epoll: fix race between ep_poll_callback(POLLFREE) and ep_free()/ep_remove()
authorOleg Nesterov <oleg@redhat.com>
Fri, 1 Sep 2017 16:55:33 +0000 (18:55 +0200)
committerLinus Torvalds <torvalds@linux-foundation.org>
Fri, 1 Sep 2017 20:07:35 +0000 (13:07 -0700)
commit138e4ad67afd5c6c318b056b4d17c17f2c0ca5c0
tree2d214e233ed80eb887d8a81e11be2613c37d9874
parent8cf9f2a29ff1265a392e5b2461c69a9e53b4539f
epoll: fix race between ep_poll_callback(POLLFREE) and ep_free()/ep_remove()

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@126.com>
Cc: stable@vger.kernel.org
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
fs/eventpoll.c