Re: For review: seccomp_user_notif(2) manual page

[Jann Horn <jannh@google.com>]

On Thu, Oct 1, 2020 at 6:58 PM Tycho Andersen <tycho@tycho.pizza> wrote:
> On Thu, Oct 01, 2020 at 05:47:54PM +0200, Jann Horn via Containers wrote:
> > On Thu, Oct 1, 2020 at 2:54 PM Christian Brauner
> > <christian.brauner@canonical.com> wrote:
> > > On Wed, Sep 30, 2020 at 05:53:46PM +0200, Jann Horn via Containers wrote:
> > > > On Wed, Sep 30, 2020 at 1:07 PM Michael Kerrisk (man-pages)
> > > > <mtk.manpages@gmail.com> wrote:
> > > > > NOTES
> > > > >        The file descriptor returned when seccomp(2) is employed with the
> > > > >        SECCOMP_FILTER_FLAG_NEW_LISTENER  flag  can  be  monitored  using
> > > > >        poll(2), epoll(7), and select(2).  When a notification  is  pend‐
> > > > >        ing,  these interfaces indicate that the file descriptor is read‐
> > > > >        able.
> > > >
> > > > We should probably also point out somewhere that, as
> > > > include/uapi/linux/seccomp.h says:
> > > >
> > > >  * Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF
> > > >  * or SECCOMP_RET_TRACE. For SECCOMP_RET_USER_NOTIF filters acting on the
> > > >  * same syscall, the most recently added filter takes precedence. This means
> > > >  * that the new SECCOMP_RET_USER_NOTIF filter can override any
> > > >  * SECCOMP_IOCTL_NOTIF_SEND from earlier filters, essentially allowing all
> > > >  * such filtered syscalls to be executed by sending the response
> > > >  * SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_TRACE can equally
> > > >  * be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.
> > > >
> > > > In other words, from a security perspective, you must assume that the
> > > > target process can bypass any SECCOMP_RET_USER_NOTIF (or
> > > > SECCOMP_RET_TRACE) filters unless it is completely prohibited from
> > > > calling seccomp(). This should also be noted over in the main
> > > > seccomp(2) manpage, especially the SECCOMP_RET_TRACE part.
> > >
> > > So I was actually wondering about this when I skimmed this and a while
> > > ago but forgot about this again... Afaict, you can only ever load a
> > > single filter with SECCOMP_FILTER_FLAG_NEW_LISTENER set. If there
> > > already is a filter with the SECCOMP_FILTER_FLAG_NEW_LISTENER property
> > > in the tasks filter hierarchy then the kernel will refuse to load a new
> > > one?
> > >
> > > static struct file *init_listener(struct seccomp_filter *filter)
> > > {
> > >         struct file *ret = ERR_PTR(-EBUSY);
> > >         struct seccomp_filter *cur;
> > >
> > >         for (cur = current->seccomp.filter; cur; cur = cur->prev) {
> > >                 if (cur->notif)
> > >                         goto out;
> > >         }
> > >
> > > shouldn't that be sufficient to guarantee that USER_NOTIF filters can't
> > > override each other for the same task simply because there can only ever
> > > be a single one?
> >
> > Good point. Exceeeept that that check seems ineffective because this
> > happens before we take the locks that guard against TSYNC, and also
> > before we decide to which existing filter we want to chain the new
> > filter. So if two threads race with TSYNC, I think they'll be able to
> > chain two filters with listeners together.
>
> Yep, seems the check needs to also be in seccomp_can_sync_threads() to
> be totally effective,
>
> > I don't know whether we want to eternalize this "only one listener
> > across all the filters" restriction in the manpage though, or whether
> > the man page should just say that the kernel currently doesn't support
> > it but that security-wise you should assume that it might at some
> > point.
>
> This requirement originally came from Andy, arguing that the semantics
> of this were/are confusing, which still makes sense to me. Perhaps we
> should do something like the below?
[...]
> +static bool has_listener_parent(struct seccomp_filter *child)
> +{
> +       struct seccomp_filter *cur;
> +
> +       for (cur = current->seccomp.filter; cur; cur = cur->prev) {
> +               if (cur->notif)
> +                       return true;
> +       }
> +
> +       return false;
> +}
[...]
> @@ -407,6 +419,11 @@ static inline pid_t seccomp_can_sync_threads(void)
[...]
> +               /* don't allow TSYNC to install multiple listeners */
> +               if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER &&
> +                   !has_listener_parent(thread->seccomp.filter))
> +                       continue;
[...]
> @@ -1462,12 +1479,9 @@ static const struct file_operations seccomp_notify_ops = {
>  static struct file *init_listener(struct seccomp_filter *filter)
[...]
> -       for (cur = current->seccomp.filter; cur; cur = cur->prev) {
> -               if (cur->notif)
> -                       goto out;
> -       }
> +       if (has_listener_parent(current->seccomp.filter))
> +               goto out;

I dislike this because it combines a non-locked check and a locked
check. And I don't think this will work in the case where TSYNC and
non-TSYNC race - if the non-TSYNC call nests around the TSYNC filter
installation, the thread that called seccomp in non-TSYNC mode will
still end up with two notifying filters. How about the following?


diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 676d4af62103..c49ad8ba0bc1 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1475,11 +1475,6 @@ static struct file *init_listener(struct
seccomp_filter *filter)
        struct file *ret = ERR_PTR(-EBUSY);
        struct seccomp_filter *cur;

-       for (cur = current->seccomp.filter; cur; cur = cur->prev) {
-               if (cur->notif)
-                       goto out;
-       }
-
        ret = ERR_PTR(-ENOMEM);
        filter->notif = kzalloc(sizeof(*(filter->notif)), GFP_KERNEL);
        if (!filter->notif)
@@ -1504,6 +1499,31 @@ static struct file *init_listener(struct
seccomp_filter *filter)
        return ret;
 }

+/*
+ * Does @new_child have a listener while an ancestor also has a listener?
+ * If so, we'll want to reject this filter.
+ * This only has to be tested for the current process, even in the TSYNC case,
+ * because TSYNC installs @child with the same parent on all threads.
+ * Note that @new_child is not hooked up to its parent at this point yet, so
+ * we use current->seccomp.filter.
+ */
+static bool has_duplicate_listener(struct seccomp_filter *new_child)
+{
+       struct seccomp_filter *cur;
+
+       /* must be protected against concurrent TSYNC */
+       lockdep_assert_held(&current->sighand->siglock);
+
+       if (!new_child->notif)
+               return false;
+       for (cur = current->seccomp.filter; cur; cur = cur->prev) {
+               if (cur->notif)
+                       return true;
+       }
+
+       return false;
+}
+
 /**
  * seccomp_set_mode_filter: internal function for setting seccomp filter
  * @flags:  flags to change filter behavior
@@ -1575,6 +1595,9 @@ static long seccomp_set_mode_filter(unsigned int flags,
        if (!seccomp_may_assign_mode(seccomp_mode))
                goto out;

+       if (has_duplicate_listener(prepared))
+               goto out;
+
        ret = seccomp_attach_filter(flags, prepared);
        if (ret)
                goto out;