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Date: Tue, 17 Nov 2015 15:14:21 -0500 (EST)
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Subject: Re: [PATCH] unix: avoid use-after-free in ep_remove_wait_queue (w/
 Fixes:)
From: David Miller <davem@davemloft.net>
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From: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Date: Mon, 16 Nov 2015 22:28:40 +0000

> An AF_UNIX datagram socket being the client in an n:1 association with
> some server socket is only allowed to send messages to the server if the
> receive queue of this socket contains at most sk_max_ack_backlog
> datagrams. This implies that prospective writers might be forced to go
> to sleep despite none of the message presently enqueued on the server
> receive queue were sent by them. In order to ensure that these will be
> woken up once space becomes again available, the present unix_dgram_poll
> routine does a second sock_poll_wait call with the peer_wait wait queue
> of the server socket as queue argument (unix_dgram_recvmsg does a wake
> up on this queue after a datagram was received). This is inherently
> problematic because the server socket is only guaranteed to remain alive
> for as long as the client still holds a reference to it. In case the
> connection is dissolved via connect or by the dead peer detection logic
> in unix_dgram_sendmsg, the server socket may be freed despite "the
> polling mechanism" (in particular, epoll) still has a pointer to the
> corresponding peer_wait queue. There's no way to forcibly deregister a
> wait queue with epoll.
> 
> Based on an idea by Jason Baron, the patch below changes the code such
> that a wait_queue_t belonging to the client socket is enqueued on the
> peer_wait queue of the server whenever the peer receive queue full
> condition is detected by either a sendmsg or a poll. A wake up on the
> peer queue is then relayed to the ordinary wait queue of the client
> socket via wake function. The connection to the peer wait queue is again
> dissolved if either a wake up is about to be relayed or the client
> socket reconnects or a dead peer is detected or the client socket is
> itself closed. This enables removing the second sock_poll_wait from
> unix_dgram_poll, thus avoiding the use-after-free, while still ensuring
> that no blocked writer sleeps forever.
> 
> Signed-off-by: Rainer Weikusat <rweikusat@mobileactivedefense.com>
> Fixes: ec0d215f9420 ("af_unix: fix 'poll for write'/connected DGRAM sockets")

So because of a corner case of epoll handling and sender socket release,
every single datagram sendmsg has to do a double lock now?

I do not dispute the correctness of your fix at this point, but that
added cost in the fast path is really too high.