WARNING: refcount bug in sock_wfree

Hello,

syzbot hit the following crash on upstream commit
79c0ef3e85c015b0921a8fd5dd539d1480e9cd6c (Mon Feb 19 19:58:19 2018 +0000)
Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net

So far this crash happened 2 times on  
https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git/master.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

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Reported-by: syzbot+98a809ad0f54884bde26@syzkaller.appspotmail.com
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IPv6: ADDRCONF(NETDEV_UP): bridge0: link is not ready
IPv6: ADDRCONF(NETDEV_UP): bridge0: link is not ready
audit: type=1400 audit(1519151768.973:11): avc:  denied  { sys_chroot }  
for  pid=4169 comm="syz-executor5" capability=18   
scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023  
tcontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023 tclass=cap_userns  
permissive=1
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 5074 at lib/refcount.c:187  
refcount_sub_and_test+0x167/0x1b0 lib/refcount.c:187
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 5074 Comm: syz-executor5 Not tainted 4.16.0-rc2+ #322
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x194/0x257 lib/dump_stack.c:53
  panic+0x1e4/0x41c kernel/panic.c:183
  __warn+0x1dc/0x200 kernel/panic.c:547
  report_bug+0x211/0x2d0 lib/bug.c:184
  fixup_bug.part.11+0x37/0x80 arch/x86/kernel/traps.c:178
  fixup_bug arch/x86/kernel/traps.c:247 [inline]
  do_error_trap+0x2d7/0x3e0 arch/x86/kernel/traps.c:296
  do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
  invalid_op+0x58/0x80 arch/x86/entry/entry_64.S:957
RIP: 0010:refcount_sub_and_test+0x167/0x1b0 lib/refcount.c:187
RSP: 0018:ffff8801cf2b6490 EFLAGS: 00010282
RAX: dffffc0000000008 RBX: 0000000000000401 RCX: ffffffff815abdbe
RDX: 0000000000000000 RSI: 1ffff10039e56c42 RDI: 1ffff10039e56c17
RBP: ffff8801cf2b6520 R08: 1ffff10039e56bd9 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 1ffff10039e56c93
R13: 00000000ffffff01 R14: 0000000000000500 R15: ffff8801ae3c69fc
  sock_wfree+0xa6/0x140 net/core/sock.c:1819
  sctp_wfree+0x2eb/0x670 net/sctp/socket.c:8065
  skb_release_head_state+0x124/0x260 net/core/skbuff.c:612
  skb_release_all+0x15/0x60 net/core/skbuff.c:625
  __kfree_skb net/core/skbuff.c:641 [inline]
  consume_skb+0x153/0x490 net/core/skbuff.c:701
  sctp_chunk_destroy net/sctp/sm_make_chunk.c:1450 [inline]
  sctp_chunk_put+0x29c/0x420 net/sctp/sm_make_chunk.c:1477
  sctp_chunk_free+0x53/0x60 net/sctp/sm_make_chunk.c:1464
  __sctp_outq_teardown+0x244/0x1230 net/sctp/outqueue.c:234
  sctp_outq_free+0x15/0x20 net/sctp/outqueue.c:291
  sctp_association_free+0x2d0/0x930 net/sctp/associola.c:356
  sctp_cmd_delete_tcb net/sctp/sm_sideeffect.c:939 [inline]
  sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1343 [inline]
  sctp_side_effects net/sctp/sm_sideeffect.c:1210 [inline]
  sctp_do_sm+0x32e3/0x6ed0 net/sctp/sm_sideeffect.c:1181
  sctp_primitive_ABORT+0xa0/0xd0 net/sctp/primitive.c:119
  sctp_close+0x266/0x9a0 net/sctp/socket.c:1539
  inet_release+0xed/0x1c0 net/ipv4/af_inet.c:427
  sock_release+0x8d/0x1e0 net/socket.c:595
  sock_close+0x16/0x20 net/socket.c:1149
  __fput+0x327/0x7e0 fs/file_table.c:209
  ____fput+0x15/0x20 fs/file_table.c:243
  task_work_run+0x199/0x270 kernel/task_work.c:113
  exit_task_work include/linux/task_work.h:22 [inline]
  do_exit+0x9bb/0x1ad0 kernel/exit.c:865
  do_group_exit+0x149/0x400 kernel/exit.c:968
  get_signal+0x73a/0x16d0 kernel/signal.c:2469
  do_signal+0x90/0x1e90 arch/x86/kernel/signal.c:809
  exit_to_usermode_loop+0x258/0x2f0 arch/x86/entry/common.c:162
  prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
  syscall_return_slowpath arch/x86/entry/common.c:265 [inline]
  do_syscall_64+0x6e5/0x940 arch/x86/entry/common.c:292
  entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x453da9
RSP: 002b:00007f297546ace8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 000000000072bf80 RCX: 0000000000453da9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 000000000072bf80
RBP: 000000000072bf80 R08: 0000000000000000 R09: 000000000072bf58
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000a3e8ef R14: 00007f297546b9c0 R15: 0000000000000001
Dumping ftrace buffer:
    (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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