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From: Andrea Shepard on 22 Jul 2010 15:30
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(a)persephoneslair.org>

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
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