Date: Tue, 4 Oct 2016 15:18:40 -0500
From: "Serge E. Hallyn" <serge@hallyn.com>
To: Tejun Heo <tj@kernel.org>
Cc: John Stultz <john.stultz@linaro.org>,
        lkml <linux-kernel@vger.kernel.org>, Li Zefan <lizefan@huawei.com>,
        Jonathan Corbet <corbet@lwn.net>, 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>
Subject: Re: [RFC][PATCH 0/2] Another pass at Android style loosening of
 cgroup attach permissions
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Quoting Tejun Heo (tj@kernel.org):
> Hello, John.
> 
> On Tue, Oct 04, 2016 at 11:01:12AM -0700, John Stultz wrote:
> > So to make sure I understand your suggestion, you're suggesting the
> > cgroupfs files like:
> > cpuctrl/tasks,
> > cpuctrl/bg_non_interactive/tasks,
> > cpuset/foreground/tasks,
> > cpuset/background/tasks,
> > etc
> > use ACL permissions to specify the specific uids that can write to
> > them? I guess this would be conceptually similar to just setting the
> > owner to the system task, no?  Though I'm not sure that would be
> 
> Yeah, finer grained but essentially just giving write perms.
> 
> > sufficient since it would still fail the
> > cgroup_procs_write_permission() checks. Or are you suggesting we add
> > extra logic to make the file owner uid as sufficient to change other
> > tasks?
> 
> Hah, now I'm not sure how this is supposed to work inside a userns as
> it's checking against GLOBAL_ROOT_UID.  cc'ing Serge.  Serge, can you
> please have a look?

Hi, thanks for the cc,

how about changing the GLOBAL_ROOT_UID check with a targeted
capability check, like

	if (!ns_capable(tcred->user_ns, CAP_SYS_NICE) &&
            !uid_eq(cred->euid, tcred->uid) &&
            !uid_eq(cred->euid, tcred->suid))
		ret = -EACCES;

where the actual capability to use may require some thought.


> But back on subject, yeah, I think a capability based approach is
> better here too.  No idea how difficult it is to add a new CAP but I
> think it's worth trying.  Can you please spin up a patch?
> 
> Thanks!
> 
> -- 
> tejun