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

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Date:   Wed, 19 Oct 2016 16:51:11 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     John Stultz <john.stultz@...aro.org>,
        lkml <linux-kernel@...r.kernel.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>,
        Ricky Zhou <rickyz@...omium.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Todd Kjos <tkjos@...gle.com>,
        Christian Poetzsch <christian.potzsch@...tec.com>,
        Amit Pundir <amit.pundir@...aro.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] cgroup: Add new capability to allow a process to migrate
 other tasks between cgroups

Hello, Andy.

On Mon, Oct 17, 2016 at 03:40:37PM -0700, Andy Lutomirski wrote:
> > @@ -2856,7 +2856,8 @@ static int cgroup_procs_write_permission(struct task_struct *task,
> >          */
> >         if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
> >             !uid_eq(cred->euid, tcred->uid) &&
> > -           !uid_eq(cred->euid, tcred->suid))
> > +           !uid_eq(cred->euid, tcred->suid) &&
> > +           !ns_capable(tcred->user_ns, CAP_CGROUP_MIGRATE))
> >                 ret = -EACCES;
> 
> This logic seems rather confused to me.  Without this patch, a user
> can write to procs if it's root *or* it matches the target uid *or* it
> matches the target suid.  How does this make sense?  How about
> ptrace_may_access(...) || ns_capable(tcred->user_ns,
> CAP_CGROUP_MIGRATE)?

Yeah, it's weird.  The problem is that there was no delegation model
defined on v1 and it used a hybrid of file + ptracey access checks.
The goal, I think, was disallowing !root user from pulling in random
tasks into a cgroup it has write access to, which was possible because
there was no isolation on the delegation boundary.

Given how long it has been out in the wild, I don't think changing the
logic is a good idea.  We should simply replace GLOBAL_ROOT_UID test
with CAT_WHATEVER_WE_PICK test and just ignore the whole thing on v2.

Thanks.

-- 
tejun