WARNING: refcount bug in rpc_kill_sb

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
0b412605ef5f5c64b31f19e2910b1d5eba9929c3 (Thu Mar 29 01:07:23 2018 +0000)
Merge tag 'drm-fixes-for-v4.16-rc8' of
git://people.freedesktop.org/~airlied/linux
syzbot dashboard link:
https://syzkaller.appspot.com/bug?extid=3e3ae51e98f9df048b65

Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:
https://syzkaller.appspot.com/x/log.txt?id=6341077023850496
Kernel config:
https://syzkaller.appspot.com/x/.config?id=-8440362230543204781
compiler: gcc (GCC) 7.1.1 20170620
CC: [anna.sc...@netapp.com bfi...@fieldses.org da...@davemloft.net
jla...@kernel.org linux-...@vger.kernel.org linu...@vger.kernel.org
net...@vger.kernel.org trond.m...@primarydata.com]

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Reported-by: syzbot+3e3ae5...@syzkaller.appspotmail.com
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------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 4488 at lib/refcount.c:187
refcount_sub_and_test+0x167/0x1b0 lib/refcount.c:187
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 4488 Comm: syz-executor0 Not tainted 4.16.0-rc7+ #5
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_sub_and_test+0x167/0x1b0 lib/refcount.c:187
RSP: 0018:ffff8801ad46f620 EFLAGS: 00010286
RAX: dffffc0000000008 RBX: 0000000000000000 RCX: ffffffff815b7bde
RDX: 0000000000000000 RSI: 1ffff10035a8de74 RDI: 1ffff10035a8de49
RBP: ffff8801ad46f6b0 R08: 0000000000000000 R09: 0000000000000000
R10: ffff8801ad46f558 R11: 0000000000000000 R12: 1ffff10035a8dec5
R13: 00000000ffffffff R14: 0000000000000001 R15: ffff8801cdbdc044
refcount_dec_and_test+0x1a/0x20 lib/refcount.c:212
put_net include/net/net_namespace.h:216 [inline]
rpc_kill_sb+0x253/0x3c0 net/sunrpc/rpc_pipe.c:1473
deactivate_locked_super+0x88/0xd0 fs/super.c:312
deactivate_super+0x141/0x1b0 fs/super.c:343
cleanup_mnt+0xb2/0x150 fs/namespace.c:1173
__cleanup_mnt+0x16/0x20 fs/namespace.c:1180
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
SYSC_exit_group kernel/exit.c:979 [inline]
SyS_exit_group+0x1d/0x20 kernel/exit.c:977
do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x4548b9
RSP: 002b:00007ffc1f7b7ad8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00000000004548b9
RDX: 000000000040e930 RSI: 0000000000a44bd0 RDI: 0000000000000045
RBP: 00000000004b9416 R08: 000000000000000b R09: 0000000000000000
R10: 0000000002037940 R11: 0000000000000246 R12: 0000000002050a00
R13: 0000000000000000 R14: 0000000000000011 R15: 0000000000045824
Dumping ftrace buffer:
(ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.