linux-kernel - Re: [PATCH v2 2/5] seccomp: Add wait_killable semantic to seccomp user notifier

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Date:   Fri, 30 Apr 2021 17:09:26 -0700
From:   Sargun Dhillon <sargun@...gun.me>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Mauricio Vásquez Bernal <mauricio@...volk.io>,
        Rodrigo Campos <rodrigo@...volk.io>,
        Tycho Andersen <tycho@...ho.pizza>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH v2 2/5] seccomp: Add wait_killable semantic to seccomp
 user notifier

On Fri, Apr 30, 2021 at 4:23 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> On Fri, Apr 30, 2021 at 1:49 PM Sargun Dhillon <sargun@...gun.me> wrote:
> >
> > The user notifier feature allows for filtering of seccomp notifications in
> > userspace. While the user notifier is handling the syscall, the notifying
> > process can be preempted, thus ending the notification. This has become a
> > growing problem, as Golang has adopted signal based async preemption[1]. In
> > this, it will preempt every 10ms, thus leaving the supervisor less than
> > 10ms to respond to a given notification. If the syscall require I/O (mount,
> > connect) on behalf of the process, it can easily take 10ms.
> >
> > This allows the supervisor to set a flag that moves the process into a
> > state where it is only killable by terminating signals as opposed to all
> > signals. The process can still be terminated before the supervisor receives
> > the notification.
>
> This is still racy, right?  If a signal arrives after the syscall
> enters the seccomp code but before the supervisor gets around to
> issuing the new ioctl, the syscall will erroneously return -EINTR,
> right?
>
> Can we please just fully fix this instead of piling a racy partial fix
> on top of an incorrect design?
>
> --Andy

I thought that you were fine with this approach. Sorry.

Maybe this is a dumb question, what's wrong with returning an EINTR if the
syscall was never observed by the supervisor?

I think that the only other reasonable design is that we add data to the
existing action which makes it sleep in wait_killable state.