WARNING: refcount bug in release_task

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 4b6c093e Merge tag 'block-5.9-2020-08-14' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=116de509900000
kernel config: https://syzkaller.appspot.com/x/.config?x=19f02fc5c511a391
dashboard link: https://syzkaller.appspot.com/bug?extid=d3114f0d2090fd19f73d
compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
CC: [ak...@linux-foundation.org chri...@brauner.io ebie...@xmission.com linux-...@vger.kernel.org mi...@kernel.org pet...@infradead.org tg...@linutronix.de]

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d3114f...@syzkaller.appspotmail.com

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refcount_t: addition on 0; use-after-free.
WARNING: CPU: 0 PID: 29013 at lib/refcount.c:25 refcount_warn_saturate+0x13d/0x1a0 lib/refcount.c:25
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 29013 Comm: syz-executor.2 Not tainted 5.8.0-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+0x1f0/0x31e lib/dump_stack.c:118
panic+0x264/0x7a0 kernel/panic.c:231
__warn+0x227/0x250 kernel/panic.c:600
report_bug+0x1b1/0x2e0 lib/bug.c:198
handle_bug+0x42/0x80 arch/x86/kernel/traps.c:234
exc_invalid_op+0x16/0x40 arch/x86/kernel/traps.c:254
asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:536
RIP: 0010:refcount_warn_saturate+0x13d/0x1a0 lib/refcount.c:25
Code: c7 43 89 15 89 31 c0 e8 a1 7f a6 fd 0f 0b eb a3 e8 58 dc d4 fd c6 05 0d ca ea 05 01 48 c7 c7 7a 89 15 89 31 c0 e8 83 7f a6 fd <0f> 0b eb 85 e8 3a dc d4 fd c6 05 f0 c9 ea 05 01 48 c7 c7 a6 89 15
RSP: 0018:ffffc9001a8a7b10 EFLAGS: 00010046
RAX: dfd733b829b0ab00 RBX: 0000000000000002 RCX: ffff88804edc0480
RDX: 0000000000000000 RSI: 0000000080000001 RDI: 0000000000000000
RBP: 0000000000000002 R08: ffffffff815e2099 R09: ffffed1015d041c3
R10: ffffed1015d041c3 R11: 0000000000000000 R12: fffffffffffffa78
R13: dffffc0000000000 R14: ffffc9001a8a7bf0 R15: ffff88804edc0480
refcount_add include/linux/refcount.h:206 [inline]
refcount_inc include/linux/refcount.h:241 [inline]
get_pid include/linux/pid.h:84 [inline]
release_task+0x118e/0x1320 kernel/exit.c:197
exit_notify kernel/exit.c:681 [inline]
do_exit+0x14d9/0x1f20 kernel/exit.c:826
do_group_exit+0x161/0x2d0 kernel/exit.c:903
get_signal+0x139b/0x1d30 kernel/signal.c:2743
arch_do_signal+0x33/0x610 arch/x86/kernel/signal.c:811
exit_to_user_mode_loop kernel/entry/common.c:135 [inline]
exit_to_user_mode_prepare+0x8d/0x1b0 kernel/entry/common.c:166
syscall_exit_to_user_mode+0x5e/0x1a0 kernel/entry/common.c:241
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45d239
Code: Bad RIP value.
RSP: 002b:00007fe8669c8cf8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 000000000118cf48 RCX: 000000000045d239
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 000000000118cf48
RBP: 000000000118cf40 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118cf4c
R13: 00007ffcda4182cf R14: 00007fe8669c99c0 R15: 000000000118cf4c
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.