linux-kernel - Re: List corruption on epoll_ctl(EPOLL_CTL_DEL) an AF

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Date:	Thu, 01 Oct 2015 11:33:59 +0100
From:	Rainer Weikusat <rweikusat@...ileactivedefense.com>
To:	Jason Baron <jbaron@...mai.com>
Cc:	Mathias Krause <minipli@...glemail.com>, netdev@...r.kernel.org,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Eric Wong <normalperson@...t.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Davide Libenzi <davidel@...ilserver.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Olivier Mauras <olivier@...ras.ch>,
	PaX Team <pageexec@...email.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"peterz\@infradead.org" <peterz@...radead.org>,
	"davem\@davemloft.net" <davem@...emloft.net>
Subject: Re: List corruption on epoll_ctl(EPOLL_CTL_DEL) an AF_UNIX socket

Jason Baron <jbaron@...mai.com> writes:
> On 09/30/2015 01:54 AM, Mathias Krause wrote:
>> On 29 September 2015 at 21:09, Jason Baron <jbaron@...mai.com> wrote:
>>> However, if we call connect on socket 's', to connect to a new socket 'o2', we
>>> drop the reference on the original socket 'o'. Thus, we can now close socket
>>> 'o' without unregistering from epoll. Then, when we either close the ep
>>> or unregister 'o', we end up with this list corruption. Thus, this is not a
>>> race per se, but can be triggered sequentially.
>> 
>> Sounds profound, but the reproducers calls connect only once per
>> socket. So there is no "connect to a new socket", no?
>> But w/e, see below.
>
> Yes, but it can be reproduced this way too. It can also happen with a
> close() on the remote peer 'o', and a send to 'o' from 's', which the
> reproducer can do as pointed out Michal. The patch I sent deals with
> both cases.

As Michal also pointed out, there's a unix_dgram_disconnected routine
being called in both cases and insofar "deregistering" anything beyond
what unix_dgram_disconnected (and - insofar I can tell this -
unix_release_sock) already do is actually required, this would be the
obvious place to add it. A good step on the way to that would be to
write (and post) some test code which actually reproduces the problem in
a predictable way.
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