From: Sargun Dhillon <sargun@sargun.me>
To: Tycho Andersen <tycho@tycho.pizza>
Cc: "Kees Cook" <keescook@chromium.org>,
LKML <linux-kernel@vger.kernel.org>,
"Linux Containers" <containers@lists.linux-foundation.org>,
"Rodrigo Campos" <rodrigo@kinvolk.io>,
"Christian Brauner" <christian.brauner@ubuntu.com>,
"Mauricio Vásquez Bernal" <mauricio@kinvolk.io>,
"Giuseppe Scrivano" <gscrivan@redhat.com>,
"Andy Lutomirski" <luto@amacapital.net>,
"Will Drewry" <wad@chromium.org>,
"Alban Crequy" <alban@kinvolk.io>
Subject: Re: [PATCH RESEND 2/5] seccomp: Add wait_killable semantic to seccomp user notifier
Date: Tue, 27 Apr 2021 09:34:26 -0700 [thread overview]
Message-ID: <CAMp4zn8MPvkWmt6Yqo+nQCd-HRNRc_=rVYAAF4LjvY2F7-OdGQ@mail.gmail.com> (raw)
In-Reply-To: <20210427134853.GA1746081@cisco>
On Tue, Apr 27, 2021 at 6:48 AM Tycho Andersen <tycho@tycho.pizza> wrote:
>
> 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
You're right.
I think an alternative solution would be to make it on a per-action
basis, and in the filter have a different action for non-preemptible
user notifications.
Since you can only install one filter, I do not think we want to make
it so we do it on a entire filter basis, in case a filter handles a combination
of preemptible and non-preemptible syscalls. For example if you mix
mount and accept.
next prev parent reply other threads:[~2021-04-27 16:35 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-26 18:06 [PATCH RESEND 0/5] Handle seccomp notification preemption Sargun Dhillon
2021-04-26 18:06 ` [PATCH RESEND 1/5] seccomp: Refactor notification handler to prepare for new semantics Sargun Dhillon
2021-04-26 18:06 ` [PATCH RESEND 2/5] seccomp: Add wait_killable semantic to seccomp user notifier Sargun Dhillon
2021-04-26 19:02 ` Tycho Andersen
2021-04-26 22:15 ` Sargun Dhillon
2021-04-27 13:48 ` Tycho Andersen
2021-04-27 16:23 ` Andy Lutomirski
2021-04-27 17:07 ` Tycho Andersen
2021-04-27 22:10 ` Sargun Dhillon
2021-04-27 23:19 ` Andy Lutomirski
2021-04-28 0:22 ` Tycho Andersen
2021-04-28 11:10 ` Rodrigo Campos
2021-04-28 13:20 ` Rodrigo Campos
2021-04-28 14:08 ` Tycho Andersen
2021-04-28 17:13 ` Sargun Dhillon
2021-04-28 3:20 ` Sargun Dhillon
2021-04-27 16:34 ` Sargun Dhillon [this message]
2021-04-26 18:06 ` [PATCH RESEND 3/5] selftests/seccomp: Add test for wait killable notifier Sargun Dhillon
2021-04-26 18:51 ` Tycho Andersen
2021-04-26 18:06 ` [PATCH RESEND 4/5] seccomp: Support atomic "addfd + send reply" Sargun Dhillon
2021-04-26 18:06 ` [PATCH RESEND 5/5] selftests/seccomp: Add test for atomic addfd+send Sargun Dhillon
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