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

Andy Lutomirski luto at amacapital.net
Tue Oct 21 05:49:02 UTC 2014


On Mon, Oct 20, 2014 at 10:42 PM, Eric W. Biederman
<ebiederm at xmission.com> wrote:
> Andy Lutomirski <luto at amacapital.net> writes:
>
>> On Mon, Oct 20, 2014 at 9:49 PM, Eric W. Biederman
>> <ebiederm at xmission.com> wrote:
>>> Andy Lutomirski <luto at amacapital.net> writes:
>>>> 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.
>>
>> This makes the security impact of cgroupns very easy to understand,
>> right?  Because there really won't be any -- cgroupns only affects
>> reads from /proc and what cgroupfs shows, but it doesn't change any
>> actual cgroups, nor does it affect any cgroup *changes*.
>
> It seems like what we have described is chcgrouproot aka chroot for
> cgroups.  At which point I think there are potentially similar security
> issues as for chroot.  Can we confuse a setuid root process if we make
> it's cgroup names look different.
>
> Of course the confusing root concern is handled by the usual namespace
> security checks that are already present.

I think that the chroot issues are mostly in two categories: setuid
confusion (not an issue here as you described) and chroot escapes.
cgroupns escapes aren't a big deal, I think -- admins should deny the
confined task the right to write to cgroupfs outside its hierarchy, by
setting cgroupfs permissions appropriately and/or avoiding mounting
cgroupfs outside the hierarchy.

>
> I do wonder if we think of this as chcgrouproot if there is a simpler
> implementation.

Could be.  I'll defer to Aditya for that one.

>
>>>> 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.
>>>
>>
>> It seems easy enough for nsenter to change the cgroup all by itself.
>
> Again.  I don't think anyone has suggested or implemented anything
> different.

The current patchset seems to punt on this decision by just failing
the setns call if the caller is outside the cgroup in question.

--Andy


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