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| H A D | skbuff.c | diff 00c5a983 Thu Jul 22 03:12:35 MDT 2010 Andrea Shepard <andrea@persephoneslair.org> net: Fix corruption of skb csum field in pskb_expand_head() of net/core/skbuff.c 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 <andrea@persephoneslair.org> Signed-off-by: David S. Miller <davem@davemloft.net> |
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