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Date: Mon, 06 Nov 2017 23:03:07 +0000
From: Ben Hutchings <ben@...adent.org.uk>
To: linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC: akpm@...ux-foundation.org,
"范龙飞" <long7573@....com>,
"Linus Torvalds" <torvalds@...ux-foundation.org>,
"Oleg Nesterov" <oleg@...hat.com>
Subject: [PATCH 3.2 077/147] epoll: fix race between ep_poll_callback(POLLFREE)
and ep_free()/ep_remove()
3.2.95-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Oleg Nesterov <oleg@...hat.com>
commit 138e4ad67afd5c6c318b056b4d17c17f2c0ca5c0 upstream.
The race was introduced by me in commit 971316f0503a ("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: 971316f0503a ("epoll: ep_unregister_pollwait() can use the freed pwq->whead")
Reported-by: 范龙飞 <long7573@....com>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2:
- Use smp_mb() and ACCESS_ONCE() instead of smp_{load_acquire,store_release}()
- EPOLLEXCLUSIVE is not supported]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -477,8 +477,14 @@ static void ep_remove_wait_queue(struct
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 = ACCESS_ONCE(pwq->whead);
+ smp_mb();
if (whead)
remove_wait_queue(whead, &pwq->wait);
rcu_read_unlock();
@@ -859,17 +865,6 @@ static int ep_poll_callback(wait_queue_t
struct epitem *epi = ep_item_from_wait(wait);
struct eventpoll *ep = epi->ep;
- 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);
/*
@@ -924,6 +919,23 @@ out_unlock:
if (pwake)
ep_poll_safewake(&ep->poll_wait);
+ 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->task_list);
+ /*
+ * ->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_mb();
+ ACCESS_ONCE(ep_pwq_from_wait(wait)->whead) = NULL;
+ }
+
return 1;
}
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