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

Tycho Andersen
2018-06-22T00:58:38Z

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:
> >
> > This patch introduces a means for syscalls matched in seccomp to notify
> > some other task that a particular filter has been triggered.
> [...]
> > +Userspace Notification
> > +======================
> > +
> > +The ``SECCOMP_RET_USER_NOTIF`` return code lets seccomp filters pass a
> > +particular syscall to userspace to be handled. This may be useful for
> > +applications like container managers, which whish to intercept particular
>
> typo: "wish"
>
> [...]
> > +passed around via ``SCM_RIGHTS`` or similar. Alternativley, a filter fd can be
>
> typo: "Alternatively"
>
> [...]
> > +It is worth noting that ``struct seccomp_data`` contains the values of register
> > +arguments to the syscall, but does not contain pointers to memory. The task's
> > +memory is accessiable to suitably privileged traces via via ``ptrace()`` or
>
> Typo: "accessible"

Thanks!

> [...]
> > +
> > +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.

Tycho