linux-kernel - Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace

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Date:   Wed, 10 Oct 2018 09:54:58 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Christian Brauner <christian@...uner.io>
Cc:     Jann Horn <jannh@...gle.com>, Paul Moore <paul@...l-moore.com>,
        Kees Cook <keescook@...omium.org>,
        Linux API <linux-api@...r.kernel.org>,
        containers@...ts.linux-foundation.org, suda.akihiro@....ntt.co.jp,
        Oleg Nesterov <oleg@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        selinux@...ho.nsa.gov, Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>
Subject: Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from
 ptrace

On Wed, Oct 10, 2018 at 05:39:57PM +0200, Christian Brauner wrote:
> On Wed, Oct 10, 2018 at 05:33:43PM +0200, Jann Horn wrote:
> > On Wed, Oct 10, 2018 at 5:32 PM Paul Moore <paul@...l-moore.com> wrote:
> > > On Tue, Oct 9, 2018 at 9:36 AM Jann Horn <jannh@...gle.com> wrote:
> > > > +cc selinux people explicitly, since they probably have opinions on this
> > >
> > > I just spent about twenty minutes working my way through this thread,
> > > and digging through the containers archive trying to get a good
> > > understanding of what you guys are trying to do, and I'm not quite
> > > sure I understand it all.  However, from what I have seen, this
> > > approach looks very ptrace-y to me (I imagine to others as well based
> > > on the comments) and because of this I think ensuring the usual ptrace
> > > access controls are evaluated, including the ptrace LSM hooks, is the
> > > right thing to do.
> > 
> > Basically the problem is that this new ptrace() API does something
> > that doesn't just influence the target task, but also every other task
> > that has the same seccomp filter. So the classic ptrace check doesn't
> > work here.
> 
> Just to throw this into the mix: then maybe ptrace() isn't the right
> interface and we should just go with the native seccomp() approach for
> now.

Please no :).

I don't buy your arguments that 3-syscalls vs. one is better. If I'm
doing this setup with a new container, I have to do
clone(CLONE_FILES), do this seccomp thing, so that my parent can pick
it up again, then do another clone without CLONE_FILES, because in the
general case I don't want to share my fd table with the container,
wait on the middle task for errors, etc. So we're still doing a bunch
of setup, and it feels more awkward than ptrace, with at least as many
syscalls, and it only works for your children.

I don't mind leaving capable(CAP_SYS_ADMIN) for the ptrace() part,
though. So if that's ok, then I think we can agree :)

Tycho