linux-kernel - Re: [PATCH 1/2] seccomp: notify user trap about unused filter

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Message-ID: <20200527153504.d7va44xn75fbzyrr@wittgenstein>
Date:   Wed, 27 May 2020 17:35:04 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Matt Denton <mpdenton@...gle.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Jann Horn <jannh@...gle.com>, Chris Palmer <palmer@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Robert Sesek <rsesek@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH 1/2] seccomp: notify user trap about unused filter

On Wed, May 27, 2020 at 09:25:59AM -0600, Tycho Andersen wrote:
> On Wed, May 27, 2020 at 01:19:01PM +0200, Christian Brauner wrote:
> > +void seccomp_filter_notify(const struct task_struct *tsk)
> > +{
> > +	struct seccomp_filter *orig = tsk->seccomp.filter;
> > +
> > +	while (orig && refcount_dec_and_test(&orig->live)) {
> > +		if (waitqueue_active(&orig->wqh))
> > +			wake_up_poll(&orig->wqh, EPOLLHUP);
> > +		orig = orig->prev;
> > +	}
> > +}
> 
> Is there a reason this can't live in put_seccomp_filter()?

put_seccomp_filter() is called from free_task() which in turn gets
called via delayed_put_task_struct through call_rcu() so the
notification will happen at some point whereas you'd often want the
notification to happen at reliable point. This is why most of that stuff
happens in release_task() which is called in the exit path when the task
is finally marked as dead. This is similar to how cgroup_release() is
called from release_task() whereas cgroup_free() is called from
free_task() or other stuff.