Re: [PATCH v2 1/2] seccomp: notify user trap about unused filter

[Jann Horn]

On Fri, May 29, 2020 at 1:11 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Thu, May 28, 2020 at 05:14:11PM +0200, Christian Brauner wrote:
> >   * @usage: reference count to manage the object lifetime.
> >   *         get/put helpers should be used when accessing an instance
> >   *         outside of a lifetime-guarded section.  In general, this
> >   *         is only needed for handling filters shared across tasks.
> > [...]
> > + * @live: Number of tasks that use this filter directly and number
> > + *     of dependent filters that have a non-zero @live counter.
> > + *     Altered during fork(), exit(), and filter installation
> > [...]
> >       refcount_set(&sfilter->usage, 1);
> > +     refcount_set(&sfilter->live, 1);
[...]
> After looking at these other lifetime management examples in the kernel,
> I'm convinced that tracking these states separately is correct, but I
> remain uncomfortable about task management needing to explicitly make
> two calls to let go of the filter.
>
> I wonder if release_task() should also detach the filter from the task
> and do a put_seccomp_filter() instead of waiting for task_free(). This
> is supported by the other place where seccomp_filter_release() is
> called:
>
> > @@ -396,6 +400,7 @@ static inline void seccomp_sync_threads(unsigned long flags)
> >                * allows a put before the assignment.)
> >               */
> >               put_seccomp_filter(thread);
> > +             seccomp_filter_release(thread);
>
> This would also remove the only put_seccomp_filter() call outside of
> seccomp.c, since the free_task() call will be removed now in favor of
> the task_release() call.
>
> So, is it safe to detach the filter in release_task()? Has dethreading
> happened yet? i.e. can we race TSYNC? -- is there a possible
> inc-from-zero?

release_task -> __exit_signal -> __unhash_process ->
list_del_rcu(&p->thread_node) drops us from the thread list under
siglock, which is the same lock TSYNC uses.

One other interesting thing that can look at seccomp state is
task_seccomp() in procfs - that can still happen at this point. At the
moment, procfs only lets you see the numeric filter state, not the
actual filter contents, so that's not a problem; but if we ever add a
procfs interface for dumping seccomp filters (in addition to the
ptrace interface that already exists), that's something to keep in
mind.

> (Actually, all our refcount_inc()s should be
> refcount_inc_not_zero() just for robustness.)

Eeeh... wouldn't that just make the code more complicated for no good reason?

> I *think* we can do it
> before the release_thread() call (instead of after cgroup_release()).