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

[Kees Cook <keescook@chromium.org>]

On Fri, May 29, 2020 at 09:56:41AM +0200, Christian Brauner wrote:
> On Thu, May 28, 2020 at 04:11:00PM -0700, Kees Cook wrote:
> > void seccomp_filter_release(const struct task_struct *tsk)
> > {
> > 	struct seccomp_filter *orig = READ_ONCE(tsk->seccomp.filter);
> > 
> > 	smp_store_release(&tsk->seccomp.filter, NULL);
> 
> I need to go through the memory ordering requirements before I can say
> yay or nay confidently to this. :)
> 
> > 	__seccomp_filter_release(orig);
> > }

The only caller will be release_task() after dethread, so honestly this
was just me being paranoid. I don't think it actually needs the
READ_ONCE() nor the store_release. I think I wrote all that before I'd
convinced myself it was safe to remove a filter then. But I'm still
suspicious given the various ways release_task() gets called... I just
know that if mode 2 is set and filter == NULL, seccomp will fail closed,
so I went the paranoid route. :)

-- 
Kees Cook