KCSAN: data-race in copy_process / release_task

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 766d004d mm, kcsan: Instrument SLAB free with ASSERT_EXCLU..
git tree: https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=1187c3e9e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=cd9253e6e662bbb3
dashboard link: https://syzkaller.appspot.com/bug?extid=8a49fcfe5a4b42e081a6
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [chri...@brauner.io ebie...@xmission.com linux-...@vger.kernel.org mi...@kernel.org ol...@redhat.com pet...@infradead.org tg...@linutronix.de el...@google.com]

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

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+8a49fc...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in copy_process / release_task

write to 0xffffffff861e7cf8 of 4 bytes by task 26874 on cpu 0:
__unhash_process kernel/exit.c:74 [inline]
__exit_signal kernel/exit.c:157 [inline]
release_task+0x74f/0xc60 kernel/exit.c:207
exit_notify kernel/exit.c:683 [inline]
do_exit+0xedc/0x18a0 kernel/exit.c:822
__module_put_and_exit+0x2c/0x30 kernel/module.c:339
cryptomgr_test+0x32/0x40 crypto/algboss.c:214
kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffffffff861e7cf8 of 4 bytes by task 7 on cpu 1:
copy_process+0x9eb/0x3b40 kernel/fork.c:1969
_do_fork+0xfe/0x7a0 kernel/fork.c:2436
kernel_thread+0xa4/0xd0 kernel/fork.c:2523
call_usermodehelper_exec_sync kernel/umh.c:135 [inline]
call_usermodehelper_exec_work+0x75/0x190 kernel/umh.c:187
process_one_work+0x413/0x8f0 kernel/workqueue.c:2264
worker_thread+0xa0/0x800 kernel/workqueue.c:2410
kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7 Comm: kworker/u4:0 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events_unbound call_usermodehelper_exec_work
==================================================================


---
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[Marco Elver]

#syz fix: fork: annotate data race in copy_process()

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