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From: Jann Horn <jannh@google.com>
To: Tycho Andersen <tycho@tycho.pizza>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
	Sargun Dhillon <sargun@sargun.me>,
	Kees Cook <keescook@chromium.org>,
	Christian Brauner <christian@brauner.io>,
	linux-man <linux-man@vger.kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Aleksa Sarai <cyphar@cyphar.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Will Drewry <wad@chromium.org>, bpf <bpf@vger.kernel.org>,
	Song Liu <songliubraving@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andy Lutomirski <luto@amacapital.net>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Giuseppe Scrivano <gscrivan@redhat.com>,
	Robert Sesek <rsesek@google.com>
Subject: Re: For review: seccomp_user_notif(2) manual page
Date: Thu, 1 Oct 2020 04:14:06 +0200	[thread overview]
Message-ID: <CAG48ez3kpEDO1x_HfvOM2R9M78Ach9O_4+Pjs-vLLfqvZL+13A@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez2xn+_KznEztJ-eVTsTzkbf9CVgPqaAk7TpRNAqbdaRoA@mail.gmail.com>

On Thu, Oct 1, 2020 at 3:52 AM Jann Horn <jannh@google.com> 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);

  reply	other threads:[~2020-10-01  2:19 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-30 11:07 Michael Kerrisk (man-pages)
2020-09-30 15:03 ` Tycho Andersen
2020-09-30 15:11   ` Tycho Andersen
2020-09-30 20:34   ` Michael Kerrisk (man-pages)
2020-09-30 23:03     ` Tycho Andersen
2020-09-30 23:11       ` Jann Horn
2020-09-30 23:24         ` Tycho Andersen
2020-10-01  1:52           ` Jann Horn
2020-10-01  2:14             ` Jann Horn [this message]
2020-10-25 16:31               ` Michael Kerrisk (man-pages)
2020-10-26 15:54                 ` Jann Horn
2020-10-27  6:14                   ` Michael Kerrisk (man-pages)
2020-10-27 10:28                     ` Jann Horn
2020-10-28  6:31                       ` Sargun Dhillon
2020-10-28  9:43                         ` Jann Horn
2020-10-28 17:43                           ` Sargun Dhillon
2020-10-28 18:20                             ` Jann Horn
2020-10-01  7:49             ` Michael Kerrisk (man-pages)
2020-10-26  0:32             ` Kees Cook
2020-10-26  9:51               ` Jann Horn
2020-10-26 10:31                 ` Jann Horn
2020-10-28 22:56                   ` Kees Cook
2020-10-29  1:11                     ` Jann Horn
     [not found]                   ` <20201029021348.GB25673@cisco>
2020-10-29  4:26                     ` Jann Horn
2020-10-28 22:53                 ` Kees Cook
2020-10-29  1:25                   ` Jann Horn
2020-10-01  7:45       ` Michael Kerrisk (man-pages)
2020-10-14  4:40         ` Michael Kerrisk (man-pages)
2020-09-30 15:53 ` Jann Horn
2020-10-01 12:54   ` Christian Brauner
2020-10-01 15:47     ` Jann Horn
2020-10-01 16:58       ` Tycho Andersen
2020-10-01 17:12         ` Christian Brauner
2020-10-14  5:41           ` Michael Kerrisk (man-pages)
2020-10-01 18:18         ` Jann Horn
2020-10-01 18:56           ` Tycho Andersen
2020-10-01 17:05       ` Christian Brauner
2020-10-15 11:24   ` Michael Kerrisk (man-pages)
2020-10-15 20:32     ` Jann Horn
2020-10-16 18:29       ` Michael Kerrisk (man-pages)
2020-10-17  0:25         ` Jann Horn
2020-10-24 12:52           ` Michael Kerrisk (man-pages)
2020-10-26  9:32             ` Jann Horn
2020-10-26  9:47               ` Michael Kerrisk (man-pages)
2020-09-30 23:39 ` Kees Cook
2020-10-15 11:24   ` Michael Kerrisk (man-pages)
2020-10-26  0:19     ` Kees Cook
2020-10-26  9:39       ` Michael Kerrisk (man-pages)
2020-10-01 12:36 ` Christian Brauner
2020-10-15 11:23   ` Michael Kerrisk (man-pages)
2020-10-01 21:06 ` Sargun Dhillon
2020-10-01 23:19   ` Tycho Andersen

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