Commit b5491137 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Thomas Gleixner
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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.

Fixes: 07d91ef5 ("futex: Prevent requeue_pi() lock nesting issue on RT")
Reported-by: default avatar <syzbot+034246a838a10d181e78@syzkaller.appspotmail.com>
Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Closes: https://lore.kernel.org/all/68b75989.050a0220.3db4df.01dd.GAE@google.com/
parent f83ec76b

kernel/futex/requeue.c

+4 −2
Original line number Diff line number Diff line
@@ -230,8 +230,9 @@ static inline 
void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,

			   struct futex_hash_bucket *hb)

{

	q->key = *key;

	struct task_struct *task;



	q->key = *key;

	__futex_unqueue(q);



	WARN_ON(!q->rt_waiter);
@@ -243,10 +244,11 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key, 
	futex_hash_get(hb);

	q->drop_hb_ref = true;

	q->lock_ptr = &hb->lock;

	task = READ_ONCE(q->task);



	/* Signal locked state to the waiter */

	futex_requeue_pi_complete(q, 1);

	wake_up_state(q->task, TASK_NORMAL);

	wake_up_state(task, TASK_NORMAL);

}



/**