Re: [PATCH RESEND 2/5] seccomp: Add wait_killable semantic to seccomp user notifier

[Tycho Andersen <tycho@tycho.pizza>]

On Mon, Apr 26, 2021 at 10:15:28PM +0000, Sargun Dhillon wrote:
> On Mon, Apr 26, 2021 at 01:02:29PM -0600, Tycho Andersen wrote:
> > On Mon, Apr 26, 2021 at 11:06:07AM -0700, Sargun Dhillon wrote:
> > > @@ -1103,11 +1111,31 @@ static int seccomp_do_user_notification(int this_syscall,
> > >  	 * This is where we wait for a reply from userspace.
> > >  	 */
> > >  	do {
> > > +		interruptible = notification_interruptible(&n);
> > > +
> > >  		mutex_unlock(&match->notify_lock);
> > > -		err = wait_for_completion_interruptible(&n.ready);
> > > +		if (interruptible)
> > > +			err = wait_for_completion_interruptible(&n.ready);
> > > +		else
> > > +			err = wait_for_completion_killable(&n.ready);
> > >  		mutex_lock(&match->notify_lock);
> > > -		if (err != 0)
> > > +
> > > +		if (err != 0) {
> > > +			/*
> > > +			 * There is a race condition here where if the
> > > +			 * notification was received with the
> > > +			 * SECCOMP_USER_NOTIF_FLAG_WAIT_KILLABLE flag, but a
> > > +			 * non-fatal signal was received before we could
> > > +			 * transition we could erroneously end our wait early.
> > > +			 *
> > > +			 * The next wait for completion will ensure the signal
> > > +			 * was not fatal.
> > > +			 */
> > > +			if (interruptible && !notification_interruptible(&n))
> > > +				continue;
> > 
> > I'm trying to understand how one would hit this race,
> > 
> 
> I'm thinking:
> P: Process that "generates" notification
> S: Supervisor
> U: User
> 
> P: Generated notification
> S: ioctl(RECV...) // With wait_killable flag.
> ...complete is called in the supervisor, but the P may not be woken up...
> U: kill -SIGTERM $P
> ...signal gets delivered to p and causes wakeup and
> wait_for_completion_interruptible returns 1...
> 
> Then you need to check the race

I see, thanks. This seems like a consequence of having the flag be
per-RECV-call vs. per-filter. Seems like it might be simpler to have
it be per-filter?

Tycho