CVE-2026-74686 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
rqspinlock: Reset tail when preserving queue on deadlock
Currently, the destruction of the waiter queue is suppressed for rqspinlock in cases where a deadlock is detected. Deadlock checks happen relatively frequently (on entry for AA, within 1ms for ABBA), and waiter threads may not be involved in locking scenarios involving deadlocks. Thus, it is useful to not flush the queue and let other waiters take a stab at acquiring the lock after we detect a deadlock and exit.
However, we need to follow the same logic as what we did previously for the waitq_timeout label: reset the tail, and if we cannot, signal the next waiter appropriately. In case of deadlocks, this signal would just mark the MCS node as unlocked, and in case of timeouts, it would signal RES_TIMEOUT_VAL. The difference thus is in the value propagated, which decides whether the queue remains active or gets flushed.
Not doing the tail reset, and waiting for the next waiter can lead to cases where we are the final waiter, and thus no next waiter arrives, leading to intermittent stalls in this path. Once the next waiter does join, we will be unblocked. In the theoretical case when the next waiter never joins, we risk stalling indefinitely.
This can only happen for ABBA deadlocks, since entry into the wait queue is guarded with AA checks. A precise sequence of executions leading up to this scenario can be:
CPU 0 holds lock A. CPU 1 holds lock B. CPU 2 attempts lock B, becomes the pending waiter for B. CPU 0 attempts lock B. B has locked+pending bits set, thus CPU 0 queues. CPU 1 attempts lock A. CPU 0 detects an ABBA deadlock.
Once deadlock detection happens for CPU 0, it will sit waiting for the next waiter in the queue to populate node->next, which will experience delays until such a waiter arrives.
Fix this by adjusting the logic for the check for deadlocks preceding the waitq_timeout label. It would make sense to consolidate code for both cases and use 'ret' to distinguish the value being propagated, but that is left as an exercise for a future refactoring task to avoid diff noise in this patch.
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Analysis
by VulDB Data Team • 08/22/2026
The Linux kernel's rqspinlock implementation addresses a critical concurrency flaw related to deadlock detection and queue management within its MCS-based locking mechanism. The vulnerability stems from the handling of waiter queues when a deadlock is detected, specifically during ABBA lock ordering violations. Under normal operation, if a thread detects that it has entered a circular wait condition involving two locks held by different CPUs, it must release resources and exit to allow other threads to proceed. However, prior to this fix, the code suppressed the destruction of the waiter queue in these deadlock scenarios. The rationale behind suppressing queue destruction was to preserve state for potential retry attempts, but this logic failed to account for a specific edge case involving tail pointer management that leads to indefinite stalls.
The technical flaw lies in the failure to reset the lock's tail field when preserving the queue upon detecting a deadlock. In an MCS spinlock, threads wait by spinning on their node's next pointer until another thread sets it. When CPU 0 detects an ABBA deadlock after queuing for lock B while holding lock A, and subsequently exits without properly resetting the tail or signaling the next waiter, it leaves the queue in an inconsistent state. Specifically, if CPU 0 is the final waiter in the queue at that moment, no subsequent thread will arrive to populate its node's next pointer. Consequently, CPU 0 remains blocked indefinitely waiting for a signal that will never come because the deadlock detection logic exited without triggering the necessary wake-up mechanism or tail reset required to unblock pending waiters.
This issue is strictly limited to ABBA deadlocks due to the specific entry guards in place for AA-style lock acquisitions. The problematic sequence involves CPU 0 holding lock A and attempting to acquire lock B, while CPU 1 holds lock B and attempts to acquire lock A. When CPU 2 also tries to acquire lock B, it becomes a pending waiter. As CPU 0 queues behind CPU 2 after detecting the contention on lock B, and then detects the ABBA deadlock with CPU 1's attempt on lock A, the system enters the flawed code path. Because the tail is not reset and no signal is sent to mark the MCS node as unlocked or propagate a timeout value, any thread waiting in that queue for CPU 0 to proceed will hang indefinitely if no new waiter joins before CPU 0 exits.
The operational impact of this vulnerability includes intermittent system stalls and potential indefinite hangs within kernel space, particularly under high contention scenarios where ABBA deadlocks are detected but the subsequent cleanup logic is flawed. These stalls can degrade overall system performance or cause service unavailability depending on which subsystems rely on these locks for critical operations. The risk is exacerbated in multi-core environments where lock contention is frequent, as the probability of encountering this specific race condition increases with the number of concurrent threads attempting to acquire multiple resources in varying orders.
To mitigate this vulnerability, the kernel developers implemented a fix that aligns the deadlock detection path with the logic used for wait queue timeouts. The solution involves resetting the tail pointer and ensuring appropriate signaling to the next waiter in line. If the system cannot reset the tail immediately, it signals the next waiter appropriately by marking the MCS node as unlocked or propagating specific values like RES_TIMEOUT_VAL depending on the context. This ensures that even if a deadlock is detected and the current thread exits, any pending waiters are properly unblocked rather than left in an indefinite waiting state.
From a security and standards perspective, this vulnerability aligns with CWE-835: Loop with Unreachable Exit Condition, as the code enters a loop where the exit condition may never be met due to missing signal propagation. It also relates to CWE-674: Uncontrolled Recursion in Loops or Recursive Functions if viewed through the lens of resource exhaustion via thread blocking. In terms of MITRE ATT&CK, this falls under T1053 Scheduled Task/Job which is not directly applicable but rather maps more closely to system availability impacts described in general software fault categories. However, for classification purposes, it represents a logic error leading to denial-of-service conditions within the kernel's synchronization primitives.
The fix emphasizes careful state management during exception handling paths in concurrent programming. It highlights the importance of ensuring that all waiters are notified when a thread responsible for advancing the queue exits unexpectedly or abnormally. Future refactoring efforts may consolidate this logic with timeout handling code to reduce complexity and prevent similar oversights, but the immediate resolution requires strict adherence to resetting tail pointers and signaling next waiters in deadlock scenarios. This ensures robustness against race conditions that could otherwise lead to persistent kernel hangs affecting system stability.