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    SubjectRe: [PATCH] unix: avoid use-after-free in ep_remove_wait_queue
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    From: Rainer Weikusat <rweikusat@mobileactivedefense.com>
    Date: Fri, 20 Nov 2015 22:07:23 +0000

    > Rainer Weikusat <rweikusat@mobileactivedefense.com> writes:
    > 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")

    Applied and queued up for -stable, thanks to you and Jason for all of
    your hard work on this fix.


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