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.