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

Tycho Andersen
2018-10-10T16:55:05Z

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@paul-moore.com> wrote:
> > > On Tue, Oct 9, 2018 at 9:36 AM Jann Horn <jannh@google.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