Re: [PATCH 0/5] RFC: CGroup Namespaces

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Andy Lutomirski (luto@amacapital.net):
> On Mon, Jul 28, 2014 at 9:51 PM, Serge E. Hallyn <serge@hallyn.com> wrote:
> > Quoting Aditya Kali (adityakali@google.com):
> >> Thank you for your review. I have tried to respond to both your emails here.
> >>
> >> On Thu, Jul 24, 2014 at 9:36 AM, Serge Hallyn <serge.hallyn@ubuntu.com> wrote:
> >> > Quoting Aditya Kali (adityakali@google.com):
> >> >> Background
> >> >>   Cgroups and Namespaces are used together to create “virtual”
> >> >>   containers that isolates the host environment from the processes
> >> >>   running in container. But since cgroups themselves are not
> >> >>   “virtualized”, the task is always able to see global cgroups view
> >> >>   through cgroupfs mount and via /proc/self/cgroup file.
> >> >>
> >> > Hi,
> >> >
> >> > A few questions/comments:
> >> >
> >> > 1. Based on this description, am I to understand that after doing a
> >> >    cgroupns unshare, 'mount -t cgroup cgroup /mnt' by default will
> >> >    still mount the global root cgroup?  Any plans on "changing" that?
> >>
> >> This is suggested in the "Possible Extensions of CGROUPNS" section.
> >> More details below.
> >>
> >> >    Will attempts to change settings of a cgroup which is not under
> >> >    our current ns be rejected?  (That should be easy to do given your
> >> >    patch 1/5).  Sorry if it's done in the set, I'm jumping around...
> >> >
> >>
> >> Currently, only 'cgroup_attach_task', 'cgroup_mkdir' and
> >> 'cgroup_rmdir' of cgroups outside of cgroupns-root are prevented. The
> >> read/write of actual cgroup properties are not prevented. Usual
> >> permission checks continue to apply for those. I was hoping that
> >> should be enough, but see more comments towards the end.
> >>
> >> > 2. What would be the reprecussions of allowing cgroupns unshare so
> >> >    long as you have ns_capable(CAP_SYS_ADMIN) to the user_ns which
> >> >    created your current ns cgroup?  It'd be a shame if that wasn't
> >> >    on the roadmap.
> >> >
> >>
> >> Its certainly on the roadmap, just that some logistics were not clear
> >> at this time. As pointed out by Andy Lutomirski on [PATCH 5/5] of this
> >> series, if we allow cgroupns creation to ns_capable(CAP_SYS_ADMIN)
> >> processes, we may need some kind of explicit permission from the
> >> cgroup subsystem to allow this. One approach could be an explicit
> >
> > So long as you do ns_capable(cgroup_ns->user_ns, CAP_SYS_ADMIN) I think
> > you're fine.
> >
> > The only real problem I can think of with unsharing a cgroup_ns is that
> > you could lock a setuid-root application someplace it wasn't expecting.
> > The above check guarantees that you were privileged enough that you'd
> > better be trusted in this user namespace.
> >
> > (Unless there is some possible interaction I'm overlooking)
> 
> I think that, if it's done this way, you'd have to unshare cgroupns
> before unsharing userns, since you forfeit that capability when you
> unshare your userns.  That means that the new cgroupns ends up being
> associated w/ the root userns, which may not be what you want.
> 
> You could unshare both namespaces in one syscall and give that some
> magic semantics, but that's kind of weird.  It would be nice if you
> could unshare your userns and temporarily retains caps in the parent,
> but there is no such mechanism right now.

Hm, good point.