From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pablo Neira Ayuso Subject: Re: [PATCH] netfilter: conntrack: drop malformed IPv4 packets Date: Wed, 4 Apr 2012 17:14:46 +0200 Message-ID: <20120404151446.GB4148@1984> References: <1333481769-16747-1-git-send-email-kadlec@blackhole.kfki.hu> <1333481769-16747-2-git-send-email-kadlec@blackhole.kfki.hu> <1333482079.18626.323.camel@edumazet-glaptop> <20120404112339.GA3319@1984> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Eric Dumazet , netfilter-devel@vger.kernel.org To: Jozsef Kadlecsik Return-path: Received: from mail.us.es ([193.147.175.20]:41232 "EHLO mail.us.es" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756686Ab2DDPOz (ORCPT ); Wed, 4 Apr 2012 11:14:55 -0400 Content-Disposition: inline In-Reply-To: Sender: netfilter-devel-owner@vger.kernel.org List-ID: On Wed, Apr 04, 2012 at 02:07:45PM +0200, Jozsef Kadlecsik wrote: > On Wed, 4 Apr 2012, Pablo Neira Ayuso wrote: > > > On Tue, Apr 03, 2012 at 09:41:19PM +0200, Eric Dumazet wrote: > > > On Tue, 2012-04-03 at 21:36 +0200, Jozsef Kadlecsik wrote: > > > > It was reported that the Linux kernel sometimes logs: > > > > > > > > klogd: [2629147.402413] kernel BUG at net / netfilter / > > > > nf_conntrack_proto_tcp.c: 447! > > > > klogd: [1072212.887368] kernel BUG at net / netfilter / > > > > nf_conntrack_proto_tcp.c: 392 > > > > > > > > ipv4_get_l4proto() in nf_conntrack_l3proto_ipv4.c and tcp_error() in > > > > nf_conntrack_proto_tcp.c should catch malformed packets, so the errors > > > > at the indicated lines - TCP options parsing - should not happen. > > > > However, tcp_error() relies on the "dataoff" offset to the TCP header, > > > > calculated by ipv4_get_l4proto(). But ipv4_get_l4proto() does not check > > > > bogus ihl values in IPv4 packets, which then can slip through tcp_error() > > > > and get caught at the TCP options parsing routines. > > > > > > > > The patch fixes ipv4_get_l4proto() by dropping packets with bogus > > > > ihl value. > > > > > > > > The patch closes netfilter bugzilla id 771. > > > > > > > > Signed-off-by: Jozsef Kadlecsik > > > > --- > > > > net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c | 8 ++++++++ > > > > 1 files changed, 8 insertions(+), 0 deletions(-) > > > > > > > > diff --git a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c > > > > index de9da21..c93d052 100644 > > > > --- a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c > > > > +++ b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c > > > > @@ -84,6 +84,14 @@ static int ipv4_get_l4proto(const struct sk_buff *skb, unsigned int nhoff, > > > > *dataoff = nhoff + (iph->ihl << 2); > > > > *protonum = iph->protocol; > > > > > > > > + /* Check bogus IP headers */ > > > > + if (*dataoff > skb->len) { > > > > + pr_debug("nf_conntrack_ipv4: drop bogus IPv4 packet: " > > > > + "nhoff %u, ihl %u, skblen %u\n", > > > > + nhoff, iph->ihl << 2, skb->len); > > > > + return -NF_ACCEPT; > > > > > > return -NF_DROP; ? > > > > The connection tracking system should not drop packets. Well, we > > sometimes do in special cases. > > > > I think -NF_ACCEPT is fine, the user can add one rule to drop INVALID > > packets (like this one) from iptables. > > That's a nontrivial question and currently we are inconsistent: in > ipv4_get_l4proto() we drop all broken packets (IP header is incomplete or > fragments) and the condition above just adds a third category. However, in > ipv6_get_l4proto() we simply mark them as INVALID and in tcp_error() we do > the same. Thanks for spotting that. > So, should we convert ipv4_get_l4proto() to mark all broken packets as > INVALID? Yes, I think we should stick to -NF_ACCEPT so we let the user drop the packet via iptables. BTW, if you send a follow-up patch for this, it would be also good to log invalid packets in ipv4_get_l4proto. There's no nf_log_packet invocation for these.