Commit 971316f0 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

epoll: ep_unregister_pollwait() can use the freed pwq->whead 



signalfd_cleanup() ensures that ->signalfd_wqh is not used, but
this is not enough. eppoll_entry->whead still points to the memory
we are going to free, ep_unregister_pollwait()->remove_wait_queue()
is obviously unsafe.

Change ep_poll_callback(POLLFREE) to set eppoll_entry->whead = NULL,
change ep_unregister_pollwait() to check pwq->whead != NULL under
rcu_read_lock() before remove_wait_queue(). We add the new helper,
ep_remove_wait_queue(), for this.

This works because sighand_cachep is SLAB_DESTROY_BY_RCU and because
->signalfd_wqh is initialized in sighand_ctor(), not in copy_sighand.
ep_unregister_pollwait()->remove_wait_queue() can play with already
freed and potentially reused ->sighand, but this is fine. This memory
must have the valid ->signalfd_wqh until rcu_read_unlock().

Reported-by: default avatarMaxime Bizon <mbizon@freebox.fr>
Cc: <stable@kernel.org>
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent d80e731e

fs/eventpoll.c

+27 −3
Original line number Diff line number Diff line
@@ -320,6 +320,11 @@ static inline int ep_is_linked(struct list_head *p) 
	return !list_empty(p);

}



static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_t *p)

{

	return container_of(p, struct eppoll_entry, wait);

}



/* Get the "struct epitem" from a wait queue pointer */

static inline struct epitem *ep_item_from_wait(wait_queue_t *p)

{
@@ -467,6 +472,18 @@ static void ep_poll_safewake(wait_queue_head_t *wq) 
	put_cpu();

}



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 (whead)

		remove_wait_queue(whead, &pwq->wait);

	rcu_read_unlock();

}



/*

 * This function unregisters poll callbacks from the associated file

 * descriptor.  Must be called with "mtx" held (or "epmutex" if called from
@@ -481,7 +498,7 @@ static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi) 
		pwq = list_first_entry(lsthead, struct eppoll_entry, llink);



		list_del(&pwq->llink);

		remove_wait_queue(pwq->whead, &pwq->wait);

		ep_remove_wait_queue(pwq);

		kmem_cache_free(pwq_cache, pwq);

	}

}
@@ -842,9 +859,16 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k 
	struct epitem *epi = ep_item_from_wait(wait);

	struct eventpoll *ep = epi->ep;



	/* the caller holds eppoll_entry->whead->lock */

	if ((unsigned long)key & POLLFREE)

	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->task_list);

	}



	spin_lock_irqsave(&ep->lock, flags);

fs/signalfd.c

+5 −1
Original line number Diff line number Diff line
@@ -33,7 +33,11 @@ 
void signalfd_cleanup(struct sighand_struct *sighand)

{

	wait_queue_head_t *wqh = &sighand->signalfd_wqh;



	/*

	 * The lockless check can race with remove_wait_queue() in progress,

	 * but in this case its caller should run under rcu_read_lock() and

	 * sighand_cachep is SLAB_DESTROY_BY_RCU, we can safely return.

	 */

	if (likely(!waitqueue_active(wqh)))

		return;

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