CVE-2026-52977

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

futex: Prevent lockup in requeue-PI during signal/ timeout wakeup

During wait-requeue-pi (task A) and requeue-PI (task B) the following

race can happen:

Task A Task B

futex_wait_requeue_pi()

futex_setup_timer()

futex_do_wait()

futex_requeue()

CLASS(hb, hb1)(&key1);

CLASS(hb, hb2)(&key2);

*timeout*

futex_requeue_pi_wakeup_sync()

requeue_state = Q_REQUEUE_PI_IGNORE

*blocks on hb->lock*

futex_proxy_trylock_atomic()

futex_requeue_pi_prepare()

Q_REQUEUE_PI_IGNORE => -EAGAIN

double_unlock_hb(hb1, hb2)

*retry*

Task B acquires both hb locks and attempts to acquire the PI-lock of the

top most waiter (task B). Task A is leaving early due to a signal/

timeout and started removing itself from the queue. It updates its

requeue_state but can not remove it from the list because this requires

the hb lock which is owned by task B.

Usually task A is able to swoop the lock after task B unlocked it.

However if task B is of higher priority then task A may not be able to

wake up in time and acquire the lock before task B gets it again.

Especially on a UP system where A is never scheduled.

As a result task A blocks on the lock and task B busy loops, trying to

make progress but live locks the system instead. Tragic.

This can be fixed by removing the top most waiter from the list in this

case. This allows task B to grab the next top waiter (if any) in the

next iteration and make progress.

Remove the top most waiter if futex_requeue_pi_prepare() fails.

Let the waiter conditionally remove itself from the list in

handle_early_requeue_pi_wakeup().