From: Rainer Weikusat Subject: Re: [PATCH v4 0/3] net: unix: fix use-after-free Date: Mon, 12 Oct 2015 13:54:02 +0100 Message-ID: <87pp0ki6xh.fsf@doppelsaurus.mobileactivedefense.com> References: <20151011.045557.2164838188213641141.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: jbaron@akamai.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, minipli@googlemail.com, normalperson@yhbt.net, eric.dumazet@gmail.com, rweikusat@mobileactivedefense.com, viro@zeniv.linux.org.uk, davidel@xmailserver.org, dave@stgolabs.net, olivier@mauras.ch, pageexec@freemail.hu, torvalds@linux-foundation.org, peterz@infradead.org, joe@perches.com To: David Miller Return-path: In-Reply-To: <20151011.045557.2164838188213641141.davem@davemloft.net> (David Miller's message of "Sun, 11 Oct 2015 04:55:57 -0700 (PDT)") Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org David Miller writes: > From: Jason Baron > Date: Fri, 9 Oct 2015 00:15:59 -0400 > >> These patches are against mainline, I can re-base to net-next, pleas= e >> let me know. >>=20 >> They have been tested against: https://lkml.org/lkml/2015/9/13/195, >> which causes the use-after-free quite quickly and here: >> https://lkml.org/lkml/2015/10/2/693. > > I'd like to understand how patches that don't even compile can be > "tested"? > > net/unix/af_unix.c: In function =E2=80=98unix_dgram_writable=E2=80=99= : > net/unix/af_unix.c:2480:3: error: =E2=80=98other_full=E2=80=99 undecl= ared (first use in this function) > net/unix/af_unix.c:2480:3: note: each undeclared identifier is report= ed only once for each function it appears in > > Could you explain how that works, I'm having a hard time understandin= g > this? This is basicallly a workaround for the problem that it's not possible to tell epoll to let go of a certain wait queue: Instead of registering the peer_wait queue via sock_poll_wait, a wait_queue_t under control of the af_unix.c code is linked onto it which relays a wake up on the peer_wait queue to the 'ordinary' wait queue associated with the polled socket via custom wake function. But (at least the code I looked it) it enqueues a unix socket on connect which has certain side effects (in particular, /dev/log will have a seriously large wait queue of entirely uninterested peers) and in many cases, this is simply not necessary, as the additional peer_wait event is only interesting in case a peer of a fan-in socket (like /dev/log) happens to be waiting for writeabilty via poll/ select/ epoll/ ... Since the wait queue handling code is now under control of the af_unix.= c code, it can remove itself from the peer_wait queue prior to dropping its reference to a peer on disconnect or on detecting a dead peer in unix_dgram_sendmsg.