[PATCH 3.15 15/26] net: Correctly set segment mac_len in skb_segment().

From: Greg Kroah-Hartman
Date: Fri Aug 08 2014 - 17:51:01 EST

3.15-stable review patch. If anyone has any objections, please let me know.


From: Vlad Yasevich <vyasevic@xxxxxxxxxx>

[ Upstream commit fcdfe3a7fa4cb74391d42b6a26dc07c20dab1d82 ]

When performing segmentation, the mac_len value is copied right
out of the original skb. However, this value is not always set correctly
(like when the packet is VLAN-tagged) and we'll end up copying a bad

One way to demonstrate this is to configure a VM which tags
packets internally and turn off VLAN acceleration on the forwarding
bridge port. The packets show up corrupt like this:
16:18:24.985548 52:54:00:ab:be:25 > 52:54:00:26:ce:a3, ethertype 802.1Q
(0x8100), length 1518: vlan 100, p 0, ethertype 0x05e0,
0x0000: 8cdb 1c7c 8cdb 0064 4006 b59d 0a00 6402 ...|...d@.....d.
0x0010: 0a00 6401 9e0d b441 0a5e 64ec 0330 14fa ..d....A.^d..0..
0x0020: 29e3 01c9 f871 0000 0101 080a 000a e833)....q.........3
0x0030: 000f 8c75 6e65 7470 6572 6600 6e65 7470 ...unetperf.netp
0x0040: 6572 6600 6e65 7470 6572 6600 6e65 7470 erf.netperf.netp
0x0050: 6572 6600 6e65 7470 6572 6600 6e65 7470 erf.netperf.netp
0x0060: 6572 6600 6e65 7470 6572 6600 6e65 7470 erf.netperf.netp

This also leads to awful throughput as GSO packets are dropped and
cause retransmissions.

The solution is to set the mac_len using the values already available
in then new skb. We've already adjusted all of the header offset, so we
might as well correctly figure out the mac_len using skb_reset_mac_len().
After this change, packets are segmented correctly and performance
is restored.

CC: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Vlad Yasevich <vyasevic@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
net/core/skbuff.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2968,9 +2968,9 @@ struct sk_buff *skb_segment(struct sk_bu
tail = nskb;

__copy_skb_header(nskb, head_skb);
- nskb->mac_len = head_skb->mac_len;

skb_headers_offset_update(nskb, skb_headroom(nskb) - headroom);
+ skb_reset_mac_len(nskb);

skb_copy_from_linear_data_offset(head_skb, -tnl_hlen,
nskb->data - tnl_hlen,

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