Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751459AbdDBMT6 (ORCPT ); Sun, 2 Apr 2017 08:19:58 -0400 Received: from mail-it0-f67.google.com ([209.85.214.67]:34927 "EHLO mail-it0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750966AbdDBMT4 (ORCPT ); Sun, 2 Apr 2017 08:19:56 -0400 Message-ID: <1491135593.10124.9.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 05:19:53 -0700 In-Reply-To: <1491134084.10124.6.camel@edumazet-glaptop3.roam.corp.google.com> References: <6c6e2f7505f969d8c2998efff24063ba@nuclearcat.com> <1491132259.10124.3.camel@edumazet-glaptop3.roam.corp.google.com> <20170402114545.GA31804@breakpoint.cc> <1491134084.10124.6.camel@edumazet-glaptop3.roam.corp.google.com> 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: 1408 Lines: 39 On Sun, 2017-04-02 at 04:54 -0700, Eric Dumazet wrote: > 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) Something like that maybe. diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c index 27241a767f17b4b27d24095a31e5e9a2d3e29ce4..1465aaf0e3a15d69d105d0a50b0429b11b6439d3 100644 --- a/net/netfilter/xt_TCPMSS.c +++ b/net/netfilter/xt_TCPMSS.c @@ -151,7 +151,9 @@ tcpmss_mangle_packet(struct sk_buff *skb, */ if (len > tcp_hdrlen) return 0; - + /* tcph->doff is 4 bits wide, do not wrap its value to 0 */ + if (tcp_hdrlen >= 15 * 4) + return 0; /* * MSS Option not found ?! add it.. */