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

From: John Stultz <john.stultz@linaro.org>
To: Casey Schaufler <casey@schaufler-ca.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>,
	lkml <linux-kernel@vger.kernel.org>, Tejun Heo <tj@kernel.org>,
	Li Zefan <lizefan@huawei.com>, Jonathan Corbet <corbet@lwn.net>,
	"open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>,
	Android Kernel Team <kernel-team@android.com>,
	Rom Lemarchand <romlem@android.com>,
	Colin Cross <ccross@android.com>,
	Dmitry Shmidt <dimitrysh@google.com>,
	Todd Kjos <tkjos@google.com>,
	Christian Poetzsch <christian.potzsch@imgtec.com>,
	Amit Pundir <amit.pundir@linaro.org>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	"Serge E . Hallyn" <serge@hallyn.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH v5] cgroup: Add new capability to allow a process to migrate other tasks between cgroups
Date: Tue, 13 Dec 2016 10:13:19 -0800	[thread overview]
Message-ID: <CALAqxLUbrO+vzrwgK8_yZS-uru7redOdJCAe2=yA34gwPGC2CQ@mail.gmail.com> (raw)
In-Reply-To: <4c60e1be-c00a-5f26-f5de-7d32b9cb0f62@schaufler-ca.com>

On Tue, Dec 13, 2016 at 9:48 AM, Casey Schaufler <casey@schaufler-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@schaufler-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@schaufler-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

thanks
-john