af_iucv: fix race when queueing skbs on the backlog queue

af_iucv: fix race when queueing skbs on the backlog queue

iucv_sock_recvmsg() and iucv_process_message()/iucv_fragment_skb race
for dequeuing an skb from the backlog queue.

If iucv_sock_recvmsg() dequeues first, iucv_process_message() calls
sock_queue_rcv_skb() with an skb that is NULL.

This results in the following kernel panic:

<1>Unable to handle kernel pointer dereference at virtual kernel address (null)
<4>Oops: 0004 [#1] PREEMPT SMP DEBUG_PAGEALLOC
<4>Modules linked in: af_iucv sunrpc qeth_l3 dm_multipath dm_mod vmur qeth ccwgroup
<4>CPU: 0 Not tainted 2.6.30 #4
<4>Process client-iucv (pid: 4787, task: 0000000034e75940, ksp: 00000000353e3710)
<4>Krnl PSW : 0704000180000000 000000000043ebca (sock_queue_rcv_skb+0x7a/0x138)
<4>           R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:0 CC:0 PM:0 EA:3
<4>Krnl GPRS: 0052900000000000 000003e0016e0fe8 0000000000000000 0000000000000000
<4>           000000000043eba8 0000000000000002 0000000000000001 00000000341aa7f0
<4>           0000000000000000 0000000000007800 0000000000000000 0000000000000000
<4>           00000000341aa7f0 0000000000594650 000000000043eba8 000000003fc2fb28
<4>Krnl Code: 000000000043ebbe: a7840006            brc     8,43ebca
<4>           000000000043ebc2: 5930c23c            c       %r3,572(%r12)
<4>           000000000043ebc6: a724004c            brc     2,43ec5e
<4>          >000000000043ebca: e3c0b0100024        stg     %r12,16(%r11)
<4>           000000000043ebd0: a7190000            lghi    %r1,0
<4>           000000000043ebd4: e310b0200024        stg     %r1,32(%r11)
<4>           000000000043ebda: c010ffffdce9        larl    %r1,43a5ac
<4>           000000000043ebe0: e310b0800024        stg     %r1,128(%r11)
<4>Call Trace:
<4>([<000000000043eba8>] sock_queue_rcv_skb+0x58/0x138)
<4> [<000003e0016bcf2a>] iucv_process_message+0x112/0x3cc [af_iucv]
<4> [<000003e0016bd3d4>] iucv_callback_rx+0x1f0/0x274 [af_iucv]
<4> [<000000000053a21a>] iucv_message_pending+0xa2/0x120
<4> [<000000000053b5a6>] iucv_tasklet_fn+0x176/0x1b8
<4> [<000000000014fa82>] tasklet_action+0xfe/0x1f4
<4> [<0000000000150a56>] __do_softirq+0x116/0x284
<4> [<0000000000111058>] do_softirq+0xe4/0xe8
<4> [<00000000001504ba>] irq_exit+0xba/0xd8
<4> [<000000000010e0b2>] do_extint+0x146/0x190
<4> [<00000000001184b6>] ext_no_vtime+0x1e/0x22
<4> [<00000000001fbf4e>] kfree+0x202/0x28c
<4>([<00000000001fbf44>] kfree+0x1f8/0x28c)
<4> [<000000000044205a>] __kfree_skb+0x32/0x124
<4> [<000003e0016bd8b2>] iucv_sock_recvmsg+0x236/0x41c [af_iucv]
<4> [<0000000000437042>] sock_aio_read+0x136/0x160
<4> [<0000000000205e50>] do_sync_read+0xe4/0x13c
<4> [<0000000000206dce>] vfs_read+0x152/0x15c
<4> [<0000000000206ed0>] SyS_read+0x54/0xac
<4> [<0000000000117c8e>] sysc_noemu+0x10/0x16
<4> [<00000042ff8def3c>] 0x42ff8def3c

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>