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Commit 138e4ad6 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
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epoll: fix race between ep_poll_callback(POLLFREE) and ep_free()/ep_remove() 



The race was introduced by me in commit 971316f0 ("epoll:
ep_unregister_pollwait() can use the freed pwq->whead").  I did not
realize that nothing can protect eventpoll after ep_poll_callback() sets
->whead = NULL, only whead->lock can save us from the race with
ep_free() or ep_remove().

Move ->whead = NULL to the end of ep_poll_callback() and add the
necessary barriers.

TODO: cleanup the ewake/EPOLLEXCLUSIVE logic, it was confusing even
before this patch.

Hopefully this explains use-after-free reported by syzcaller:

	BUG: KASAN: use-after-free in debug_spin_lock_before
	...
	 _raw_spin_lock_irqsave+0x4a/0x60 kernel/locking/spinlock.c:159
	 ep_poll_callback+0x29f/0xff0 fs/eventpoll.c:1148

this is spin_lock(eventpoll->lock),

	...
	Freed by task 17774:
	...
	 kfree+0xe8/0x2c0 mm/slub.c:3883
	 ep_free+0x22c/0x2a0 fs/eventpoll.c:865

Fixes: 971316f0 ("epoll: ep_unregister_pollwait() can use the freed pwq->whead")
Reported-by: default avatar范龙飞 <long7573@126.com>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8cf9f2a2

fs/eventpoll.c

+26 −16
Original line number Diff line number Diff line
@@ -600,8 +600,13 @@ static void ep_remove_wait_queue(struct eppoll_entry *pwq) 
	wait_queue_head_t *whead;



	rcu_read_lock();

	/* If it is cleared by POLLFREE, it should be rcu-safe */

	whead = rcu_dereference(pwq->whead);

	/*

	 * If it is cleared by POLLFREE, it should be rcu-safe.

	 * If we read NULL we need a barrier paired with

	 * smp_store_release() in ep_poll_callback(), otherwise

	 * we rely on whead->lock.

	 */

	whead = smp_load_acquire(&pwq->whead);

	if (whead)

		remove_wait_queue(whead, &pwq->wait);

	rcu_read_unlock();
@@ -1134,17 +1139,6 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v 
	struct eventpoll *ep = epi->ep;

	int ewake = 0;



	if ((unsigned long)key & POLLFREE) {

		ep_pwq_from_wait(wait)->whead = NULL;

		/*

		 * whead = NULL above can race with ep_remove_wait_queue()

		 * which can do another remove_wait_queue() after us, so we

		 * can't use __remove_wait_queue(). whead->lock is held by

		 * the caller.

		 */

		list_del_init(&wait->entry);

	}



	spin_lock_irqsave(&ep->lock, flags);



	ep_set_busy_poll_napi_id(epi);
@@ -1228,10 +1222,26 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v 
	if (pwake)

		ep_poll_safewake(&ep->poll_wait);



	if (epi->event.events & EPOLLEXCLUSIVE)

		return ewake;

	if (!(epi->event.events & EPOLLEXCLUSIVE))

		ewake = 1;



	if ((unsigned long)key & POLLFREE) {

		/*

		 * If we race with ep_remove_wait_queue() it can miss

		 * ->whead = NULL and do another remove_wait_queue() after

		 * us, so we can't use __remove_wait_queue().

		 */

		list_del_init(&wait->entry);

		/*

		 * ->whead != NULL protects us from the race with ep_free()

		 * or ep_remove(), ep_remove_wait_queue() takes whead->lock

		 * held by the caller. Once we nullify it, nothing protects

		 * ep/epi or even wait.

		 */

		smp_store_release(&ep_pwq_from_wait(wait)->whead, NULL);

	}



	return 1;

	return ewake;

}



/*