From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrea Shepard Subject: [PATCH] Fix corruption of skb csum field in pskb_expand_head() of net/core/skbuff.c Date: Thu, 22 Jul 2010 12:12:35 -0700 Message-ID: <20100722191234.GA832@cronus.persephoneslair.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org To: davem@davemloft.net Return-path: Received: from cronus.persephoneslair.org ([216.254.15.24]:45140 "EHLO cronus.persephoneslair.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750776Ab0GVTXh (ORCPT ); Thu, 22 Jul 2010 15:23:37 -0400 Content-Disposition: inline Sender: netdev-owner@vger.kernel.org List-ID: Make pskb_expand_head() check ip_summed to make sure csum_start is really csum_start and not csum before adjusting it. This fixes a bug I encountered using a Sun Quad-Fast Ethernet card and VLANs. On my configuration, the sunhme driver produces skbs with differing amounts of headroom on receive depending on the packet size. See line 2030 of drivers/net/sunhme.c; packets smaller than RX_COPY_THRESHOLD have 52 bytes of headroom but packets larger than that cutoff have only 20 bytes. When these packets reach the VLAN driver, vlan_check_reorder_header() calls skb_cow(), which, if the packet has less than NET_SKB_PAD (== 32) bytes of headroom, uses pskb_expand_head() to make more. Then, pskb_expand_head() needs to adjust a lot of offsets into the skb, including csum_start. Since csum_start is a union with csum, if the packet has a valid csum value this will corrupt it, which was the effect I observed. The sunhme hardware computes receive checksums, so the skbs would be created by the driver with ip_summed == CHECKSUM_COMPLETE and a valid csum field, and then pskb_expand_head() would corrupt the csum field, leading to an "hw csum error" message later on, for example in icmp_rcv() for pings larger than the sunhme RX_COPY_THRESHOLD. On the basis of the comment at the beginning of include/linux/skbuff.h, I believe that the csum_start skb field is only meaningful if ip_csummed is CSUM_PARTIAL, so this patch makes pskb_expand_head() adjust it only in that case to avoid corrupting a valid csum value. Please see my more in-depth disucssion of tracking down this bug for more details if you like: http://puellavulnerata.livejournal.com/112186.html http://puellavulnerata.livejournal.com/112567.html http://puellavulnerata.livejournal.com/112891.html http://puellavulnerata.livejournal.com/113096.html http://puellavulnerata.livejournal.com/113591.html I am not subscribed to this list, so please CC me on replies. Signed-off-by: Andrea Shepard diff -Nau linux-2.6.34.1/net/core/skbuff.c linux-2.6.34.1-skb-csum/net/core/skbuff.c --- linux-2.6.34.1/net/core/skbuff.c 2010-07-05 11:24:10.000000000 -0700 +++ linux-2.6.34.1-skb-csum/net/core/skbuff.c 2010-07-21 18:42:33.000000000 -0700 @@ -854,7 +854,9 @@ skb->network_header += off; if (skb_mac_header_was_set(skb)) skb->mac_header += off; - skb->csum_start += nhead; + /* Only adjust this if it actually is csum_start rather than csum */ + if (skb->ip_summed == CHECKSUM_PARTIAL) + skb->csum_start += nhead; skb->cloned = 0; skb->hdr_len = 0; skb->nohdr = 0; -- Andrea Shepard