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From:   Soheil Hassas Yeganeh <soheil.kdev@gmail.com>
To:     viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        dave@stgolabs.net, Soheil Hassas Yeganeh <soheil@google.com>,
        Guantao Liu <guantaol@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Willem de Bruijn <willemb@google.com>,
        Khazhismel Kumykov <khazhy@google.com>
Subject: [PATCH 1/2] epoll: check ep_events_available() upon timeout
Date:   Wed, 28 Oct 2020 14:02:01 -0400
Message-Id: <20201028180202.952079-1-soheil.kdev@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
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From: Soheil Hassas Yeganeh <soheil@google.com>

After abc610e01c66, we break out of the ep_poll loop upon timeout,
without checking whether there is any new events available.  Prior to
that patch-series we always called ep_events_available() after
exiting the loop.

This can cause races and missed wakeups. For example, consider
the following scenario reported by Guantao Liu:

Suppose we have an eventfd added using EPOLLET to an epollfd.

Thread 1: Sleeps for just below 5ms and then writes to an eventfd.
Thread 2: Calls epoll_wait with a timeout of 5 ms. If it sees an
          event of the eventfd, it will write back on that fd.
Thread 3: Calls epoll_wait with a negative timeout.

Prior to abc610e01c66, it is guaranteed that Thread 3 will wake up
either by Thread 1 or Thread 2.  After abc610e01c66, Thread 3 can
be blocked indefinitely if Thread 2 sees a timeout right before
the write to the eventfd by Thread 1. Thread 2 will be woken up from
schedule_hrtimeout_range and, with evail 0, it will not call
ep_send_events().

To fix this issue, while holding the lock, try to remove the thread that
timed out the wait queue and check whether it was woken up or not.

Fixes: abc610e01c66 ("fs/epoll: avoid barrier after an epoll_wait(2) timeout")
Reported-by: Guantao Liu <guantaol@google.com>
Tested-by: Guantao Liu <guantaol@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Khazhismel Kumykov <khazhy@google.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
---
 fs/eventpoll.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 4df61129566d..11388436b85a 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1907,7 +1907,21 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 
 		if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS)) {
 			timed_out = 1;
-			break;
+			__set_current_state(TASK_RUNNING);
+			/*
+			 * Acquire the lock and try to remove this thread from
+			 * the wait queue. If this thread is not on the wait
+			 * queue, it has woken up after its timeout ended
+			 * before it could re-acquire the lock. In that case,
+			 * try to harvest some events.
+			 */
+			write_lock_irq(&ep->lock);
+			if (!list_empty(&wait.entry))
+				__remove_wait_queue(&ep->wq, &wait);
+			else
+				eavail = 1;
+			write_unlock_irq(&ep->lock);
+			goto send_events;
 		}
 
 		/* We were woken up, thus go and try to harvest some events */
-- 
2.29.0.rc2.309.g374f81d7ae-goog