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

[Christian Brauner <christian.brauner@ubuntu.com>]

On Fri, May 29, 2020 at 01:32:03AM +0200, Jann Horn wrote:
> On Fri, May 29, 2020 at 1:11 AM Kees Cook <keescook@chromium.org> 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.

We should move us after write_unlock_irq(&tasklist_lock). We're after
__exit_signal() so we're unhashed and can't be discovered by tsync too
anymore and we also don't require the tasklist_lock to be held:

diff --git a/kernel/exit.c b/kernel/exit.c
index b332e3635eb5..5490cc04f436 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -193,8 +193,6 @@ void release_task(struct task_struct *p)

        cgroup_release(p);

        write_lock_irq(&tasklist_lock);
        ptrace_release_task(p);
        thread_pid = get_pid(p->thread_pid);
@@ -220,6 +218,7 @@ void release_task(struct task_struct *p)
        }

        write_unlock_irq(&tasklist_lock);
+       seccomp_filter_release(p);
        proc_flush_pid(thread_pid);
        put_pid(thread_pid);
        release_thread(p);

Christian