x25: Fix NULL dereference in x25_recvmsg

x25: Fix NULL dereference in x25_recvmsg

commit cb101ed2 in 3.0 introduced a bug in x25_recvmsg()
When passed bogus junk from userspace, x25->neighbour can be NULL,
as shown in this oops..

BUG: unable to handle kernel NULL pointer dereference at 000000000000001c
IP: [<ffffffffa05482bd>] x25_recvmsg+0x4d/0x280 [x25]
PGD 1015f3067 PUD 105072067 PMD 0
Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
CPU 0
Pid: 27928, comm: iknowthis Not tainted 3.1.0+ #2 Gigabyte Technology Co., Ltd. GA-MA78GM-S2H/GA-MA78GM-S2H
RIP: 0010:[<ffffffffa05482bd>]  [<ffffffffa05482bd>] x25_recvmsg+0x4d/0x280 [x25]
RSP: 0018:ffff88010c0b7cc8  EFLAGS: 00010282
RAX: 0000000000000000 RBX: ffff88010c0b7d78 RCX: 0000000000000c02
RDX: ffff88010c0b7d78 RSI: ffff88011c93dc00 RDI: ffff880103f667b0
RBP: ffff88010c0b7d18 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff880103f667b0
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS:  00007f479ce7f700(0000) GS:ffff88012a600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 000000000000001c CR3: 000000010529e000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process iknowthis (pid: 27928, threadinfo ffff88010c0b6000, task ffff880103faa4f0)
Stack:
 0000000000000c02 0000000000000c02 ffff88010c0b7d18 ffffff958153cb37
 ffffffff8153cb60 0000000000000c02 ffff88011c93dc00 0000000000000000
 0000000000000c02 ffff88010c0b7e10 ffff88010c0b7de8 ffffffff815372c2
Call Trace:
 [<ffffffff8153cb60>] ? sock_update_classid+0xb0/0x180
 [<ffffffff815372c2>] sock_aio_read.part.10+0x142/0x150
 [<ffffffff812d6752>] ? inode_has_perm+0x62/0xa0
 [<ffffffff815372fd>] sock_aio_read+0x2d/0x40
 [<ffffffff811b05e2>] do_sync_read+0xd2/0x110
 [<ffffffff812d3796>] ? security_file_permission+0x96/0xb0
 [<ffffffff811b0a91>] ? rw_verify_area+0x61/0x100
 [<ffffffff811b103d>] vfs_read+0x16d/0x180
 [<ffffffff811b109d>] sys_read+0x4d/0x90
 [<ffffffff81657282>] system_call_fastpath+0x16/0x1b
Code: 8b 66 20 4c 8b 32 48 89 d3 48 89 4d b8 45 89 c7 c7 45 cc 95 ff ff ff 4d 85 e4 0f 84 ed 01 00 00 49 8b 84 24 18 05 00 00 4c 89 e7
 78 1c 01 45 19 ed 31 f6 e8 d5 37 ff e0 41 0f b6 44 24 0e 41

Signed-off-by: Dave Jones <davej@redhat.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>