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

Jann Horn
2018-06-21T23:22:03Z

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"

[...]
> +
> +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"?

> + mutex_lock(&match->notify_lock);
> + if (err < 0)
> + goto remove_list;
> + }
> +
> + ret = n.val;
> + err = n.error;
> +
> +remove_list:
> + list_del(&n.list);
> +out:
> + mutex_unlock(&match->notify_lock);
> + syscall_set_return_value(current, task_pt_regs(current),
> + err, ret);
> +}