Re: KCSAN: data-race in taskstats_exit / taskstats_exit

[Balbir Singh <bsingharora@gmail.com>]

On Fri, 2019-10-04 at 21:26 -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    b4bd9343 x86, kcsan: Enable KCSAN for x86
> git tree:       https://github.com/google/ktsan.git kcsan
> console output: https://syzkaller.appspot.com/x/log.txt?x=125329db600000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80
> dashboard link: https://syzkaller.appspot.com/bug?extid=c5d03165a1bd1dead0c1
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> 
> 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+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KCSAN: data-race in taskstats_exit / taskstats_exit
> 
> write to 0xffff8881157bbe10 of 8 bytes by task 7951 on cpu 0:
>   taskstats_tgid_alloc kernel/taskstats.c:567 [inline]
>   taskstats_exit+0x6b7/0x717 kernel/taskstats.c:596
>   do_exit+0x2c2/0x18e0 kernel/exit.c:864
>   do_group_exit+0xb4/0x1c0 kernel/exit.c:983
>   get_signal+0x2a2/0x1320 kernel/signal.c:2734
>   do_signal+0x3b/0xc00 arch/x86/kernel/signal.c:815
>   exit_to_usermode_loop+0x250/0x2c0 arch/x86/entry/common.c:159
>   prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
>   syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
>   do_syscall_64+0x2d7/0x2f0 arch/x86/entry/common.c:299
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> read to 0xffff8881157bbe10 of 8 bytes by task 7949 on cpu 1:
>   taskstats_tgid_alloc kernel/taskstats.c:559 [inline]
>   taskstats_exit+0xb2/0x717 kernel/taskstats.c:596
>   do_exit+0x2c2/0x18e0 kernel/exit.c:864
>   do_group_exit+0xb4/0x1c0 kernel/exit.c:983
>   __do_sys_exit_group kernel/exit.c:994 [inline]
>   __se_sys_exit_group kernel/exit.c:992 [inline]
>   __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:992
>   do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 

Sorry I've been away and just catching up with email

I don't think this is a bug, if I interpret the report correctly it shows a
race

static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk)
{
	struct signal_struct *sig = tsk->signal;
	struct taskstats *stats;

#1	if (sig->stats || thread_group_empty(tsk)) <- the check of sig->stats
		goto ret;

	/* No problem if kmem_cache_zalloc() fails */
	stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);

	spin_lock_irq(&tsk->sighand->siglock);
	if (!sig->stats) {
#2		sig->stats = stats; <- here in setting sig->stats
		stats = NULL;
	}
	spin_unlock_irq(&tsk->sighand->siglock);

	if (stats)
		kmem_cache_free(taskstats_cache, stats);
ret:
	return sig->stats;
}

The worst case scenario is that we might see sig->stats as being NULL when two
threads belonging to the same tgid. We do free up stats if we got that wrong

Am I misinterpreting the report?

Balbir Singh.