CVE-2025-21897

Description

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.

Affected products

Version statuses are supplied by the assigning organization. Consult the original record for detailed ranges and platform conditions.

References

• https://git.kernel.org/stable/c/5324c459f90d16b0c43a78b494c598915d782b7a ↗
• https://git.kernel.org/stable/c/de60a31cb0bcacfaf9487546eac5e70e0a9c66d7 ↗
• https://git.kernel.org/stable/c/8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5 ↗