Re: For review: seccomp_user_notif(2) manual page

From: Jann Horn
Date: Wed Sep 30 2020 - 22:15:32 EST

On Thu, Oct 1, 2020 at 3:52 AM Jann Horn <jannh@xxxxxxxxxx> 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
> 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 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...

And I guess this would be the UAPI-compatible version - not actually
as terrible as I thought it might be. Do y'all want this? If so, feel
free to either turn this into a proper patch with Co-developed-by, or
tell me that I should do it and I'll try to get around to turning it
into something proper.

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 676d4af62103..d08c453fcc2c 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -138,7 +138,7 @@ struct seccomp_kaddfd {
* @notifications: A list of struct seccomp_knotif elements.
*/
struct notification {
- struct semaphore request;
+ bool canceled_reqs;
u64 next_id;
struct list_head notifications;
};
@@ -859,7 +859,6 @@ 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);

@@ -901,8 +900,20 @@ static int seccomp_do_user_notification(int this_syscall,
* *reattach* to a notifier right now. If one is added, we'll need to
* keep track of the notif itself and make sure they match here.
*/
- if (match->notif)
+ if (match->notif) {
list_del(&n.list);
+
+ /*
+ * We are stuck with a UAPI that requires that after a spurious
+ * wakeup, SECCOMP_IOCTL_NOTIF_RECV must return immediately.
+ * This is the tracking for that, keeping track of whether we
+ * canceled a request after waking waiters, but before userspace
+ * picked up the notification.
+ */
+ if (n.state == SECCOMP_NOTIFY_INIT)
+ match->notif->canceled_reqs = true;
+ }
+
out:
mutex_unlock(&match->notify_lock);

@@ -1178,6 +1189,7 @@ static long seccomp_notify_recv(struct
seccomp_filter *filter,
void __user *buf)
{
struct seccomp_knotif *knotif = NULL, *cur;
+ DECLARE_WAITQUEUE(wait, current);
struct seccomp_notif unotif;
ssize_t ret;

@@ -1190,11 +1202,9 @@ 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 +1212,32 @@ 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 (filter->notif->canceled_reqs) {
+ ret = -ENOENT;
+ goto out;
+ } else {
+ /* 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;
+ }
}

unotif.id = knotif->id;
@@ -1220,6 +1248,8 @@ static long seccomp_notify_recv(struct
seccomp_filter *filter,
wake_up_poll(&filter->wqh, EPOLLOUT | EPOLLWRNORM);
ret = 0;
out:
+ filter->notif->canceled_reqs = false;
+ finish_wait(&filter->wqh, &wait);
mutex_unlock(&filter->notify_lock);

if (ret == 0 && copy_to_user(buf, &unotif, sizeof(unotif))) {
@@ -1233,10 +1263,8 @@ 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);
}

@@ -1485,7 +1513,6 @@ 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);