net: frag, fix race conditions in LRU list maintenance

This patch fixes race between inet_frag_lru_move() and inet_frag_lru_add()
which was introduced in commit 3ef0eb0db4bf92c6d2510fe5c4dc51852746f206
("net: frag, move LRU list maintenance outside of rwlock")

One cpu already added new fragment queue into hash but not into LRU.
Other cpu found it in hash and tries to move it to the end of LRU.
This leads to NULL pointer dereference inside of list_move_tail().

Another possible race condition is between inet_frag_lru_move() and
inet_frag_lru_del(): move can happens after deletion.

This patch initializes LRU list head before adding fragment into hash and
inet_frag_lru_move() doesn't touches it if it's empty.

I saw this kernel oops two times in a couple of days.

[119482.128853] BUG: unable to handle kernel NULL pointer dereference at           (null)
[119482.132693] IP: [<ffffffff812ede89>] __list_del_entry+0x29/0xd0
[119482.136456] PGD 2148f6067 PUD 215ab9067 PMD 0
[119482.140221] Oops: 0000 [#1] SMP
[119482.144008] Modules linked in: vfat msdos fat 8021q fuse nfsd auth_rpcgss nfs_acl nfs lockd sunrpc ppp_async ppp_generic bridge slhc stp llc w83627ehf hwmon_vid snd_hda_codec_hdmi snd_hda_codec_realtek kvm_amd k10temp kvm snd_hda_intel snd_hda_codec edac_core radeon snd_hwdep ath9k snd_pcm ath9k_common snd_page_alloc ath9k_hw snd_timer snd soundcore drm_kms_helper ath ttm r8169 mii
[119482.152692] CPU 3
[119482.152721] Pid: 20, comm: ksoftirqd/3 Not tainted 3.9.0-zurg-00001-g9f95269 #132 To Be Filled By O.E.M. To Be Filled By O.E.M./RS880D
[119482.161478] RIP: 0010:[<ffffffff812ede89>]  [<ffffffff812ede89>] __list_del_entry+0x29/0xd0
[119482.166004] RSP: 0018:ffff880216d5db58  EFLAGS: 00010207
[119482.170568] RAX: 0000000000000000 RBX: ffff88020882b9c0 RCX: dead000000200200
[119482.175189] RDX: 0000000000000000 RSI: 0000000000000880 RDI: ffff88020882ba00
[119482.179860] RBP: ffff880216d5db58 R08: ffffffff8155c7f0 R09: 0000000000000014
[119482.184570] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88020882ba00
[119482.189337] R13: ffffffff81c8d780 R14: ffff880204357f00 R15: 00000000000005a0
[119482.194140] FS:  00007f58124dc700(0000) GS:ffff88021fcc0000(0000) knlGS:0000000000000000
[119482.198928] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[119482.203711] CR2: 0000000000000000 CR3: 00000002155f0000 CR4: 00000000000007e0
[119482.208533] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[119482.213371] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[119482.218221] Process ksoftirqd/3 (pid: 20, threadinfo ffff880216d5c000, task ffff880216d3a9a0)
[119482.223113] Stack:
[119482.228004]  ffff880216d5dbd8 ffffffff8155dcda 0000000000000000 ffff000200000001
[119482.233038]  ffff8802153c1f00 ffff880000289440 ffff880200000014 ffff88007bc72000
[119482.238083]  00000000000079d5 ffff88007bc72f44 ffffffff00000002 ffff880204357f00
[119482.243090] Call Trace:
[119482.248009]  [<ffffffff8155dcda>] ip_defrag+0x8fa/0xd10
[119482.252921]  [<ffffffff815a8013>] ipv4_conntrack_defrag+0x83/0xe0
[119482.257803]  [<ffffffff8154485b>] nf_iterate+0x8b/0xa0
[119482.262658]  [<ffffffff8155c7f0>] ? inet_del_offload+0x40/0x40
[119482.267527]  [<ffffffff815448e4>] nf_hook_slow+0x74/0x130
[119482.272412]  [<ffffffff8155c7f0>] ? inet_del_offload+0x40/0x40
[119482.277302]  [<ffffffff8155d068>] ip_rcv+0x268/0x320
[119482.282147]  [<ffffffff81519992>] __netif_receive_skb_core+0x612/0x7e0
[119482.286998]  [<ffffffff81519b78>] __netif_receive_skb+0x18/0x60
[119482.291826]  [<ffffffff8151a650>] process_backlog+0xa0/0x160
[119482.296648]  [<ffffffff81519f29>] net_rx_action+0x139/0x220
[119482.301403]  [<ffffffff81053707>] __do_softirq+0xe7/0x220
[119482.306103]  [<ffffffff81053868>] run_ksoftirqd+0x28/0x40
[119482.310809]  [<ffffffff81074f5f>] smpboot_thread_fn+0xff/0x1a0
[119482.315515]  [<ffffffff81074e60>] ? lg_local_lock_cpu+0x40/0x40
[119482.320219]  [<ffffffff8106d870>] kthread+0xc0/0xd0
[119482.324858]  [<ffffffff8106d7b0>] ? insert_kthread_work+0x40/0x40
[119482.329460]  [<ffffffff816c32dc>] ret_from_fork+0x7c/0xb0
[119482.334057]  [<ffffffff8106d7b0>] ? insert_kthread_work+0x40/0x40
[119482.338661] Code: 00 00 55 48 8b 17 48 b9 00 01 10 00 00 00 ad de 48 8b 47 08 48 89 e5 48 39 ca 74 29 48 b9 00 02 20 00 00 00 ad de 48 39 c8 74 7a <4c> 8b 00 4c 39 c7 75 53 4c 8b 42 08 4c 39 c7 75 2b 48 89 42 08
[119482.343787] RIP  [<ffffffff812ede89>] __list_del_entry+0x29/0xd0
[119482.348675]  RSP <ffff880216d5db58>
[119482.353493] CR2: 0000000000000000

Oops happened on this path:
ip_defrag() -> ip_frag_queue() -> inet_frag_lru_move() -> list_move_tail() -> __list_del_entry()

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Eric Dumazet <edumazet@google.com>
Cc: David S. Miller <davem@davemloft.net>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>