CVE-2026-68200 in Linux
Summary
by MITRE • 08/10/2026
In the Linux kernel, the following vulnerability has been resolved:
ALSA: timer: don't re-enter an instance callback that is still running
The userspace-driven timer (utimer) TRIGGER ioctl calls snd_timer_interrupt() directly with no serialization, so two threads triggering the same utimer can run snd_timer_interrupt() on one snd_timer concurrently.
snd_timer_process_callbacks() drops timer->lock around each instance callback and marks the in-flight callback with the single SNDRV_TIMER_IFLG_CALLBACK bit; snd_timer_close_locked() waits on that bit to drain an in-flight callback before freeing the instance. The bit cannot represent two concurrent callbacks: when a second interrupt re-queues an instance whose callback is still running, both run at once, the first to finish clears the bit, and the close-path drain then frees the instance (and its callback_data) while the other callback is still live - a use-after-free reachable by any user able to open /dev/snd/timer, both via a user timer instance and via a sequencer queue timer bound to the utimer.
snd_timer_interrupt() sets IFLG_CALLBACK before dropping timer->lock, so a concurrent interrupt already observes it under the lock. Skip re-queuing an instance (and its slaves) to the ack/sack list while its callback is in flight; the accumulated pticks are delivered on the next tick, so no event is lost.
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Analysis
by VulDB Data Team • 08/10/2026
The vulnerability identified in the Linux kernel's Advanced Linux Sound Architecture (ALSA) subsystem represents a critical race condition affecting the timer module implementation. This flaw specifically impacts the userspace-driven timer (utimer) functionality where multiple concurrent threads attempting to trigger the same timer instance can cause simultaneous execution of timer callbacks, leading to potential system instability and security risks. The issue stems from insufficient serialization mechanisms within the ALSA timer subsystem, creating a scenario where concurrent access patterns bypass normal synchronization protocols.
The technical root cause lies in how the snd_timer_interrupt() function handles concurrent invocations without proper locking mechanisms. When userspace applications invoke the TRIGGER ioctl on a utimer instance, the system directly calls snd_timer_interrupt() without adequate serialization measures. This design flaw allows multiple threads to execute the timer interrupt handler simultaneously on the same timer instance, creating a scenario where two separate callback executions can occur concurrently within the same timer context.
The vulnerability manifests through the improper handling of the SNDRV_TIMER_IFLG_CALLBACK flag which serves as a marker for in-flight callbacks. While the system does implement basic callback tracking by setting this flag before dropping the timer lock, the design fails to account for multiple concurrent callback scenarios. The snd_timer_process_callbacks() function temporarily releases the timer->lock during individual instance callback execution while marking the running callback with the single callback bit flag. However, this approach cannot effectively track multiple simultaneous callbacks from different interrupt sources.
The operational impact of this vulnerability extends beyond simple system instability into potential security implications. When snd_timer_close_locked() attempts to drain in-flight callbacks before freeing memory resources, it relies on the callback flag to determine completion status. If a second interrupt occurs while the first callback is still executing, both callbacks can run concurrently, with the first to finish clearing the flag and allowing resource deallocation. This creates a use-after-free condition where callback data structures may be freed while still referenced by an active callback execution, potentially enabling arbitrary code execution or privilege escalation.
The exploitation pathway for this vulnerability requires an attacker to have access to the /dev/snd/timer device and be able to open both a user timer instance and a sequencer queue timer bound to the utimer. This access pattern allows the attacker to trigger the race condition through concurrent thread operations, leveraging the lack of proper serialization between different interrupt sources targeting the same timer instance. The vulnerability affects systems utilizing ALSA audio subsystems and represents a significant concern for embedded systems or servers where multiple processes might concurrently access audio timer functionality.
The fix implemented addresses the core synchronization issue by preventing re-queuing of timer instances whose callbacks are already in flight. When a timer interrupt occurs while another callback for the same instance is still executing, the system now skips re-queuing operations to prevent concurrent execution scenarios. This approach ensures that accumulated timer ticks are delivered on subsequent timer events rather than being lost, maintaining proper timer functionality while preventing the race condition. The solution aligns with established security practices for concurrent access control and resource management, following principles similar to those outlined in CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) and addresses patterns described in ATT&CK technique T1059 (Command and Scripting Interpreter) where improper synchronization can lead to privilege escalation scenarios.
This vulnerability demonstrates the critical importance of proper locking mechanisms in kernel subsystems, particularly when dealing with shared resources that may be accessed from multiple concurrent contexts. The design flaw highlights how seemingly simple operations can create complex concurrency issues when proper serialization is not implemented, affecting both system stability and security posture. The solution maintains backward compatibility while strengthening the timer subsystem's resilience against race conditions that could otherwise lead to system crashes or more severe exploitation outcomes.