Andy Lutomirski [off-list ref] writes:

On Sun, Oct 19, 2014 at 9:55 PM, Eric W.Biederman [off-list ref] wrote:

quoted

On October 19, 2014 1:26:29 PM CDT, Andy Lutomirski [off-list ref] wrote:

quoted

quoted

Is the idea
that you want a privileged user wrt a cgroupns's userns to be able to
use this?  If so:

Yes, that current_cred() thing is bogus.  (Actually, this is probably
exploitable right now if any cgroup.procs inode anywhere on the system
lets non-root write.)  (Can we have some kernel debugging option that
makes any use of current_cred() in write(2) warn?)

We really need a weaker version of may_ptrace for this kind of stuff.
Maybe the existing may_ptrace stuff is okay, actually.  But this is
completely missing group checks, cap checks, capabilities wrt the
userns, etc.

Also, I think that, if this version of the patchset allows non-init
userns to unshare cgroupns, then the issue of what permission is
needed to lock the cgroup hierarchy like that needs to be addressed,
because unshare(CLONE_NEWUSER|CLONE_NEWCGROUP) will effectively pin
the calling task with no permission required.  Bolting on a fix later
will be a mess.

I imagine the pinning would be like the userns.

Ah but there is a potentially serious issue with the pinning.
With pinning we can make it impossible for root to move us to a different cgroup.

I am not certain how serious that is but it bears thinking about.
If we don't implement pinning we should be able to implent everything with just filesystem mount options, and no new namespace required.

Sigh.

I am too tired tonight to see the end game in this.

Possible solution:

Ditch the pinning.  That is, if you're outside a cgroupns (or you have
a non-ns-confined cgroupfs mounted), then you can move a task in a
cgroupns outside of its root cgroup.  If you do this, then the task
thinks its cgroup is something like "../foo" or "../../foo".

Of the possible solutions that seems attractive to me, simply because
we sometimes want to allow clever things to occur.

Does anyone know of a reason (beyond pretty printing) why we need
cgroupns to restrict the subset of cgroups processes can be in?

I would expect permissions on the cgroup directories themselves, and
limited visiblilty would be (in general) to achieve the desired
visiblity.

While we're at it, consider making setns for a cgroupns *not* change
the caller's cgroup.  Is there any reason it really needs to?

setns doesn't but nsenter is going to need to change the cgroup 
if the pinning requirement is kept.  nsenenter is going to want to
change the cgroup if the pinning requirement is dropped.

Eric