CVE-2026-89517

In the Linux kernel, the following vulnerability has been resolved:

sched ext: Fix rq->core pick corruption under core scheduling

Core scheduling's pick next task() picks what to run on every SMT sibling of the core in a single pass under the shared core-wide rq lock. The selection state is consistent only while the lock is held continuously, so ->pick task() originally could not release it. However, since 4c95380701f5 ("sched/ext: Fold balance scx() into pick task scx()"), sched ext runs dispatch from inside the pick and dispatching can drop the rq lock. To support this, pick next task() has been updated to restart the whole selection when a pick returns RETRY TASK after releasing the lock.

When selections on the same core interleave through the dropped lock, they corrupt each other's state: one clears the other's rq->core pick leading to a NULL deref, or invalidates its keep-the-previous-task decision leaving a dequeued task running, which deadlocks the next wakeup and matches the reported hard hangs. A cookied ping-pong load on an SMT machine makes the interleavings frequent and kills the kernel within seconds.

Fix it by making the pick return RETRY TASK whenever dispatch released the rq lock, so that a selection only ever commits picks made under a continuously held lock. The previous patch's rq->scx.lock drop seq counts the releases. A dispatch that touched nothing never releases the lock and its verdict, including "nothing to run", stands: retries are bounded, each following a dispatch that actually did something, and an idle CPU does not loop.

If another dispatch is already in flight on the rq, skip dispatching and pick from what is already queued locally - the in-flight dispatch has released the lock, so its own selection will retry and re-pick this rq, while returning RETRY TASK here would only spin on the lock that dispatch needs to finish.

Balance callbacks must run in the context that queued them, so they can only be queued on the CPU's own rq. When dispatching for another rq, run the deferred work directly instead - that rq may consume all its picks through the core-sched fast path and never queue the callback itself.

The put prev task scx() warning about a runnable task being left behind assumed that dispatch ran as part of the very pick that is switching away. That now only holds on the non-core path, so gate it and drop the cookie-match test, which is always true without core scheduling, from its condition.