CVE-2025-21897 in Linux
Summary
by MITRE • 04/01/2025
In the Linux kernel, the following vulnerability has been resolved:
sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance()
a6250aa251ea ("sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx()") added a workaround to handle the cases where pick_task_scx() is called without prececing balance_scx() which is due to a fair class bug where pick_taks_fair() may return NULL after a true return from balance_fair().
The workaround detects when pick_task_scx() is called without preceding balance_scx() and emulates SCX_RQ_BAL_KEEP and triggers kicking to avoid stalling. Unfortunately, the workaround code was testing whether @prev was on SCX to decide whether to keep the task running. This is incorrect as the task may be on SCX but no longer runnable.
This could lead to a non-runnable task to be returned from pick_task_scx() which cause interesting confusions and failures. e.g. A common failure mode is the task ending up with (!on_rq && on_cpu) state which can cause potential wakers to busy loop, which can easily lead to deadlocks.
Fix it by testing whether @prev has SCX_TASK_QUEUED set. This makes @prev_on_scx only used in one place. Open code the usage and improve the comment while at it.
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Analysis
by VulDB Data Team • 02/01/2026
The vulnerability CVE-2025-21897 resides within the Linux kernel's scheduler extension subsystem, specifically affecting the sched_ext module's task selection mechanism. This issue manifests when the pick_task_scx() function is invoked without a preceding balance_scx() call, a scenario that arises from a deeper bug in the fair scheduling class where pick_task_fair() may return NULL following a successful balance_fair() operation. The original fix implemented a workaround that detected when pick_task_scx() was called without balance_scx() and attempted to emulate SCX_RQ_BAL_KEEP behavior while triggering task kicking to prevent system stalls. However, this workaround contained a critical logical flaw in its implementation.
The technical flaw stems from an incorrect conditional test that evaluated whether the @prev task was on SCX to determine if the task should remain running, rather than properly checking if the task was actually queued for execution. This improper validation means that tasks which are no longer runnable but still appear to be on SCX could be incorrectly selected by pick_task_scx(), leading to a fundamental violation of scheduling state consistency. The vulnerability creates a scenario where non-runnable tasks are returned from the scheduler, causing tasks to enter inconsistent states such as (!on_rq && on_cpu) which represents a critical condition where a task appears to be running on a CPU but is not actually queued on any runqueue.
The operational impact of this vulnerability extends beyond simple scheduling confusion to potentially cause severe system instability including deadlock conditions. When tasks enter the problematic (!on_rq && on_cpu) state, potential wakers may enter busy loop patterns attempting to manage the inconsistent scheduling state, leading to resource exhaustion and system hangs. This behavior directly violates the fundamental scheduling principles defined in the Linux kernel's scheduler design and can result in complete system lockups under certain workloads. The vulnerability affects systems utilizing the sched_ext framework and demonstrates a failure in proper state management within the scheduler's task selection logic.
The fix addresses this issue by implementing a more accurate test that checks whether the prev task has the SCX_TASK_QUEUED flag set, which properly indicates whether a task is actually queued for execution rather than merely being associated with the SCX subsystem. This change eliminates the flawed logic that was testing for SCX presence instead of actual queue status, thereby ensuring that only properly queued tasks are returned from pick_task_scx(). The solution also includes code refactoring that removes the indirect usage of prev_on_scx by opening up the conditional logic and improving the accompanying documentation. This fix aligns with security best practices for kernel scheduling components and addresses the underlying CWE category related to improper handling of task states in concurrent systems. The remediation ensures proper adherence to the Linux kernel's scheduler design principles and prevents the dangerous state transitions that could lead to system instability and potential denial of service conditions.
This vulnerability demonstrates the critical importance of proper state validation in kernel-level scheduling components and highlights the potential for seemingly minor logic flaws to cascade into severe system stability issues. The fix represents a targeted correction that maintains the intended workaround functionality while eliminating the fundamental logical error that caused the incorrect task selection behavior. The solution exemplifies proper kernel security practices by ensuring that task state transitions are properly validated before being exposed to scheduling consumers, thereby preventing the propagation of inconsistent scheduling states throughout the kernel's execution environment.