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

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Date:   Tue, 13 Dec 2016 10:32:14 -0800
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     Michael Kerrisk <mtk.manpages@...il.com>,
        lkml <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Rom Lemarchand <romlem@...roid.com>,
        Colin Cross <ccross@...roid.com>,
        Dmitry Shmidt <dimitrysh@...gle.com>,
        Todd Kjos <tkjos@...gle.com>,
        Christian Poetzsch <christian.potzsch@...tec.com>,
        Amit Pundir <amit.pundir@...aro.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Kees Cook <keescook@...omium.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...capital.net>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v5] cgroup: Add new capability to allow a process to
 migrate other tasks between cgroups

On 12/13/2016 10:13 AM, John Stultz wrote:
> On Tue, Dec 13, 2016 at 9:48 AM, Casey Schaufler <casey@...aufler-ca.com> wrote:
>> On 12/13/2016 9:24 AM, John Stultz wrote:
>>> On Tue, Dec 13, 2016 at 9:17 AM, Casey Schaufler <casey@...aufler-ca.com> wrote:
>>>> On 12/13/2016 8:49 AM, John Stultz wrote:
>>>>> On Tue, Dec 13, 2016 at 8:39 AM, Casey Schaufler <casey@...aufler-ca.com> 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

Then we need to see what those as-yet-unimplemented systems
require and how to address them. I don't think that taking
the "someone might want it" approach is really appropriate.

>
> thanks
> -john
>