CVE-2026-68201 in Linux
Summary
by MITRE • 08/10/2026
In the Linux kernel, the following vulnerability has been resolved:
ALSA: timer: drain a slave's callback before its master detaches it
snd_timer_close_locked() drains the closing instance's own in-flight callback (IFLG_CALLBACK) before freeing it, but not its slaves'. When a master instance is closed, remove_slave_links() clears each slave's ->timer; the slave's own close then reads timer == NULL and takes the branch that skips the drain entirely (snd_timer_stop_slave() also no-ops on a NULL timer). So a slave whose callback is still running when the master is closed is freed underneath the live callback, leading to use-after-free.
Drain the slaves too before remove_slave_links() severs them. snd_timer_stop() has already taken this instance off the active list, so no new slave callback can be queued. Take the slaves off the ack list so a pending one can't fire either, then wait for any that is already in flight.
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Analysis
by VulDB Data Team • 08/11/2026
This vulnerability exists within the Advanced Linux Sound Architecture (ALSA) timer subsystem of the Linux kernel and represents a critical use-after-free condition that could potentially lead to system instability or privilege escalation. The flaw occurs during the cleanup process when a master timer instance is closed, specifically in how the kernel handles the detachment and cleanup of slave timer instances that are linked to it. The vulnerability is categorized under CWE-415 as an improper cleanup of memory resources, where the system attempts to access memory after it has been freed.
The technical implementation flaw stems from the asymmetric handling of timer callback cleanup between master and slave instances within the snd_timer_close_locked() function. When a master timer instance closes, the remove_slave_links() function properly clears each slave's timer pointer but fails to drain any in-flight callbacks that may still be executing on those slave instances. This creates a race condition where slave timer callbacks can continue running even after their associated timer structures have been freed and potentially reallocated by the kernel memory allocator. The function snd_timer_stop_slave() is designed to handle slave cleanup, but it no-ops when timer == NULL, meaning that any callback currently executing on a slave timer cannot be properly terminated before the underlying memory is freed.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable privilege escalation attacks through controlled use-after-free conditions. An attacker who can manipulate timer instances could potentially trigger the race condition by creating master-slave timer relationships and then closing the master while slave callbacks are still executing. This scenario allows for arbitrary code execution in kernel space, as the freed memory structure can be overwritten with attacker-controlled data before being accessed again by the running callback. The vulnerability aligns with ATT&CK technique T1068 which covers local privilege escalation through kernel vulnerabilities, and specifically targets the kernel's memory management subsystem where attackers could exploit the improper cleanup of timer structures.
The fix implemented addresses this issue by ensuring comprehensive cleanup of all slave timer instances before severing their links to the master. The solution requires draining slave callbacks before remove_slave_links() severs them, taking slaves off the acknowledgment list to prevent pending callbacks from firing, and then waiting for any callbacks that are already in flight to complete. This approach follows proper resource management principles by ensuring that all references to a resource are properly terminated before the resource itself is freed, preventing the use-after-free condition. The fix maintains the existing kernel timer behavior while adding the necessary synchronization to prevent race conditions during cleanup operations and ensures that all timer callbacks are properly terminated before memory deallocation occurs.
This vulnerability demonstrates the complexity of managing reference counting and cleanup in kernel subsystems where objects can have multiple dependencies and references. The ALSA timer subsystem must maintain proper synchronization between master and slave timer instances, as the closure of one instance affects the lifecycle of others within the same hierarchical structure. The solution implements a proper drain sequence that ensures no callbacks can execute on freed memory, which is essential for maintaining kernel stability and preventing potential security exploits in environments where unprivileged users might be able to create and manipulate timer instances through the ALSA interface.