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author | Oleg Nesterov <oleg@redhat.com> | 2017-09-01 18:55:33 +0200 |
---|---|---|
committer | Linus Torvalds <torvalds@linux-foundation.org> | 2017-09-01 13:07:35 -0700 |
commit | 138e4ad67afd5c6c318b056b4d17c17f2c0ca5c0 (patch) | |
tree | 2d214e233ed80eb887d8a81e11be2613c37d9874 | |
parent | 8cf9f2a29ff1265a392e5b2461c69a9e53b4539f (diff) | |
download | lwn-138e4ad67afd5c6c318b056b4d17c17f2c0ca5c0.tar.gz lwn-138e4ad67afd5c6c318b056b4d17c17f2c0ca5c0.zip |
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>
-rw-r--r-- | fs/eventpoll.c | 42 |
1 files changed, 26 insertions, 16 deletions
diff --git a/fs/eventpoll.c b/fs/eventpoll.c index e767e4389cb1..adbe328b957c 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -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 @@ out_unlock: 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; } /* |