Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754155AbbKQQNt (ORCPT ); Tue, 17 Nov 2015 11:13:49 -0500 Received: from prod-mail-xrelay05.akamai.com ([23.79.238.179]:60087 "EHLO prod-mail-xrelay05.akamai.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751458AbbKQQNp (ORCPT ); Tue, 17 Nov 2015 11:13:45 -0500 Subject: Re: [PATCH] unix: avoid use-after-free in ep_remove_wait_queue (w/ Fixes:) To: Rainer Weikusat References: <20151012120249.GB16370@unicorn.suse.cz> <1444652071.27760.156.camel@edumazet-glaptop2.roam.corp.google.com> <563CC002.5050307@akamai.com> <87ziyrcg67.fsf@doppelsaurus.mobileactivedefense.com> <87fv0fnslr.fsf_-_@doppelsaurus.mobileactivedefense.com> <877flp34fl.fsf@doppelsaurus.mobileactivedefense.com> <87a8qhspfm.fsf@doppelsaurus.mobileactivedefense.com> <876111wpza.fsf@doppelsaurus.mobileactivedefense.com> <87ziydvasn.fsf_-_@doppelsaurus.mobileactivedefense.com> Cc: Dmitry Vyukov , syzkaller , Michal Kubecek , Al Viro , "linux-fsdevel@vger.kernel.org" , LKML , David Miller , Hannes Frederic Sowa , David Howells , Paul Moore , salyzyn@android.com, sds@tycho.nsa.gov, ying.xue@windriver.com, netdev , Kostya Serebryany , Alexander Potapenko , Andrey Konovalov , Sasha Levin , Julien Tinnes , Kees Cook , Mathias Krause From: Jason Baron Message-ID: <564B5237.8080803@akamai.com> Date: Tue, 17 Nov 2015 11:13:43 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <87ziydvasn.fsf_-_@doppelsaurus.mobileactivedefense.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2880 Lines: 58 On 11/16/2015 05:28 PM, 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. > > Signed-off-by: Rainer Weikusat > Fixes: ec0d215f9420 ("af_unix: fix 'poll for write'/connected DGRAM sockets") > --- > > Additional remark about "5456f09aaf88/ af_unix: fix unix_dgram_poll() > behavior for EPOLLOUT event": This shouldn't be an issue anymore with > this change despite it restores the "only when writable" behaviour" as > the wake up relay will also be set up once _dgram_sendmsg returned > EAGAIN for a send attempt on a n:1 connected socket. > > Hi, My only comment was about potentially avoiding the double lock in the write path, otherwise this looks ok to me. Thanks, -Jason -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/