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

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Date:   Tue, 13 Feb 2018 13:32:26 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Tycho Andersen <tycho@...ho.ws>
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 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. :)

-Kees

-- 
Kees Cook
Pixel Security