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SubjectWARNING: refcount bug in put_watch

syzbot found the following crash on:

HEAD commit: 282ffdf3 Add linux-next specific files for 20191205
git tree: linux-next
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:
C reproducer:

IMPORTANT: if you fix the bug, please add the following tag to the commit:

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 9318 at lib/refcount.c:28
refcount_warn_saturate+0x1dc/0x1f0 lib/refcount.c:28
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 9318 Comm: syz-executor327 Not tainted
5.4.0-next-20191205-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x197/0x210 lib/dump_stack.c:118
panic+0x2e3/0x75c kernel/panic.c:221
__warn.cold+0x2f/0x3e kernel/panic.c:582
report_bug+0x289/0x300 lib/bug.c:195
fixup_bug arch/x86/kernel/traps.c:174 [inline]
fixup_bug arch/x86/kernel/traps.c:169 [inline]
do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:267
do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:286
invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
RIP: 0010:refcount_warn_saturate+0x1dc/0x1f0 lib/refcount.c:28
Code: e9 d8 fe ff ff 48 89 df e8 01 52 23 fe e9 85 fe ff ff e8 b7 ae e5 fd
48 c7 c7 e0 b7 6f 88 c6 05 5d d5 ec 06 01 e8 b3 5a b6 fd <0f> 0b e9 ac fe
ff ff 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 55 48
RSP: 0018:ffffc90001e77bf0 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff815e5896 RDI: fffff520003cef70
RBP: ffffc90001e77c00 R08: ffff8880a7154640 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000003
R13: ffff8880950c5e58 R14: ffff8880950c5e00 R15: ffffffff8a0218c0
refcount_sub_and_test include/linux/refcount.h:261 [inline]
refcount_dec_and_test include/linux/refcount.h:281 [inline]
kref_put include/linux/kref.h:64 [inline]
put_watch+0xa2/0xb0 drivers/misc/watch_queue.c:633
watch_queue_clear drivers/misc/watch_queue.c:826 [inline]
watch_queue_release+0x34b/0xc40 drivers/misc/watch_queue.c:842
__fput+0x2ff/0x890 fs/file_table.c:280
____fput+0x16/0x20 fs/file_table.c:313
task_work_run+0x145/0x1c0 kernel/task_work.c:113
exit_task_work include/linux/task_work.h:22 [inline]
do_exit+0x8e7/0x2ef0 kernel/exit.c:797
do_group_exit+0x135/0x360 kernel/exit.c:895
__do_sys_exit_group kernel/exit.c:906 [inline]
__se_sys_exit_group kernel/exit.c:904 [inline]
__x64_sys_exit_group+0x44/0x50 kernel/exit.c:904
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
RIP: 0033:0x43edb8
Code: Bad RIP value.
RSP: 002b:00007ffc74c09078 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 000000000043edb8
RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
RBP: 00000000004be5c8 R08: 00000000000000e7 R09: ffffffffffffffd0
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00000000006d0180 R14: 0000000000000000 R15: 0000000000000000
Kernel Offset: disabled
Rebooting in 86400 seconds..

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