The critical path is `__hrtimer_rearm_deferred`. If this routine is interrupted by the timer interrupt handler while holding `cpu_base->lock`, it deadlocks. It's a lock order inversion issue. It's just a deadlock. There is no other memory corruption.
I also checked that the path from `__exit_to_user_mode_loop` to `__hrtimer_rearm_deferred` is the only path in this scenario. Then I looked for another path: `hrtimer_rearm_deferred_user_irq`.
So, I am confident in my analysis.
|
| 35/2 |
2026/08/02 00:51 |
tool |
read-file |
0m
Args: map[File:include/linux/hrtimer_rearm.h FirstLine:28 LineCount:20]
Results: map[Contents: 28: /* Invoked from the exit to user before invoking exit_to_user_mode_loop() */
29: static __always_inline bool
30: hrtimer_rearm_deferred_user_irq(unsigned long *tif_work, const unsigned long tif_mask)
31: {
32: /* Help the compiler to optimize the function out for syscall returns */
33: if (!(tif_mask & _TIF_HRTIMER_REARM))
34: return false;
35: /*
36: * Rearm the timer if none of the resched flags is set before going into
37: * the loop which re-enables interrupts.
38: */
39: if (unlikely((*tif_work & TIF_REARM_MASK) == _TIF_HRTIMER_REARM)) {
40: clear_thread_flag(TIF_HRTIMER_REARM);
41: __hrtimer_rearm_deferred();
42: /* Don't go into the loop if HRTIMER_REARM was the only flag */
43: *tif_work &= ~TIF_HRTIMER_REARM;
44: return !*tif_work;
45: }
46: return false;
47: }
]
|
| 36/2 |
2026/08/02 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 32775
output: 40
thoughts: 52
Thoughts: Here's my summary, as the expert:
**Investigating the Timing of `hrtimer_rearm_deferred_user_irq`**
Okay, so I'm seeing that `hrtimer_rearm_deferred_user_irq` is being called *before* `exit_to_user_mode_loop`. That's an interesting sequencing choice, and the fact that interrupts are disabled during this sequence is crucial. My first instinct, given this context, is to double-check the interaction with `irqentry_exit_to_user_mode_prepare`. I need to understand how the IRQ handling, especially deferred ones, is being set up in relation to the transition back to user space. It feels like there might be some kind of subtle synchronization or race condition that I want to rule out, or a deliberate performance optimization that I should understand. I'm especially interested in how the deferred IRQ rearming interacts with the context switch implications of `exit_to_user_mode_loop`. The goal is to fully understand and verify the interrupt setup and timing within this path.
|
| 37/2 |
2026/08/02 00:51 |
tool |
read-file |
0m
Args: map[File:include/linux/irq-entry-common.h FirstLine:200 LineCount:50]
Results: map[Contents: 200:
201: /* Flush pending rcuog wakeup before the last need_resched() check */
202: tick_nohz_user_enter_prepare();
203:
204: ti_work = read_thread_flags();
205: if (unlikely(ti_work & work_mask)) {
206: if (!hrtimer_rearm_deferred_user_irq(&ti_work, work_mask))
207: ti_work = exit_to_user_mode_loop(regs, ti_work);
208: }
209:
210: arch_exit_to_user_mode_prepare(regs, ti_work);
211: }
212:
213: static __always_inline void __exit_to_user_mode_validate(void)
214: {
215: /* Ensure that kernel state is sane for a return to userspace */
216: kmap_assert_nomap();
217: lockdep_assert_irqs_disabled();
218: lockdep_sys_exit();
219: }
220:
221: /**
222: * syscall_exit_to_user_mode_prepare - call exit_to_user_mode_loop() if required
223: * @regs: Pointer to pt_regs on entry stack
224: *
225: * Wrapper around __exit_to_user_mode_prepare() to separate the exit work for
226: * syscalls and interrupts.
227: */
228: static __always_inline void syscall_exit_to_user_mode_prepare(struct pt_regs *regs)
229: {
230: __exit_to_user_mode_prepare(regs, EXIT_TO_USER_MODE_WORK_SYSCALL);
231: rseq_syscall_exit_to_user_mode();
232: __exit_to_user_mode_validate();
233: }
234:
235: /**
236: * irqentry_exit_to_user_mode_prepare - call exit_to_user_mode_loop() if required
237: * @regs: Pointer to pt_regs on entry stack
238: *
239: * Wrapper around __exit_to_user_mode_prepare() to separate the exit work for
240: * syscalls and interrupts.
241: */
242: static __always_inline void irqentry_exit_to_user_mode_prepare(struct pt_regs *regs)
243: {
244: __exit_to_user_mode_prepare(regs, EXIT_TO_USER_MODE_WORK_IRQ);
245: rseq_irqentry_exit_to_user_mode();
246: __exit_to_user_mode_validate();
247: }
248:
249: /**
]
|
| 38/2 |
2026/08/02 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 33934
output: 37
thoughts: 1255
Thoughts: **Analysis of the `hrtimer_rearm_deferred_tif` Deadlock**
Alright, let's break this down. I've traced the execution path and it's clear: `__exit_to_user_mode_prepare` initiates a process that can lead to a deadlock. It starts with `hrtimer_rearm_deferred_user_irq` being called with interrupts disabled. If that call returns false, we enter `exit_to_user_mode_loop`, which eventually gets to `__exit_to_user_mode_loop`. Within this loop, `local_irq_enable()` is called, and *then* `rseq_grant_slice_extension` gets called. Critically, `__rseq_grant_slice_extension` re-enables interrupts before calling `hrtimer_rearm_deferred_tif`.
Now, `hrtimer_test_and_clear_rearm_deferred_tif`, *inside* `hrtimer_rearm_deferred_tif`, asserts that interrupts are disabled via `lockdep_assert_irqs_disabled()`. But, hey, we just *enabled* them! So that warning is triggered. Then we proceed to `__hrtimer_rearm_deferred`, which grabs `cpu_base->lock` with a raw spinlock. The problem is now painfully obvious: if a timer interrupt happens to fire *while* this lock is held (and interrupts are enabled), we have a deadlock.
So the core issue is exactly as I expected: an interrupt is enabled during a critical section that is meant to run with interrupts disabled. However, while the analysis confirms a deadlock, the possibility of memory corruption due to this bug seems unlikely. There is no way to exploit this in the context of memory corruption.
The next question: can this be triggered by unprivileged users? And the answer appears to be yes. `rseq` is accessible to unprivileged users, and by controlling a few variables, it's possible to set `usr_ctrl.request = 1`. The user then just needs to wait for a timer interrupt that sets `TIF_NEED_RESCHED`, which can be forced by a CPU intensive operation. Once that happens, `__exit_to_user_mode_loop` is called, sees `TIF_NEED_RESCHED`, enables interrupts, and calls `rseq_grant_slice_extension`. `__rseq_grant_slice_extension` grants the extension and returns true. Then the troublesome `hrtimer_rearm_deferred_tif` is called.
The user now has a window to trigger the deadlock. Specifically, it's during this code:
```c
guard(raw_spinlock)(&cpu_base->lock);
if (cpu_base->deferred_needs_update) {
hrtimer_update_base(cpu_base);
expires_next = hrtimer_update_next_event(cpu_base);
} else {
/* No timer added/removed. Use the cached value */
expires_next = cpu_base->deferred_expires_next;
}
hrtimer_rearm(cpu_base, expires_next, true);
```
That's a relatively small window, but the user *can* control the scheduling through `rseq` and can repeatedly set `usr_ctrl.request` to 1 in a tight loop and keep getting preempted. So, eventually, a timer interrupt will likely hit that window, and trigger the deadlock. And the `Tainted: [L]=SOFTLOCKUP` message from syzkaller confirms that it *is* hitting the deadlock, which means it is a DoS, which is what I suspected.
Let's quickly check for other consequences beyond the deadlock... `__hrtimer_rearm_deferred` calls `hrtimer_rearm`. So, ultimately, it's just this deadlock that we have to worry about.
|
| 39/2 |
2026/08/02 00:51 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/hrtimer.c Name:hrtimer_rearm]
Results: map[SourceCode:2124: /*
2125: * Very similar to hrtimer_force_reprogram(), except it deals with
2126: * deferred_rearm and hang_detected.
2127: */
2128: static void hrtimer_rearm(struct hrtimer_cpu_base *cpu_base, ktime_t expires_next, bool deferred)
2129: {
2130: cpu_base->expires_next = expires_next;
2131: cpu_base->deferred_rearm = false;
2132:
2133: if (unlikely(cpu_base->hang_detected)) {
2134: /*
2135: * Give the system a chance to do something else than looping
2136: * on hrtimer interrupts.
2137: */
2138: expires_next = ktime_add_ns(ktime_get(),
2139: min(100 * NSEC_PER_MSEC, cpu_base->max_hang_time));
2140: }
2141: hrtimer_rearm_event(expires_next, deferred);
2142: }
SourceFile:kernel/time/hrtimer.c]
|
| 40/2 |
2026/08/02 00:51 |
llm |
expert |
1m
Model:
Tokens:
input: 36423
output: 39
thoughts: 17
Thoughts: Okay, here's my interpretation of that thought process, from an expert's perspective:
**Reframing the Timer Arming Process**
Alright, so `hrtimer_rearm_event` is the key function when we need to reschedule a high-resolution timer event. Now, the crucial thing here is that within that context, the code calls `tick_program_event`. This immediately triggers a mental mapping. We're not just re-arming the HR timer in isolation. We are directly interacting with the broader tick infrastructure, and programming the tick device's event queue itself. The implication here is tight integration and a synchronized approach to managing system time. I need to keep this in mind when I start to debug issues related to timer drift, or interrupt latency, as it indicates a hand-off of responsibility at some point. It's not just the HR timer; it's the underlying clock source and its relationship to the broader scheduler.
|
| 41/2 |
2026/08/02 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/hrtimer.c Name:hrtimer_rearm_event]
Results: map[SourceCode: 704: static inline void hrtimer_rearm_event(ktime_t expires_next, bool deferred)
705: {
706: trace_hrtimer_rearm(expires_next, deferred);
707: tick_program_event(expires_next, 1);
708: }
SourceFile:kernel/time/hrtimer.c]
|
| 42/2 |
2026/08/02 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 36795
output: 21
thoughts: 405
Thoughts: **My Analysis of the Clock Event Driver Deadlock**
Okay, so here's the situation as I see it. The `tick_program_event` function is programming the clock event device, as expected. The critical issue arises when interrupts are enabled during this process. An interrupt firing simultaneously creates a race condition, potentially leading to another attempt to program the clock event device within the interrupt handler, which is problematic. However, the *core* problem centers on the deadlock involving `cpu_base->lock`.
Let's break down the impact. We're *not* looking at an exploitable vulnerability in the traditional sense. There's no memory corruption, no information leakage, and definitely no privilege escalation. However, we're definitely looking at a Denial of Service scenario β a soft lockup. The interesting thing is that an unprivileged user can trigger this because of the availability of rseq. It is also usable in a user namespace. It won't be triggered by a virtual machine guest on the host, since the problem originates from host user space. Therefore, this issue is not caused by the virtual machine guest nor can it be triggered remotely or via a peripheral or filesystem.
Initially, I almost went down the path of worrying about memory corruption given the lock being held with interrupts enabled. But then, as I reviewed the interrupt handler's actions, I realized it's a textbook spinlock deadlock. The interrupt handler attempts to acquire `cpu_base->lock` using `raw_spin_lock_irqsave`, but the lock is already held. It spins indefinitely, because the interrupt handler will wait forever for the lock to become available, because the interrupt handler is running on the same CPU, so it will never release the lock. This stalls the CPU within the interrupt handler. Consequently, the soft lockup detector (NMI watchdog or similar) kicks in. Depending on the `panic_on_softlockup` setting, the system either panics immediately (a more severe DoS) or simply prints a warning and leaves the CPU deadlocked (a DoS but potentially less disruptive). There's *no* memory corruption, because the interrupt handler doesn't actually get far enough to modify any data structures; it's stuck in a spinloop.
Finally, I need to verify that rseq_grant_slice_extension is enabled. Its availability depends on `CONFIG_RSEQ_SLICE_EXTENSION`. I need to review the configuration for this.
|
| 43/2 |
2026/08/02 00:52 |
tool |
grepper |
0m
Args: map[Expression:config RSEQ_SLICE_EXTENSION]
Results: map[Output:init/Kconfig=1983=config RSEQ
--
init/Kconfig-1996-
init/Kconfig:1997:config RSEQ_SLICE_EXTENSION
init/Kconfig-1998- bool "Enable rseq-based time slice extension mechanism"
]
|
| 44/2 |
2026/08/02 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 37813
output: 33
thoughts: 18
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Diving into the Kernel Configuration Landscape**
Okay, so I need to understand something related to kernel configuration. Let's see... a specific area I'm thinking about is probably best understood by examining the `init/Kconfig` file. Given my prior experience, I anticipate the relevant section is likely to be deeper in the file rather than right at the start. So, I'll start by taking a look around line 1997. It's a reasonably large file, so getting that line as a starting point provides a good scope for a targeted investigation. Hopefully this will get me closer to figuring out the answer to my question.
|
| 45/2 |
2026/08/02 00:52 |
tool |
read-file |
0m
Args: map[File:init/Kconfig FirstLine:1990 LineCount:20]
Results: map[Contents:1990: user-space cache for the current CPU number value, which
1991: speeds up getting the current CPU number from user-space,
1992: as well as an ABI to speed up user-space operations on
1993: per-CPU data.
1994:
1995: If unsure, say Y.
1996:
1997: config RSEQ_SLICE_EXTENSION
1998: bool "Enable rseq-based time slice extension mechanism"
1999: depends on RSEQ && HIGH_RES_TIMERS && GENERIC_ENTRY && HAVE_GENERIC_TIF_BITS
2000: help
2001: Allows userspace to request a limited time slice extension when
2002: returning from an interrupt to user space via the RSEQ shared
2003: data ABI. If granted, that allows to complete a critical section,
2004: so that other threads are not stuck on a conflicted resource,
2005: while the task is scheduled out.
2006:
2007: If unsure, say N.
2008:
2009: config RSEQ_STATS
]
|
| 46/2 |
2026/08/02 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 38342
output: 34
thoughts: 864
Thoughts: **Analyzing the Bug: A Deep Dive into a Potential Deadlock**
Okay, so I'm looking at this bug, and it seems like it's an optional feature, but unprivileged users *could* trigger it if it's enabled. The syzkaller kernel report shows it's enabled there, so that's something to consider. Let's walk through the questions posed about the vulnerability.
First, is it exploitable for memory corruption, privilege escalation, or information leaks? Hmm, no, that's false. The report says it's a deadlock, so no to those things.
Next, Denial of Service? Yep, true. Soft lockup, so that checks out.
Can an unprivileged process trigger it? Yes, since `rseq` is accessible to those users.
How about within a user namespace with all capabilities? True, again.
Can a KVM guest trigger this? No.
What about a confidential computing KVM host? Also, no.
Does it involve Ethernet traffic? Nope.
Other remote traffic? Negative.
Peripheral device trigger? No.
Malicious filesystem? Not here.
Alright, now let's dig deeper and see if there are any other pathways to memory corruption. The warning message is `lockdep_assert_irqs_disabled()` in `hrtimer_test_and_clear_rearm_deferred_tif`. If lockdep is off, that's essentially a nothing burger. Then, the code calls `__hrtimer_rearm_deferred`. It uses `cpu_base->lock` with `guard(raw_spinlock)(&cpu_base->lock)`. That's a raw spinlock. So if the lock is held, it just spins. If an interrupt happens while the lock is held, the `hrtimer_interrupt` will call `raw_spin_lock_irqsave(&cpu_base->lock, flags)`. This will cause the lock to be held. It spins forever. Classic deadlock. The CPU is stuck. No memory corruption. Just a DoS.
But wait, what if the interrupt occurs on a *different* CPU? In that case, the other CPU will spin, waiting for the first CPU to release the lock, but the first CPU will release the lock very quickly. The other CPU gets the lock quickly and continues. Itβs only a deadlock if it's the *same* CPU. Since `__hrtimer_rearm_deferred` uses `this_cpu_ptr(&hrtimer_bases)`, it takes the lock of the local CPU. And timer interrupts for this CPU will be delivered to this CPU. So if a timer interrupt fires on this CPU while the lock is held, we deadlock. The window is small, but possible.
Any other consequences? What if `__hrtimer_rearm_deferred` is called with preemption enabled? `__exit_to_user_mode_loop` calls `local_irq_enable()`. Does that mean it can be preempted? Yes, `local_irq_enable()` enables interrupts, and if preemption is enabled, it *can* be preempted. Wait, is `__exit_to_user_mode_loop` called with preemption disabled? Gotta check `exit_to_user_mode_loop`.
|
| 47/2 |
2026/08/02 00:52 |
tool |
read-file |
0m
Args: map[File:kernel/entry/common.c FirstLine:90 LineCount:50]
Results: map[Contents: 90: }
91:
92: /**
93: * exit_to_user_mode_loop - do any pending work before leaving to user space
94: * @regs: Pointer to pt_regs on entry stack
95: * @ti_work: TIF work flags as read by the caller
96: */
97: __always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
98: unsigned long ti_work)
99: {
100: for (;;) {
101: ti_work = __exit_to_user_mode_loop(regs, ti_work);
102:
103: if (likely(!rseq_exit_to_user_mode_restart(regs, ti_work)))
104: return ti_work;
105: ti_work = read_thread_flags();
106: }
107: }
108:
109: noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
110: {
111: if (user_mode(regs)) {
112: irqentry_state_t ret = {
113: .exit_rcu = false,
114: };
115:
116: irqentry_enter_from_user_mode(regs);
117: return ret;
118: }
119:
120: return irqentry_enter_from_kernel_mode(regs);
121: }
122:
123: /**
124: * arch_irqentry_exit_need_resched - Architecture specific need resched function
125: *
126: * Invoked from raw_irqentry_exit_cond_resched() to check if resched is needed.
127: * Defaults return true.
128: *
129: * The main purpose is to permit arch to avoid preemption of a task from an IRQ.
130: */
131: static inline bool arch_irqentry_exit_need_resched(void);
132:
133: #ifndef arch_irqentry_exit_need_resched
134: static inline bool arch_irqentry_exit_need_resched(void) { return true; }
135: #endif
136:
137: void raw_irqentry_exit_cond_resched(void)
138: {
139: if (!preempt_count()) {
]
|
| 48/2 |
2026/08/02 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 40680
output: 35
thoughts: 226
Thoughts: **My Analysis of the `__exit_to_user_mode_loop` Function**
Okay, so I'm looking at this `__exit_to_user_mode_loop` function and trying to understand its scheduling logic. The core idea seems to be this `while` loop, checking `ti_work` for the `EXIT_TO_USER_MODE_WORK_LOOP` flag. Within that, it's enabling local interrupts, which makes sense for responding to incoming events.
Now, the interesting part. It checks for either `_TIF_NEED_RESCHED` or `_TIF_NEED_RESCHED_LAZY`. If *either* of those are set, it calls `rseq_grant_slice_extension`. And here's the kicker: *if* that function *doesn't* return true (meaning it needs the scheduler), it calls `schedule()`. But the critical piece is it does *not* call `schedule()` if it returns true! I need to be careful with this, so I will analyze this more.
Now the question is, *why*? The code has already enabled preemption, but I also see this `__hrtimer_rearm_deferred` path. And I know from memory that this can be a path that does something, because it takes `cpu_base->lock` with `guard(raw_spinlock)`. And I'm pretty sure that any call to `raw_spin_lock` disables preemption, which brings me to a question.
Hold on... does `raw_spin_lock` disable preemption? Yes, it does. Right, so if `rseq_grant_slice_extension` is returning true, it's *not* calling `schedule()` and *that's* important.
|
| 49/2 |
2026/08/02 00:52 |
tool |
read-file |
0m
Args: map[File:include/linux/spinlock.h FirstLine:180 LineCount:50]
Results: map[Contents: 180: extern void do_raw_spin_lock(raw_spinlock_t *lock) __acquires(lock);
181: extern int do_raw_spin_trylock(raw_spinlock_t *lock) __cond_acquires(true, lock);
182: extern void do_raw_spin_unlock(raw_spinlock_t *lock) __releases(lock);
183: #else
184: static inline void do_raw_spin_lock(raw_spinlock_t *lock) __acquires(lock)
185: {
186: __acquire(lock);
187: arch_spin_lock(&lock->raw_lock);
188: mmiowb_spin_lock();
189: }
190:
191: static inline int do_raw_spin_trylock(raw_spinlock_t *lock)
192: __cond_acquires(true, lock)
193: {
194: int ret = arch_spin_trylock(&(lock)->raw_lock);
195:
196: if (ret)
197: mmiowb_spin_lock();
198:
199: return ret;
200: }
201:
202: static inline void do_raw_spin_unlock(raw_spinlock_t *lock) __releases(lock)
203: {
204: mmiowb_spin_unlock();
205: arch_spin_unlock(&lock->raw_lock);
206: __release(lock);
207: }
208: #endif
209:
210: /*
211: * Define the various spin_lock methods. Note we define these
212: * regardless of whether CONFIG_SMP or CONFIG_PREEMPTION are set. The
213: * various methods are defined as nops in the case they are not
214: * required.
215: */
216: #define raw_spin_trylock(lock) _raw_spin_trylock(lock)
217:
218: #define raw_spin_lock(lock) _raw_spin_lock(lock)
219:
220: #ifdef CONFIG_DEBUG_LOCK_ALLOC
221: # define raw_spin_lock_nested(lock, subclass) \
222: _raw_spin_lock_nested(lock, subclass)
223:
224: # define raw_spin_lock_nest_lock(lock, nest_lock) \
225: do { \
226: typecheck(struct lockdep_map *, &(nest_lock)->dep_map);\
227: _raw_spin_lock_nest_lock(lock, &(nest_lock)->dep_map); \
228: } while (0)
229: #else
]
|
| 50/2 |
2026/08/02 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 42141
output: 37
thoughts: 12
Thoughts: Okay, here's my attempt at summarizing that thought process, tailored for an expert audience:
**Examining the Raw Spin Lock**
Alright, let's dive right in. My first thought is to zero in on the `_raw_spin_lock` construct. Given the context, I'm assuming we're dealing with low-level concurrency mechanisms. The prefix `_raw` immediately flags a potential area for close inspection. It usually implies that this isn't a high-level abstraction or a convenience wrapper. We're likely dealing with the bare-bones implementation of a spinlock, probably directly interfacing with atomic instructions and memory barriers. The "raw" nature of it suggests that any higher-level locking discipline, any protection against priority inversion, or maybe even any kind of instrumentation around it has been bypassed. So, my mental checklist now includes: potential livelock concerns, the necessity for correct memory ordering, and any risks associated with it being used in an interrupt context. My expertise tells me, understanding the implementation of `_raw_spin_lock` would reveal critical insight into the underlying concurrency model, and the assumptions on which the correctness of the surrounding code relies. This is the place to start drilling down.
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2026/08/02 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/spinlock.h Name:_raw_spin_lock]
Results: map[SourceCode: 156: noinline void __lockfunc _raw_spin_lock(raw_spinlock_t *lock)
157: {
158: __raw_spin_lock(lock);
159: }
SourceFile:kernel/locking/spinlock.c]
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2026/08/02 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 42534
output: 37
thoughts: 12
Thoughts: Here's my thought process, summarized as you requested:
**Deconstructing `__raw_spin_lock`**
Okay, so I need to take a look at `__raw_spin_lock`. I'm familiar with spinlocks, but I want to refresh my memory and make sure I understand the low-level mechanics of *this* specific implementation. Let's see what the code reveals. I want to identify any specific optimizations or platform-specific considerations that might be in play. I'm focusing on the *raw* aspect β is it bypassing any higher-level abstractions? What are the implications for contention and performance in a heavily threaded environment? The name suggests it's a fundamental building block; I need to understand it fully before I move on.
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2026/08/02 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/spinlock.c Name:__raw_spin_lock]
Results: map[SourceCode: 154: static inline void __raw_spin_lock(raw_spinlock_t *lock)
155: __acquires(lock) __no_context_analysis
156: {
157: preempt_disable();
158: spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
159: LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);
160: }
SourceFile:include/linux/spinlock_api_smp.h]
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| 54/2 |
2026/08/02 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 42909
output: 66
thoughts: 588
Thoughts: **My Analysis of the Deadlock Vulnerability**
Okay, so I've worked through this. My thinking is, `__raw_spin_lock` *does* call `preempt_disable()`, which disables preemption while the `cpu_base->lock` is held. But, and this is the crux, interrupts are *still* enabled. This is where the problem lies.
If an interrupt fires while the lock is held, and specifically if it's a timer interrupt, the system will call `hrtimer_interrupt`. And, crucially, that will try to acquire the *same* lock again with `raw_spin_lock_irqsave(&cpu_base->lock, flags)`. Boom. We've got a classic deadlock right there, spinning forever. The evidence lines up, and the soft lockup confirms the scenario.
So, is there a way to *exploit* this? Probably not. Generally, deadlocks are DoS vulnerabilities. They don't typically lead to privilege escalation or memory corruption. The "Exploitable" question gets a resounding "False" from me.
Let me just quickly review the questions to make sure I'm on the same page.
* **Exploitable:** False. No privilege escalation or memory corruption.
* **Denial of Service:** True. Deadlock is DoS.
* **Accessible From Unprivileged Processes:** True. The trigger is accessible via the `rseq` system call, which is usable by regular users.
* **Accessible From User Namespaces:** True. It's accessible even within a user namespace.
* **VM Guest Trigger:** False. Not a KVM guest trigger.
* **VM Host Trigger in The Confidential Computing Context:** False. Not relevant to a confidential computing guest.
* **Ethernet Network Trigger:** False. Not triggered by network traffic.
* **Other Remote Trigger:** False. Not a remote trigger.
* **Peripheral Trigger:** False. Not a peripheral device trigger.
* **Malicious Filesystem Trigger:** False. Not a filesystem trigger.
I'm feeling pretty solid in this. The deadlock is clear because of the spinlock taken with interrupts enabled in the timer handler. The `lockdep_assert_irqs_disabled()` check is violated, which is what's expected in this situation. That `syzkaller` report with the `Tainted: [L]=SOFTLOCKUP` confirms it. The trigger is the `rseq_grant_slice_extension`, which is reachable from a typical user. The impact is a local Denial of Service.
I'm confident in these findings. Time to use the `set-results` tool and move on.
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2026/08/02 00:53 |
tool |
set-results |
0m
Args: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
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| 56/2 |
2026/08/02 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 44209
output: 657
thoughts: 456
Thoughts: **Finalizing the Report**
Okay, the analysis is complete, and it's time to craft the final response. My focus will be on clearly articulating the core of this bug: its mechanism, how it's activated, and most importantly, why it's a DoS and not a privilege escalation vulnerability.
Here's the breakdown of my thought process:
First, the bug mechanism. I've pinpointed the crux of the issue within the `__exit_to_user_mode_loop` function. The process is initiated when `TIF_NEED_RESCHED` or `TIF_NEED_RESCHED_LAZY` are active, leading to the enabling of interrupts and a call to `rseq_grant_slice_extension()`. This function, in turn, can trigger `hrtimer_rearm_deferred_tif()` and then `__hrtimer_rearm_deferred()`, if an rseq slice extension is requested. The critical flaw lies in `__hrtimer_rearm_deferred()`'s use of a `guard(raw_spinlock)` to protect `hrtimer_bases.lock`. This lock, while disabling preemption, *leaves interrupts enabled*. This detail is fundamental! If a timer interrupt fires on the *same* CPU while this lock is held, the `hrtimer_interrupt()` handler attempts to acquire the *same* `hrtimer_bases.lock` again using `raw_spin_lock_irqsave()`. The inevitable result is a spinlock deadlock.
Second, the trigger. An unprivileged user can actively provoke this deadlock scenario. The user simply needs to register an `rseq` area, request a slice extension via `usr_ctrl.request = 1`, and then wait. When the kernel returns to userspace, it will traverse the vulnerable code path. The real trick here is that the race window is small, but by repeatedly triggering this, the likelihood of a conflict increases until the lock is held when the timer interrupt fires.
Finally, the impact. This is where I'll be clear and firm. The outcome is a CPU deadlock β a classic soft lockup. While this undeniably results in a Denial of Service (DoS) due to the affected CPU becoming unresponsive, it does *not* lead to any memory corruption, information leakage, or privilege escalation. Itβs important to emphasize that this is *not* an exploitable vulnerability in the sense of allowing unauthorized access or control over the system. It's a resource exhaustion issue limited to a DoS.
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2026/08/02 00:53 |
action |
format-explanation |
0m
Results: map[Explanation:Based on the analysis of the bug report and the kernel source code, here is the
assessment of the vulnerability:
### Bug Mechanism
The bug is a classic spinlock deadlock caused by taking a lock with interrupts
enabled, which is also acquired by an interrupt handler on the same CPU.
1. When a task is returning to user space, `__exit_to_user_mode_loop()` checks
for pending work. If rescheduling is needed (`TIF_NEED_RESCHED`), it enables
interrupts via `local_irq_enable()` and calls `rseq_grant_slice_extension()`.
2. `rseq_grant_slice_extension()` checks if the user-space process requested an
`rseq` time slice extension. If requested, it grants the extension and calls
`hrtimer_rearm_deferred_tif()`.
3. This eventually calls `__hrtimer_rearm_deferred()`, which acquires the local
CPU's `hrtimer_bases.lock` using `guard(raw_spinlock)`. Crucially,
`raw_spinlock` disables preemption but **leaves interrupts enabled**.
4. The `lockdep_assert_irqs_disabled()` warning in
`hrtimer_test_and_clear_rearm_deferred_tif()` correctly flags this precondition
violation.
5. If a timer interrupt fires on the same CPU during the small window while
`hrtimer_bases.lock` is held, the interrupt handler `hrtimer_interrupt()` will
execute and attempt to acquire the exact same lock using
`raw_spin_lock_irqsave()`.
6. The interrupt handler will spin forever waiting for the lock to be released,
but the lock holder was interrupted and cannot make progress, resulting in a
deadlock. The syzkaller report confirms this by showing `Tainted:
[L]=SOFTLOCKUP`.
### Security Impact
* **Exploitable (False):** The bug is a pure deadlock. It does not cause any
memory corruption (like use-after-free or out-of-bounds access), information
leaks, or control-flow hijacking. Therefore, it cannot be exploited for
privilege escalation.
* **Denial of Service (True):** The deadlock causes a CPU soft lockup. If
`panic_on_softlockup` is set, the system will crash; otherwise, the CPU will be
permanently stuck, degrading system performance and eventually leading to a full
system hang.
* **Accessible From Unprivileged Processes (True):** The `rseq` (Restartable
Sequences) system call and its time slice extension feature are available to
unprivileged user-space processes. A malicious unprivileged user can repeatedly
request slice extensions and force preemptions to reliably hit the race window
and trigger the deadlock.
* **Accessible From User Namespaces (True):** Since it requires no privileges,
it is trivially accessible from within user namespaces.
* **Other Triggers (False):** The bug is located in the core kernel entry/exit
and timer code. It is not reachable via network packets, malicious filesystems,
peripheral devices, or from a VM guest (a guest can only crash itself, not the
host).]
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