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

    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()).

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