The problem occurs when iptables constructs the tcp reset packet.
It doesn't initialize the pointer to the tcp header within the skb.
When the skb is passed to the ixgbe driver for transmit, the ixgbe
driver attempts to access the tcp header and crashes.
Currently, other drivers (such as our 1G e1000e or igb drivers) don't
access the tcp header on transmit unless the TSO option is turned on.

<1>BUG: unable to handle kernel NULL pointer dereference at 0000000d
<1>IP: [<d081621c>] ixgbe_xmit_frame_ring+0x8cc/0x2260 [ixgbe]
<4>*pdpt = 0000000085e5d001 *pde = 0000000000000000
<0>Oops: 0000 [#1] SMP
[...]
<4>Pid: 0, comm: swapper Tainted: P            2.6.35.12 #1 Greencity/Thurley
<4>EIP: 0060:[<d081621c>] EFLAGS: 00010246 CPU: 16
<4>EIP is at ixgbe_xmit_frame_ring+0x8cc/0x2260 [ixgbe]
<4>EAX: c7628820 EBX: 00000007 ECX: 00000000 EDX: 00000000
<4>ESI: 00000008 EDI: c6882180 EBP: dfc6b000 ESP: ced95c48
<4> DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
<0>Process swapper (pid: 0, ti=ced94000 task=ced73bd0 task.ti=ced94000)
<0>Stack:
<4> cbec7418 c779e0d8 c77cc888 c77cc8a8 0903010a 00000000 c77c0008 00000002
<4><0> cd4997c0 00000010 dfc6b000 00000000 d0d176c9 c77cc8d8 c6882180 cbec7318
<4><0> 00000004 00000004 cbec7230 cbec7110 00000000 cbec70c0 c779e000 00000002
<0>Call Trace:
<4> [<d0d176c9>] ? 0xd0d176c9
<4> [<d0d18a4d>] ? 0xd0d18a4d
<4> [<411e243e>] ? dev_hard_start_xmit+0x218/0x2d7
<4> [<411f03d7>] ? sch_direct_xmit+0x4b/0x114
<4> [<411f056a>] ? __qdisc_run+0xca/0xe0
<4> [<411e28b0>] ? dev_queue_xmit+0x2d1/0x3d0
<4> [<411e8120>] ? neigh_resolve_output+0x1c5/0x20f
<4> [<411e94a1>] ? neigh_update+0x29c/0x330
<4> [<4121cf29>] ? arp_process+0x49c/0x4cd
<4> [<411f80c9>] ? nf_hook_slow+0x3f/0xac
<4> [<4121ca8d>] ? arp_process+0x0/0x4cd
<4> [<4121ca8d>] ? arp_process+0x0/0x4cd
<4> [<4121c6d5>] ? T.901+0x38/0x3b
<4> [<4121c918>] ? arp_rcv+0xa3/0xb4
<4> [<4121ca8d>] ? arp_process+0x0/0x4cd
<4> [<411e1173>] ? __netif_receive_skb+0x32b/0x346
<4> [<411e19e1>] ? netif_receive_skb+0x5a/0x5f
<4> [<411e1ea9>] ? napi_skb_finish+0x1b/0x30
<4> [<d0816eb4>] ? ixgbe_xmit_frame_ring+0x1564/0x2260 [ixgbe]
<4> [<41013468>] ? lapic_next_event+0x13/0x16
<4> [<410429b2>] ? clockevents_program_event+0xd2/0xe4
<4> [<411e1b03>] ? net_rx_action+0x55/0x127
<4> [<4102da1a>] ? __do_softirq+0x77/0xeb
<4> [<4102dab1>] ? do_softirq+0x23/0x27
<4> [<41003a67>] ? do_IRQ+0x7d/0x8e
<4> [<41002a69>] ? common_interrupt+0x29/0x30
<4> [<41007bcf>] ? mwait_idle+0x48/0x4d
<4> [<4100193b>] ? cpu_idle+0x37/0x4c
<0>Code: df 09 d7 0f 94 c2 0f b6 d2 e9 e7 fb ff ff 31 db 31 c0 e9 38
ff ff ff 80 78 06 06 0f 85 3e fb ff ff 8b 7c 24 38 8b 8f b8 00 00 00
<0f> b6 51 0d f6 c2 01 0f 85 27 fb ff ff 80 e2 02 75 0d 8b 6c 24
<0>EIP: [<d081621c>] ixgbe_xmit_frame_ring+0x8cc/0x2260 [ixgbe] SS:ESP

Signed-off-by: Mukund Jampala <jbmukund@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>