Re: For review: seccomp_user_notif(2) manual page

From: Kees Cook
Date: Sun Oct 25 2020 - 20:35:55 EST

On Thu, Oct 01, 2020 at 03:52:02AM +0200, Jann Horn wrote:
> On Thu, Oct 1, 2020 at 1:25 AM Tycho Andersen <tycho@tycho.pizza> wrote:
> > On Thu, Oct 01, 2020 at 01:11:33AM +0200, Jann Horn wrote:
> > > On Thu, Oct 1, 2020 at 1:03 AM Tycho Andersen <tycho@tycho.pizza> wrote:
> > > > On Wed, Sep 30, 2020 at 10:34:51PM +0200, Michael Kerrisk (man-pages) wrote:
> > > > > On 9/30/20 5:03 PM, Tycho Andersen wrote:
> > > > > > On Wed, Sep 30, 2020 at 01:07:38PM +0200, Michael Kerrisk (man-pages) wrote:
> > > > > >> ┌─────────────────────────────────────────────────────┐
> > > > > >> │FIXME │
> > > > > >> ├─────────────────────────────────────────────────────┤
> > > > > >> │From my experiments, it appears that if a SEC‐ │
> > > > > >> │COMP_IOCTL_NOTIF_RECV is done after the target │
> > > > > >> │process terminates, then the ioctl() simply blocks │
> > > > > >> │(rather than returning an error to indicate that the │
> > > > > >> │target process no longer exists). │
> > > > > >
> > > > > > Yeah, I think Christian wanted to fix this at some point,
> > > > >
> > > > > Do you have a pointer that discussion? I could not find it with a
> > > > > quick search.
> > > > >
> > > > > > but it's a
> > > > > > bit sticky to do.
> > > > >
> > > > > Can you say a few words about the nature of the problem?
> > > >
> > > > I remembered wrong, it's actually in the tree: 99cdb8b9a573 ("seccomp:
> > > > notify about unused filter"). So maybe there's a bug here?
> > >
> > > That thing only notifies on ->poll, it doesn't unblock ioctls; and
> > > Michael's sample code uses SECCOMP_IOCTL_NOTIF_RECV to wait. So that
> > > commit doesn't have any effect on this kind of usage.
> >
> > Yes, thanks. And the ones stuck in RECV are waiting on a semaphore so
> > we don't have a count of all of them, unfortunately.
> >
> > We could maybe look inside the wait_list, but that will probably make
> > people angry :)
>
> The easiest way would probably be to open-code the semaphore-ish part,
> and let the semaphore and poll share the waitqueue. The current code
> kind of mirrors the semaphore's waitqueue in the wqh - open-coding the
> entire semaphore would IMO be cleaner than that. And it's not like
> semaphore semantics are even a good fit for this code anyway.
>
> Let's see... if we didn't have the existing UAPI to worry about, I'd
> do it as follows (*completely* untested). That way, the ioctl would
> block exactly until either there actually is a request to deliver or
> there are no more users of the filter. The problem is that if we just
> apply this patch, existing users of SECCOMP_IOCTL_NOTIF_RECV that use
> an event loop and don't set O_NONBLOCK will be screwed. So we'd

Wait, why? Do you mean a ioctl calling loop (rather than a poll event
loop)? I think poll would be fine, but a "try calling RECV and expect to
return ENOENT" loop would change. But I don't think anyone would do this
exactly because it _currently_ acts like O_NONBLOCK, yes?

> probably also have to add some stupid counter in place of the
> semaphore's counter that we can use to preserve the old behavior of
> returning -ENOENT once for each cancelled request. :(

I only see this in Debian Code Search:
https://sources.debian.org/src/crun/0.15+dfsg-1/src/libcrun/seccomp_notify.c/?hl=166#L166
which is using epoll_wait():
https://sources.debian.org/src/crun/0.15+dfsg-1/src/libcrun/container.c/?hl=1326#L1326

I expect LXC is using it. :)

Let's change it ASAP! ;)

-Kees

>
> I guess this is a nice point in favor of Michael's usual complaint
> that if there are no man pages for a feature by the time the feature
> lands upstream, there's a higher chance that the UAPI will suck
> forever...
>
>
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 676d4af62103..f0f4c68e0bc6 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -138,7 +138,6 @@ struct seccomp_kaddfd {
> * @notifications: A list of struct seccomp_knotif elements.
> */
> struct notification {
> - struct semaphore request;
> u64 next_id;
> struct list_head notifications;
> };
> @@ -859,7 +858,6 @@ static int seccomp_do_user_notification(int this_syscall,
> list_add(&n.list, &match->notif->notifications);
> INIT_LIST_HEAD(&n.addfd);
>
> - up(&match->notif->request);
> wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
> mutex_unlock(&match->notify_lock);
>
> @@ -1175,9 +1173,10 @@ find_notification(struct seccomp_filter *filter, u64 id)
>
>
> static long seccomp_notify_recv(struct seccomp_filter *filter,
> - void __user *buf)
> + void __user *buf, bool blocking)
> {
> struct seccomp_knotif *knotif = NULL, *cur;
> + DECLARE_WAITQUEUE(wait, current);
> struct seccomp_notif unotif;
> ssize_t ret;
>
> @@ -1190,11 +1189,9 @@ static long seccomp_notify_recv(struct
> seccomp_filter *filter,
>
> memset(&unotif, 0, sizeof(unotif));
>
> - ret = down_interruptible(&filter->notif->request);
> - if (ret < 0)
> - return ret;
> -
> mutex_lock(&filter->notify_lock);
> +
> +retry:
> list_for_each_entry(cur, &filter->notif->notifications, list) {
> if (cur->state == SECCOMP_NOTIFY_INIT) {
> knotif = cur;
> @@ -1202,14 +1199,32 @@ static long seccomp_notify_recv(struct
> seccomp_filter *filter,
> }
> }
>
> - /*
> - * If we didn't find a notification, it could be that the task was
> - * interrupted by a fatal signal between the time we were woken and
> - * when we were able to acquire the rw lock.
> - */
> if (!knotif) {
> - ret = -ENOENT;
> - goto out;
> + /* This has to happen before checking &filter->users. */
> + prepare_to_wait(&filter->wqh, &wait, TASK_INTERRUPTIBLE);
> +
> + /*
> + * If all users of the filter are gone, throw an error instead
> + * of pointlessly continuing to block.
> + */
> + if (refcount_read(&filter->users) == 0) {
> + ret = -ENOTCON;
> + goto out;
> + }
> + if (blocking) {
> + /* No notifications pending - wait for one,
> then retry. */
> + mutex_unlock(&filter->notify_lock);
> + schedule();
> + mutex_lock(&filter->notify_lock);
> + if (signal_pending(current)) {
> + ret = -EINTR;
> + goto out;
> + }
> + goto retry;
> + } else {
> + ret = -ENOENT;
> + goto out;
> + }
> }
>
> unotif.id = knotif->id;
> @@ -1220,6 +1235,7 @@ static long seccomp_notify_recv(struct
> seccomp_filter *filter,
> wake_up_poll(&filter->wqh, EPOLLOUT | EPOLLWRNORM);
> ret = 0;
> out:
> + finish_wait(&filter->wqh, &wait);
> mutex_unlock(&filter->notify_lock);
>
> if (ret == 0 && copy_to_user(buf, &unotif, sizeof(unotif))) {
> @@ -1233,10 +1249,8 @@ static long seccomp_notify_recv(struct
> seccomp_filter *filter,
> */
> mutex_lock(&filter->notify_lock);
> knotif = find_notification(filter, unotif.id);
> - if (knotif) {
> + if (knotif)
> knotif->state = SECCOMP_NOTIFY_INIT;
> - up(&filter->notif->request);
> - }
> mutex_unlock(&filter->notify_lock);
> }
>
> @@ -1412,11 +1426,12 @@ static long seccomp_notify_ioctl(struct file
> *file, unsigned int cmd,
> {
> struct seccomp_filter *filter = file->private_data;
> void __user *buf = (void __user *)arg;
> + bool blocking = !(file->f_flags & O_NONBLOCK);
>
> /* Fixed-size ioctls */
> switch (cmd) {
> case SECCOMP_IOCTL_NOTIF_RECV:
> - return seccomp_notify_recv(filter, buf);
> + return seccomp_notify_recv(filter, buf, blocking);
> case SECCOMP_IOCTL_NOTIF_SEND:
> return seccomp_notify_send(filter, buf);
> case SECCOMP_IOCTL_NOTIF_ID_VALID_WRONG_DIR:
> @@ -1485,7 +1500,6 @@ static struct file *init_listener(struct
> seccomp_filter *filter)
> if (!filter->notif)
> goto out;
>
> - sema_init(&filter->notif->request, 0);
> filter->notif->next_id = get_random_u64();
> INIT_LIST_HEAD(&filter->notif->notifications);

--
Kees Cook