Re: [PATCH v4 1/4] seccomp: add a return code to trap to userspace

Tycho Andersen
2018-06-22T01:39:23Z

On Fri, Jun 22, 2018 at 03:28:24AM +0200, Jann Horn wrote:
> On Fri, Jun 22, 2018 at 2:58 AM Tycho Andersen <tycho@tycho.ws> wrote:
> >
> > On Fri, Jun 22, 2018 at 01:21:47AM +0200, Jann Horn wrote:
> > > On Fri, Jun 22, 2018 at 12:05 AM Tycho Andersen <tycho@tycho.ws> wrote:
> > > [...]
> > > > +
> > > > +static void seccomp_do_user_notification(int this_syscall,
> > > > + struct seccomp_filter *match,
> > > > + const struct seccomp_data *sd)
> > > > +{
> > > > + int err;
> > > > + long ret = 0;
> > > > + struct seccomp_knotif n = {};
> > > > +
> > > > + mutex_lock(&match->notify_lock);
> > > > + err = -ENOSYS;
> > > > + if (!match->has_listener)
> > > > + goto out;
> > > > +
> > > > + n.pid = task_pid(current);
> > > > + n.state = SECCOMP_NOTIFY_INIT;
> > > > + n.data = sd;
> > > > + n.id = seccomp_next_notify_id(match);
> > > > + init_completion(&n.ready);
> > > > +
> > > > + list_add(&n.list, &match->notifications);
> > > > + wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
> > > > +
> > > > + mutex_unlock(&match->notify_lock);
> > > > + up(&match->request);
> > > > +
> > > > + err = wait_for_completion_interruptible(&n.ready);
> > > > + mutex_lock(&match->notify_lock);
> > > > +
> > > > + /*
> > > > + * Here it's possible we got a signal and then had to wait on the mutex
> > > > + * while the reply was sent, so let's be sure there wasn't a response
> > > > + * in the meantime.
> > > > + */
> > > > + if (err < 0 && n.state != SECCOMP_NOTIFY_REPLIED) {
> > > > + /*
> > > > + * We got a signal. Let's tell userspace about it (potentially
> > > > + * again, if we had already notified them about the first one).
> > > > + */
> > > > + if (n.state == SECCOMP_NOTIFY_SENT) {
> > > > + n.state = SECCOMP_NOTIFY_INIT;
> > > > + up(&match->request);
> > > > + }
> > > > + mutex_unlock(&match->notify_lock);
> > > > + err = wait_for_completion_killable(&n.ready);
> > >
> > > Does this mean that when you get a signal that isn't SIGKILL,
> > > wait_for_completion_interruptible() will bail out with -ERESTARTSYS,
> > > but then you hang on this wait_for_completion_killable()? I don't
> > > understand what's going on here. What's the point of using
> > > wait_for_completion_interruptible() when you'll just hang on another
> > > wait on the same "struct completion"?
> >
> > This is the implementation of this suggestion by Andy:
> > https://lkml.org/lkml/2018/3/15/1122
> >
> > The idea is to alert the listener that there was a signal exactly
> > once, in case it's in the middle of processing a request it could bail
> > out and do something else. So the killable wait is intended to ignore
> > other (non-fatal) signals after the first one and wait for whatever
> > the handler decides to do with the signal it received.
>
> How can the listener tell that a signal arrived? When the first
> non-fatal signal comes in, you just set the state to
> SECCOMP_NOTIFY_INIT if it was SECCOMP_NOTIFY_SENT, right? So the
> listener will potentially see the request twice, but with no
> additional indicator that a signal arrived? And in particular, if the
> listener doesn't read the request before the signal arrives, it will
> only see the request once, just as if it was a normal request with no
> signals involved?

I was thinking just parsing /proc/pid/status (given that people are
already going to be mapping things in /proc/pid/map_files to read
arguments and stuff, I didn't think too much of it),

> Would it perhaps make sense to add a field to struct seccomp_notif
> that indicates whether the notification is for a normal syscall or a
> canceled syscall?

Sure, I'll add a __u32 signal and set it to the value of the signal if
we got one.

Thanks!

Tycho