Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751406AbdDBLys (ORCPT ); Sun, 2 Apr 2017 07:54:48 -0400 Received: from mail-it0-f66.google.com ([209.85.214.66]:34832 "EHLO mail-it0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750940AbdDBLyq (ORCPT ); Sun, 2 Apr 2017 07:54:46 -0400 Message-ID: <1491134084.10124.6.camel@edumazet-glaptop3.roam.corp.google.com> Subject: Re: KASAN, xt_TCPMSS finally found nasty use-after-free bug? 4.10.8 From: Eric Dumazet To: Florian Westphal Cc: Denys Fedoryshchenko , Linux Kernel Network Developers , Pablo Neira Ayuso , Patrick McHardy , Jozsef Kadlecsik , netfilter-devel@vger.kernel.org, coreteam@netfilter.org, linux-kernel@vger.kernel.org Date: Sun, 02 Apr 2017 04:54:44 -0700 In-Reply-To: <20170402114545.GA31804@breakpoint.cc> References: <6c6e2f7505f969d8c2998efff24063ba@nuclearcat.com> <1491132259.10124.3.camel@edumazet-glaptop3.roam.corp.google.com> <20170402114545.GA31804@breakpoint.cc> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.10.4-0ubuntu2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 803 Lines: 20 On Sun, 2017-04-02 at 13:45 +0200, Florian Westphal wrote: > Eric Dumazet wrote: > > - for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) { > > + for (i = sizeof(struct tcphdr); i < tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) { > > if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) { > > u_int16_t oldmss; > > maybe I am low on caffeeine but this looks fine, for tcp header with > only tcpmss this boils down to "20 <= 24 - 4" so we acccess offsets 20-23 which seems ok. I am definitely low on caffeine ;) An issue in this function is that we might add the missing MSS option, without checking that TCP options are already full. But this should not cause a KASAN splat, only some malformed TCP packet (tcph->doff would wrap)