linux-kernel - Re: [syzbot] [kernel?] WARNING in signal_wake_up

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Date: Wed, 10 Jan 2024 17:03:20 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: syzbot <syzbot+c6d438f2d77f96cae7c2@...kaller.appspotmail.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, luto@...nel.org,
	michael.christie@...cle.com, mst@...hat.com, peterz@...radead.org,
	syzkaller-bugs@...glegroups.com, tglx@...utronix.de
Subject: Re: [syzbot] [kernel?] WARNING in signal_wake_up_state

On 01/09, Linus Torvalds wrote:
>
> Oleg/Eric, can you make any sense of this?
>
> On Tue, 9 Jan 2024 at 10:18, syzbot
> <syzbot+c6d438f2d77f96cae7c2@...kaller.appspotmail.com> wrote:
> >
> > The issue was bisected to:
> >
> > commit f9010dbdce911ee1f1af1398a24b1f9f992e0080
>
> Hmm. This smells more like a "that triggers the problem" than a cause.
>
> Because the warning itself is
>
> > WARNING: CPU: 1 PID: 5069 at kernel/signal.c:771 signal_wake_up_state+0xfa/0x120 kernel/signal.c:771
>
> That's
>
>         lockdep_assert_held(&t->sighand->siglock);

I have a fever, possibly I am totally confused, but this commit added

+               /* Don't require de_thread to wait for the vhost_worker */
+               if ((t->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER)
+                       count++;

into zap_other_threads().

So it seems the caller can do unshare_sighand() before vhost thread exits and
actually unshare ->sighand because oldsighand->count > 1.

This is already very wrong (plus it seems this breaks the signal->notify_count
logic). IIRC I even tried to argue with this change... not sure.

And this can explain the warning, this task can start the coredump after exec
and hit vhost_worker with the old sighand != current->sighand.

Oleg.