CVE-2026-74594 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
sched/psi: Shut down rtpoll_timer in psi_cgroup_free()
psi_schedule_rtpoll_work() is called locklessly from the scheduler hotpath and can race psi_trigger_destroy() taking down the last rtpoll trigger under rtpoll_trigger_lock:
psi_schedule_rtpoll_work() psi_trigger_destroy()
rcu_read_lock(); task = rcu_dereference(rtpoll_task); rcu_assign_pointer(rtpoll_task, NULL); timer_delete(&rtpoll_timer); mod_timer(&rtpoll_timer, ...); rcu_read_unlock(); synchronize_rcu(); kthread_stop(task_to_destroy);
The group can then be freed with the re-armed timer still pending, and poll_timer_fn() runs on freed memory.
461daba06bdc ("psi: eliminate kthread_worker from psi trigger scheduling mechanism") deleted the timer synchronously after the synchronize_rcu(), which prevented this but raced trigger creation instead: the deletion could cancel the timer that a new trigger set armed during the grace period and, as creation also reinitialized the timer at the time, corrupt it. 8f91efd870ea ("psi: Fix race between psi_trigger_create/destroy") moved the initialization into group_init() and the deletion into the locked section, trading the creation races for the window above.
Neither placement in the destruction path works. A pending timer firing while the group is alive is harmless though. poll_timer_fn() just wakes the rtpoll waitqueue and doesn't re-arm itself. Bind the timer to the group's lifetime instead and shut it down in psi_cgroup_free(). Nothing can arm it by then. timer_shutdown_sync() because the timer is never armed again.
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Analysis
by VulDB Data Team • 08/22/2026
The Linux kernel subsystem responsible for Pressure Stall Information, specifically within the scheduling policy interface, contains a critical race condition involving the lifecycle management of polling timers and associated kernel threads. This vulnerability arises from an improper synchronization mechanism when handling the destruction of PSI triggers versus their creation in the scheduler hotpath. The core issue stems from psi_schedule_rtpoll_work being invoked locklessly during normal CPU scheduling operations while simultaneously, another execution path may be destroying a trigger via psi_trigger_destroy under rtpoll_trigger_lock. This concurrency creates a window where the timer can be re-armed or accessed after its associated data structures have been freed, leading to use-after-free conditions that compromise system stability and security.
The technical flaw is rooted in the asynchronous nature of RCU grace periods combined with kernel timer management. When psi_schedule_rtpoll_work executes under an RCU read lock, it dereferences rtpoll_task and potentially modifies or schedules rtpoll_timer. Concurrently, psi_trigger_destroy sets rtpoll_task to NULL via rcu_assign_pointer, deletes the timer using timer_delete, waits for the RCU grace period with synchronize_rcu, and then stops the associated kernel thread. However, if a new trigger is created during this window, it may reinitialize or arm the same timer instance before the old one is fully cleaned up. Previous attempts to fix this by moving initialization into group_init and deletion into locked sections failed because they either canceled timers belonging to newly created triggers or left pending timers active after their owning cgroup was freed. The fundamental error is that the timer's lifetime was not strictly bound to the lifecycle of its parent cgroup structure, allowing it to fire on memory that has already been returned to the allocator.
The operational impact of this vulnerability includes kernel panics, system crashes, and potential arbitrary code execution if an attacker can exploit the use-after-free condition in poll_timer_fn. Since poll_timer_fn wakes a waitqueue without rearming itself, a single race might not always cause immediate corruption, but repeated races or specific timing conditions can lead to memory corruption of critical kernel structures. This affects system reliability, particularly on systems with high CPU contention and frequent PSI trigger creation and destruction cycles. The vulnerability undermines the integrity of resource monitoring mechanisms, which are often used by container orchestration tools and security auditing frameworks, potentially leading to denial-of-service or privilege escalation if exploited in conjunction with other vulnerabilities.
To mitigate this risk, the fix involves binding the rtpoll_timer strictly to the lifetime of the cgroup structure. The timer is now shut down synchronously within psi_cgroup_free using timer_shutdown_sync, ensuring that no pending timers remain active after the group's memory is freed. This approach eliminates the race window by guaranteeing that once a cgroup begins its free process, any associated polling mechanisms are immediately and safely disabled before any RCU grace periods or thread stops occur. Administrators should apply kernel updates that include this patch to prevent potential stability issues and security exploits related to PSI timer management.
This vulnerability aligns with CWE-416 Use After Free, as the core issue involves accessing memory after it has been freed due to improper synchronization of resource lifecycles. It also relates to CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, specifically regarding race conditions between lockless scheduler paths and locked destruction routines. In terms of MITRE ATT&CK, this could be leveraged in techniques associated with Defense Evasion or Privilege Escalation if an attacker can trigger the race condition to crash security monitoring services or corrupt kernel memory for code execution. The fix emphasizes the importance of strict lifecycle management for asynchronous resources like timers and kthreads within high-performance kernel subsystems.