Re: [PATCH v5] cgroup: Add new capability to allow a process to migrate other tasks between cgroups

[John Stultz]

On Tue, Dec 13, 2016 at 9:48 AM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
> On 12/13/2016 9:24 AM, John Stultz wrote:
>> On Tue, Dec 13, 2016 at 9:17 AM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>>> On 12/13/2016 8:49 AM, John Stultz wrote:
>>>> On Tue, Dec 13, 2016 at 8:39 AM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>>>>> On 12/13/2016 1:47 AM, Michael Kerrisk (man-pages) wrote:
>>>>>> How about CAP_CGROUP_CONTROL or some such, with the idea that this
>>>>>> might be a capability that allows the holder to step outside usual
>>>>>> cgroup rules? At the moment, that capability would allow only one such
>>>>>> step, but maybe there would be others in the future.
>>>>> I agree, but want to put it more strongly. The granularity of
>>>>> capabilities can never be fine enough for some people, and this
>>>>> is an example of a case where you're going a bit too far. If the
>>>>> use case is Android as you say, you don't need this. As my friends
>>>>> on the far side of the aisle would say, "just write SELinux policy"
>>>>> to correctly control access as required.
>>>> So.. The trouble is that while selinux is good for restricting
>>>> permissions, the in-kernel permission checks here are already too
>>>> restrictive.
>>> Why did the original authors of cgroups make it that restrictive?
>>> If there isn't a good reason, loosen it up. If there is a good
>>> reason, then pay heed to it.
>> That's what this patch is proposing. And I agree with Michael that the
>> newly proposed cap was a bit to narrowly focused on my immediate use
>> case, and broadening it to CGROUP_CONTROL is smart. Then that
>> capability could be further restricted w/ selinux policy, as you
>> suggest.
>
> Adding a new capability is unnecessary. The current use of CAP_SYS_NICE,
> while arguably obscure, provides as much "security" as a new capability
> does. While cgroups are a wonderful thing, they don't need a separate
> capability.

The trouble is that CAP_SYS_NICE or _RESOURCE (which was tried in an
earlier version of this patch) aren't necessarily appropriate for
non-android systems. See Andy's objection here:
https://lkml.org/lkml/2016/11/8/946

thanks
-john