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Date: Wed, 11 Nov 2015 13:28:18 +0100
From: Hannes Frederic Sowa <hannes@...essinduktion.org>
To: Rainer Weikusat <rweikusat@...ileactivedefense.com>
Cc: Jason Baron <jbaron@...mai.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
syzkaller <syzkaller@...glegroups.com>,
Michal Kubecek <mkubecek@...e.cz>,
Al Viro <viro@...iv.linux.org.uk>,
linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
David Miller <davem@...emloft.net>,
David Howells <dhowells@...hat.com>,
Paul Moore <paul@...l-moore.com>, salyzyn@...roid.com,
sds@...ho.nsa.gov, ying.xue@...driver.com,
netdev <netdev@...r.kernel.org>,
Kostya Serebryany <kcc@...gle.com>,
Alexander Potapenko <glider@...gle.com>,
Andrey Konovalov <andreyknvl@...gle.com>,
Sasha Levin <sasha.levin@...cle.com>,
Julien Tinnes <jln@...gle.com>,
Kees Cook <keescook@...gle.com>,
Mathias Krause <minipli@...glemail.com>
Subject: Re: [PATCH] unix: avoid use-after-free in ep_remove_wait_queue
Hello,
On Tue, Nov 10, 2015, at 22:55, Rainer Weikusat wrote:
> 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.
This whole patch seems pretty complicated to me.
Can't we just remove the unix_recvq_full checks alltogether and unify
unix_dgram_poll with unix_poll?
If we want to be cautious we could simply make unix_max_dgram_qlen limit
the number of skbs which are in flight from a sending socket. The skb
destructor can then decrement this. This seems much simpler.
Would this work?
Thanks,
Hannes
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