linux-kernel - Re: [RFC 3/3] seccomp: add a way to get a listener fd from ptrace

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Date:   Wed, 14 Feb 2018 08:33:59 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>
Subject: Re: [RFC 3/3] seccomp: add a way to get a listener fd from ptrace

On Tue, Feb 13, 2018 at 01:32:26PM -0800, Kees Cook wrote:
> On Sun, Feb 4, 2018 at 2:49 AM, Tycho Andersen <tycho@...ho.ws> wrote:
> > As an alternative to SECCOMP_FILTER_FLAG_GET_LISTENER, perhaps a ptrace()
> > version which can acquire filters is useful. There are at least two reasons
> > this is preferable, even though it uses ptrace:
> >
> > 1. You can control tasks that aren't cooperating with you
> > 2. You can control tasks whose filters block sendmsg() and socket(); if the
> >    task installs a filter which blocks these calls, there's no way with
> >    SECCOMP_FILTER_FLAG_GET_LISTENER to get the fd out to the privileged task.
> 
> I got worried for a second that this would get us into a many-to-many
> state, but I see init_listener enforces a single listener per filter.
> Whew. Seems legit. :)

Yes, although if you sendmsg() the listener fd, you could still get
into that state, so it's still maybe a concern?

Tycho