[RFC PATCH 1/2] devices cgroup: allow can_attach() if ns_capable

[Serge Hallyn]

Quoting Tejun Heo (tj at kernel.org):
> On Tue, Jul 23, 2013 at 2:38 PM, Serge Hallyn <serge.hallyn at ubuntu.com> wrote:
> > This doesn't delegate it into the container.  It allows me, on the host,
> > to set the cgroup for a container.
> 
> Hmmm? I'm a bit confused. Isn't the description saying that the patch
> allows pseudo-root in userns to change cgroup membership even if it
> isn't actually root?

If task A is uid 1000 on the host, and creates task B as uid X in a new
user namespace, then task A, still being uid 1000 on the host, is
privileged with respect to B and his namespace - i.e.
ns_capable(B->userns, CAP_SYS_ADMIN) is true.

> Besides, I find the whole check rather bogus and would actually much
> prefer just nuking the check and just follow the standard permission
> checks.

I'd be ok with that - but there's one case I'm not sure about:  If PAM
sets me up with /sys/fs/cgroup/devices/serge owned by me, then if I'm
thinking right, removing can_attach would mean I could move init into
/sys/fs/cgroup/devices/serge...

Is there something else stopping that from happening?

-serge