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SubjectWARNING in task_participate_group_stop (2)

syzbot hit the following crash on upstream commit
bf8f5de17442bba5f811e7e724980730e079ee11 (Sat Apr 28 17:05:04 2018 +0000)
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So far this crash happened 2 times on upstream.
C reproducer:
syzkaller reproducer:
Raw console output:
Kernel config:
compiler: gcc (GCC) 8.0.1 20180413 (experimental)

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random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
WARNING: CPU: 1 PID: 4520 at kernel/signal.c:351
task_participate_group_stop+0x205/0x260 kernel/signal.c:351
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 4520 Comm: 3 Not tainted 4.17.0-rc2+ #22
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+0x1b9/0x294 lib/dump_stack.c:113
panic+0x22f/0x4de kernel/panic.c:184
__warn.cold.8+0x163/0x1b3 kernel/panic.c:536
report_bug+0x252/0x2d0 lib/bug.c:186
fixup_bug arch/x86/kernel/traps.c:178 [inline]
do_error_trap+0x1de/0x490 arch/x86/kernel/traps.c:296
do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:992
RIP: 0010:task_participate_group_stop+0x205/0x260 kernel/signal.c:351
RSP: 0018:ffff8801ae7b7880 EFLAGS: 00010093
RAX: ffff8801d96200c0 RBX: 0000000000000000 RCX: ffffffff81498b5b
RDX: 0000000000000000 RSI: ffffffff81498c75 RDI: 0000000000000005
RBP: ffff8801ae7b78a0 R08: ffff8801d96200c0 R09: ffffed0036c48971
R10: ffffed0036c48971 R11: ffff8801b6244b8b R12: ffff8801d926ecd8
R13: ffff8801d926ec40 R14: 0000000000040000 R15: dffffc0000000000
do_signal_stop+0x502/0xa20 kernel/signal.c:2176
get_signal+0x9b4/0x1960 kernel/signal.c:2353
do_signal+0x98/0x2040 arch/x86/kernel/signal.c:810
exit_to_usermode_loop+0x28a/0x310 arch/x86/entry/common.c:162
prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
syscall_return_slowpath arch/x86/entry/common.c:265 [inline]
do_syscall_64+0x6ac/0x800 arch/x86/entry/common.c:290
RIP: 0033:0x127
RSP: 002b:00007ffda970b440 EFLAGS: 00000200 ORIG_RAX: 000000000000003b
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
Dumping ftrace buffer:
(ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..

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