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().