Re: [PATCHv1 7/8] cgroup: cgroup namespace setns support

On Sat, Oct 18, 2014 at 10:23 PM, Eric W. Biederman
[off-list ref] wrote:

"Serge E. Hallyn" [off-list ref] writes:

quoted

Quoting Aditya Kali (adityakali-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org):

quoted

On Thu, Oct 16, 2014 at 2:12 PM, Serge E. Hallyn [off-list ref] wrote:

quoted

Quoting Aditya Kali (adityakali-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org):

quoted

setns on a cgroup namespace is allowed only if
* task has CAP_SYS_ADMIN in its current user-namespace and
  over the user-namespace associated with target cgroupns.
* task's current cgroup is descendent of the target cgroupns-root
  cgroup.

What is the point of this?

If I'm a user logged into
/lxc/c1/user.slice/user-1000.slice/session-c12.scope and I start
a container which is in
/lxc/c1/user.slice/user-1000.slice/session-c12.scope/x1
then I will want to be able to enter the container's cgroup.
The container's cgroup root is under my own (satisfying the
below condition0 but my cgroup is not a descendent of the
container's cgroup.

This condition is there because we don't want to do implicit cgroup
changes when a process attaches to another cgroupns. cgroupns tries to
preserve the invariant that at any point, your current cgroup is
always under the cgroupns-root of your cgroup namespace. But in your
example, if we allow a process in "session-c12.scope" container to
attach to cgroupns root'ed at "session-c12.scope/x1" container
(without implicitly moving its cgroup), then this invariant won't
hold.

Oh, I see.  Guess that should be workable.  Thanks.

Which has me looking at what the rules are for moving through
the cgroup hierarchy.

As long as we have write access to cgroup.procs and are allowed
to open the file for write, we can move any of our own tasks
into the cgroup.  So the cgroup namespace rules don't seem
to be a problem.

Andy can you please take a look at the permission checks in
__cgroup_procs_write.

The actual requirements for calling that function haven't changed,
right?  IOW, what does this have to do with cgroupns?  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.

--Andy

As I read the code I see 3 security gaffaws in the permssion check.
- Using current->cred instead of file->f_cred.
- Not checking tcred->euid.
- Checking GLOBAL_ROOT_UID instead of having a capable call.

The file permission on cgroup.procs seem just sufficient to keep
to keep those bugs from being easily exploitable.

Eric

-- 
Andy Lutomirski
AMA Capital Management, LLC