WARNING: refcount bug in sk_alloc

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
c2a9838452a4d71f76103c18c926468a9ea05713 (Fri Mar 30 05:27:12 2018 +0000)
Merge tag 'for-4.16/dm-fixes-4' of  
git://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=8362f345b3edaf37e986

So far this crash happened 27 times on bpf-next, upstream.
Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5065618571132928
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-8440362230543204781
compiler: gcc (GCC) 7.1.1 20170620

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Reported-by: syzbot+8362f345b3edaf37e986@xxxxxxxxxxxxxxxxxxxxxxxxx
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device lo entered promiscuous mode
------------[ cut here ]------------
refcount_t: increment on 0; use-after-free.
WARNING: CPU: 0 PID: 4461 at lib/refcount.c:153 refcount_inc+0x47/0x50  
lib/refcount.c:153
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 4461 Comm: syz-executor3 Not tainted 4.16.0-rc7+ #6
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+0x194/0x24d lib/dump_stack.c:53
 panic+0x1e4/0x41c kernel/panic.c:183
 __warn+0x1dc/0x200 kernel/panic.c:547
 report_bug+0x1f4/0x2b0 lib/bug.c:186
 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+0x1b/0x40 arch/x86/entry/entry_64.S:986
RIP: 0010:refcount_inc+0x47/0x50 lib/refcount.c:153
RSP: 0018:ffff8801af79f860 EFLAGS: 00010286
RAX: dffffc0000000008 RBX: ffff8801d26bc104 RCX: ffffffff815b7bde
RDX: 0000000000000000 RSI: 1ffff10035ef3ebc RDI: 1ffff10035ef3e91
RBP: ffff8801af79f868 R08: ffffffff87b3b658 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801af79faf8
R13: ffff8801ba2f3513 R14: ffff8801d26bc100 R15: ffff8801ba2f3501
 get_net include/net/net_namespace.h:198 [inline]
 sk_alloc+0x3f9/0x1440 net/core/sock.c:1537
 inet_create+0x47c/0xf50 net/ipv4/af_inet.c:320
 __sock_create+0x4d4/0x850 net/socket.c:1285
 sock_create net/socket.c:1325 [inline]
 SYSC_socket net/socket.c:1355 [inline]
 SyS_socket+0xeb/0x1d0 net/socket.c:1335
 do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x4573e7
RSP: 002b:00007fffdeb62308 EFLAGS: 00000206 ORIG_RAX: 0000000000000029
RAX: ffffffffffffffda RBX: 00000000000003f3 RCX: 00000000004573e7
RDX: 0000000000000006 RSI: 0000000000000001 RDI: 0000000000000002
RBP: 00007fffdeb629b0 R08: 0000000000000000 R09: 0000000000000001
R10: 000000000000000a R11: 0000000000000206 R12: 0000000000000c1e
R13: 0000000000000c1e R14: 0000000000000015 R15: 000000000004415e
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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