path: root/net
diff options
authorAndrea Shepard <andrea@persephoneslair.org>2010-07-22 09:12:35 +0000
committerDavid S. Miller <davem@davemloft.net>2010-07-22 13:25:18 -0700
commit00c5a9834b476a138158fb17d576da751727a9f1 (patch)
treea06b8b779d11784bb29f380e00c438e16c28f41a /net
parent8a35747a5d13b99e076b0222729e0caa48cb69b6 (diff)
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>
Diffstat (limited to 'net')
1 files changed, 3 insertions, 1 deletions
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 34432b4e96b..c699159b3ed 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -843,7 +843,9 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
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;