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

Eric W. Biederman ebiederm at xmission.com
Tue Oct 21 05:42:26 UTC 2014


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:
>>
>>> On Sun, Oct 19, 2014 at 9:55 PM, Eric W.Biederman <ebiederm at xmission.com> wrote:
>>>>
>>>>
>>>> On October 19, 2014 1:26:29 PM CDT, Andy Lutomirski <luto at amacapital.net> wrote:
>>
>>>>> 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.
>
> 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 do wonder if we think of this as chcgrouproot if there is a simpler
implementation.

>>> 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.

Eric


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