l2tp: l2tp_ip - fix possible oops on packet receive

l2tp: l2tp_ip - fix possible oops on packet receive

When a packet is received on an L2TP IP socket (L2TPv3 IP link
encapsulation), the l2tpip socket's backlog_rcv function calls
xfrm4_policy_check(). This is not necessary, since it was called
before the skb was added to the backlog. With CONFIG_NET_NS enabled,
xfrm4_policy_check() will oops if skb->dev is null, so this trivial
patch removes the call.

This bug has always been present, but only when CONFIG_NET_NS is
enabled does it cause problems. Most users are probably using UDP
encapsulation for L2TP, hence the problem has only recently
surfaced.

EIP: 0060:[<c12bb62b>] EFLAGS: 00210246 CPU: 0
EIP is at l2tp_ip_recvmsg+0xd4/0x2a7
EAX: 00000001 EBX: d77b5180 ECX: 00000000 EDX: 00200246
ESI: 00000000 EDI: d63cbd30 EBP: d63cbd18 ESP: d63cbcf4
 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
Call Trace:
 [<c1218568>] sock_common_recvmsg+0x31/0x46
 [<c1215c92>] __sock_recvmsg_nosec+0x45/0x4d
 [<c12163a1>] __sock_recvmsg+0x31/0x3b
 [<c1216828>] sock_recvmsg+0x96/0xab
 [<c10b2693>] ? might_fault+0x47/0x81
 [<c10b2693>] ? might_fault+0x47/0x81
 [<c1167fd0>] ? _copy_from_user+0x31/0x115
 [<c121e8c8>] ? copy_from_user+0x8/0xa
 [<c121ebd6>] ? verify_iovec+0x3e/0x78
 [<c1216604>] __sys_recvmsg+0x10a/0x1aa
 [<c1216792>] ? sock_recvmsg+0x0/0xab
 [<c105a99b>] ? __lock_acquire+0xbdf/0xbee
 [<c12d5a99>] ? do_page_fault+0x193/0x375
 [<c10d1200>] ? fcheck_files+0x9b/0xca
 [<c10d1259>] ? fget_light+0x2a/0x9c
 [<c1216bbb>] sys_recvmsg+0x2b/0x43
 [<c1218145>] sys_socketcall+0x16d/0x1a5
 [<c11679f0>] ? trace_hardirqs_on_thunk+0xc/0x10
 [<c100305f>] sysenter_do_call+0x12/0x38
Code: c6 05 8c ea a8 c1 01 e8 0c d4 d9 ff 85 f6 74 07 3e ff 86 80 00 00 00 b9 17 b6 2b c1 ba 01 00 00 00 b8 78 ed 48 c1 e8 23 f6 d9 ff <ff> 76 0c 68 28 e3 30 c1 68 2d 44 41 c1 e8 89 57 01 00 83 c4 0c

Signed-off-by: James Chapman <jchapman@katalix.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>