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

From: Tycho Andersen <tycho@tycho.ws>
To: Kees Cook <keescook@chromium.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Oleg Nesterov <oleg@redhat.com>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	"Serge E . Hallyn" <serge@hallyn.com>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Tyler Hicks <tyhicks@canonical.com>,
	Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Subject: Re: [RFC 3/3] seccomp: add a way to get a listener fd from ptrace
Date: Wed, 14 Feb 2018 08:33:59 -0700	[thread overview]
Message-ID: <20180214153359.6wj6wclsqvgj4jlt@smitten> (raw)
In-Reply-To: <CAGXu5jLS2dzCjZOKa-W4kUdOPoJkRAq5Rsw1t5jX99v34yaoQw@mail.gmail.com>

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@tycho.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