In the Linux kernel, the following vulnerability has been resolved: futex: Prevent use-after...

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

futex: Prevent use-after-free during requeue-PI

syzbot managed to trigger the following race:

T1 T2

futex_wait_requeue_pi()
futex_do_wait()
schedule()
futex_requeue()
futex_proxy_trylock_atomic()
futex_requeue_pi_prepare()
requeue_pi_wake_futex()
futex_requeue_pi_complete()
/ preempt /

     * timeout/ signal wakes T1 *

futex_requeue_pi_wakeup_sync() // Q_REQUEUE_PI_LOCKED
futex_hash_put()
// back to userland, on stack futex_q is garbage

                                  /* back */
                                 wake_up_state(q->task, TASK_NORMAL);

In this scenario futex_wait_requeue_pi() is able to leave without using
futex_q::lock_ptr for synchronization.

This can be prevented by reading futex_q::task before updating the
futex_q::requeue_state. A reference on the task_struct is not needed
because requeue_pi_wake_futex() is invoked with a spinlock_t held which
implies a RCU read section.

Even if T1 terminates immediately after, the task_struct will remain valid
during T2's wake_up_state(). A READ_ONCE on futex_q::task before
futex_requeue_pi_complete() is enough because it ensures that the variable
is read before the state is updated.

Read futex_q::task before updating the requeue state, use it for the
following wakeup.