linux-kernel - Re: [PATCH] unix: avoid use-after-free in ep_remove_wait

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Date:	Wed, 11 Nov 2015 16:12:27 +0000
From:	Rainer Weikusat <rweikusat@...ileactivedefense.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	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

Hannes Frederic Sowa <hannes@...essinduktion.org> writes:
> 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 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?

In the way this is intended to work, cf

http://marc.info/?t=115627606000002&r=1&w=2

only if the limit would also apply to sockets which didn't sent anything
so far. Which means it'll end up in the exact same situation as before:
Sending something using a certain socket may not be possible because of
data sent by other sockets, so either, code trying to send using this
sockets ends up busy-waiting for "space again available" despite it's
trying to use select/ poll/ epolll/ $whatnot to get notified of this
condition and sleep until then or this notification needs to be
propagated to sleeping threads which didn't get to send anything yet.
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