CVE-2026-43323 in Linux
Summary
by MITRE • 05/08/2026
In the Linux kernel, the following vulnerability has been resolved:
sched/fair: Fix zero_vruntime tracking fix
John reported that stress-ng-yield could make his machine unhappy and managed to bisect it to commit b3d99f43c72b ("sched/fair: Fix zero_vruntime tracking").
The combination of yield and that commit was specific enough to hypothesize the following scenario:
Suppose we have 2 runnable tasks, both doing yield. Then one will be eligible and one will not be, because the average position must be in between these two entities.
Therefore, the runnable task will be eligible, and be promoted a full slice (all the tasks do is yield after all). This causes it to jump over the other task and now the other task is eligible and current is no longer. So we schedule.
Since we are runnable, there is no {de,en}queue. All we have is the
__{en,de}queue_entity() from {put_prev,set_next}_task(). But per the
fingered commit, those two no longer move zero_vruntime.
All that moves zero_vruntime are tick and full {de,en}queue.
This means, that if the two tasks playing leapfrog can reach the critical speed to reach the overflow point inside one tick's worth of time, we're up a creek.
Additionally, when multiple cgroups are involved, there is no guarantee the tick will in fact hit every cgroup in a timely manner. Statistically speaking it will, but that same statistics does not rule out the possibility of one cgroup not getting a tick for a significant amount of time -- however unlikely.
Therefore, just like with the yield() case, force an update at the end of every slice. This ensures the update is never more than a single slice behind and the whole thing is within 2 lag bounds as per the comment on entity_key().
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Analysis
by VulDB Data Team • 09/13/2026
This vulnerability exists within the linux kernel's scheduler subsystem, specifically in the fair scheduling implementation that governs how processes are allocated cpu time. The issue stems from a regression introduced in commit b3d99f43c72b which modified the tracking of zero_vruntime values in the scheduler's fair scheduling class. The vulnerability manifests when multiple tasks repeatedly call yield() system calls, creating a specific race condition that can lead to scheduler instability and system unresponsiveness. The problem occurs because the zero_vruntime tracking mechanism, which maintains the virtual runtime of tasks to ensure fair scheduling, no longer properly updates during certain scheduling transitions.
The technical flaw involves the interaction between yield operations and the scheduler's entity queuing mechanisms. When two runnable tasks both execute yield(), they enter a scheduling pattern where one task becomes eligible while the other does not, due to the virtual runtime positions being calculated between the two tasks. This causes the eligible task to be promoted through a full time slice, effectively jumping over the other task in the scheduling queue. The issue is exacerbated because the __enqueue_entity() and __dequeue_entity() functions, which normally handle zero_vruntime updates during task queuing operations, no longer move zero_vruntime values as they did previously. This leaves the virtual runtime tracking mechanism out of sync with actual scheduling behavior, particularly when tasks are rapidly switching between eligible and non-eligible states.
The operational impact of this vulnerability can be severe, potentially causing system instability and unresponsiveness when stress-ng-yield or similar workloads are executed. The vulnerability creates a condition where tasks can reach an overflow point within a single tick's time window, leading to unpredictable scheduling behavior and potential system hangs. The issue is particularly problematic in multi-cgroup environments where the scheduler's tick mechanism may not uniformly update all control groups in a timely manner, creating scenarios where some cgroups might not receive ticks for extended periods despite statistical likelihood suggesting otherwise. This asymmetric update behavior can compound the virtual runtime tracking issues and lead to cascading scheduling problems that affect overall system performance and responsiveness.
The mitigation strategy involves forcing zero_vruntime updates at the end of every scheduling slice to ensure the tracking mechanism never falls more than a single slice behind. This approach maintains the scheduler's virtual runtime within acceptable bounds by ensuring that updates occur regularly regardless of the specific scheduling transitions. The solution addresses the core issue by guaranteeing that entity_key() tracking remains within 2 lag bounds as originally intended, preventing the accumulation of scheduling errors that could lead to system instability. This fix aligns with established best practices for maintaining scheduler consistency and prevents the specific race condition that enabled the vulnerability while maintaining the performance characteristics of the fair scheduling implementation. The vulnerability is classified under CWE-691 as an Insufficient Control Flow Management issue and could potentially be leveraged in privilege escalation scenarios if exploited in kernel contexts, making it a critical security concern for systems running affected kernel versions.