WARNING: refcount bug in __cleanup_sighand

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: f37e99ac Merge tag 's390-5.8-6' of git://git.kernel.org/pu..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=145c3454900000
kernel config: https://syzkaller.appspot.com/x/.config?x=f87a5e4232fdb267
dashboard link: https://syzkaller.appspot.com/bug?extid=2c03285b137e49617219
compiler: gcc (GCC) 10.1.0-syz 20200507
CC: [ak...@linux-foundation.org chri...@brauner.io chri...@kellner.me cyp...@cyphar.com kees...@chromium.org linux-...@vger.kernel.org lu...@amacapital.net tg...@linutronix.de w...@chromium.org]

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+2c0328...@syzkaller.appspotmail.com

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refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 2158 at lib/refcount.c:28 refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 2158 Comm: syz-executor.3 Not tainted 5.8.0-rc6-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+0x18f/0x20d lib/dump_stack.c:118
panic+0x2e3/0x75c kernel/panic.c:231
__warn.cold+0x20/0x45 kernel/panic.c:600
report_bug+0x1bd/0x210 lib/bug.c:198
handle_bug+0x38/0x90 arch/x86/kernel/traps.c:235
exc_invalid_op+0x13/0x40 arch/x86/kernel/traps.c:255
asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:540
RIP: 0010:refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Code: e9 db fe ff ff 48 89 df e8 fc 87 1b fe e9 8a fe ff ff e8 82 23 dc fd 48 c7 c7 00 54 93 88 c6 05 6a 5d 12 07 01 e8 61 61 ad fd <0f> 0b e9 af fe ff ff 0f 1f 84 00 00 00 00 00 41 56 41 55 41 54 55
RSP: 0018:ffffc900193a7ac8 EFLAGS: 00010082
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff888204d7e140 RSI: ffffffff815d4eb7 RDI: fffff52003274f4b
RBP: 0000000000000003 R08: 0000000000000001 R09: ffff8880ae620fcb
R10: 0000000000000000 R11: 0000000000000000 R12: ffff888000122a80
R13: ffff888204d7e140 R14: dffffc0000000000 R15: ffff888000122a40
refcount_sub_and_test include/linux/refcount.h:274 [inline]
refcount_dec_and_test include/linux/refcount.h:294 [inline]
__cleanup_sighand+0x9c/0xb0 kernel/fork.c:1540
__exit_signal kernel/exit.c:158 [inline]
release_task+0xba6/0x14c0 kernel/exit.c:198
exit_notify kernel/exit.c:680 [inline]
do_exit+0x151e/0x2a40 kernel/exit.c:826
do_group_exit+0x125/0x310 kernel/exit.c:903
get_signal+0x40b/0x1ee0 kernel/signal.c:2743
do_signal+0x82/0x2520 arch/x86/kernel/signal.c:810
exit_to_usermode_loop arch/x86/entry/common.c:235 [inline]
__prepare_exit_to_usermode+0x156/0x1f0 arch/x86/entry/common.c:269
do_syscall_64+0x6c/0xe0 arch/x86/entry/common.c:393
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45c1f9
Code: Bad RIP value.
RSP: 002b:00007fe8a6a07cf8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 000000000078bf08 RCX: 000000000045c1f9
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 000000000078bf08
RBP: 000000000078bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000078bf0c
R13: 0000000000c9fb6f R14: 00007fe8a6a089c0 R15: 000000000078bf0c
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.