Date:   Mon, 5 Dec 2016 20:00:11 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     John Stultz <john.stultz@...aro.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Mickaël Salaün <mic@...ikod.net>,
        Daniel Mack <daniel@...que.org>,
        "David S. Miller" <davem@...emloft.net>, kafai@...com,
        Florian Westphal <fw@...len.de>,
        Harald Hoyer <harald@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        Sargun Dhillon <sargun@...gun.me>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        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>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [RESEND][PATCH v4] cgroup: Use CAP_SYS_RESOURCE to allow a
 process to migrate other tasks between cgroups

On Mon, Dec 05, 2016 at 04:36:51PM -0800, Andy Lutomirski wrote:
> On Mon, Dec 5, 2016 at 4:28 PM, John Stultz <john.stultz@...aro.org> wrote:
> > On Tue, Nov 22, 2016 at 4:57 PM, John Stultz <john.stultz@...aro.org> wrote:
> >> On Tue, Nov 8, 2016 at 4:12 PM, Andy Lutomirski <luto@...capital.net> wrote:
> >>> On Tue, Nov 8, 2016 at 4:03 PM, Alexei Starovoitov
> >>> <alexei.starovoitov@...il.com> wrote:
> >>>> On Tue, Nov 08, 2016 at 03:51:40PM -0800, Andy Lutomirski wrote:
> >>>>>
> >>>>> I hate to say it, but I think I may see a problem.  Current
> >>>>> developments are afoot to make cgroups do more than resource control.
> >>>>> For example, there's Landlock and there's Daniel's ingress/egress
> >>>>> filter thing.  Current cgroup controllers can mostly just DoS their
> >>>>> controlled processes.  These new controllers (or controller-like
> >>>>> things) can exfiltrate data and change semantics.
> >>>>>
> >>>>> Does anyone have a security model in mind for these controllers and
> >>>>> the cgroups that they're attached to?  I'm reasonably confident that
> >>>>> CAP_SYS_RESOURCE is not the answer...
> >>>>
> >>>> and specifically the answer is... ?
> >>>> Also would be great if you start with specifying the question first
> >>>> and the problem you're trying to solve.
> >>>>
> >>>
> >>> I don't have a good answer right now.  Here are some constraints, though:
> >>>
> >>> 1. An insufficiently privileged process should not be able to move a
> >>> victim into a dangerous cgroup.
> >>>
> >>> 2. An insufficiently privileged process should not be able to move
> >>> itself into a dangerous cgroup and then use execve to gain privilege
> >>> such that the execve'd program can be compromised.
> >>>
> >>> 3. An insufficiently privileged process should not be able to make an
> >>> existing cgroup dangerous in a way that could compromise a victim in
> >>> that cgroup.
> >>>
> >>> 4. An insufficiently privileged process should not be able to make a
> >>> cgroup dangerous in a way that bypasses protections that would
> >>> otherwise protect execve() as used by itself or some other process in
> >>> that cgroup.
> >>>
> >>> Keep in mind that "dangerous" may apply to a cgroup's descendents in
> >>> addition to the cgroup being controlled.
> >>
> >> Sorry for taking awhile to get back to you here.  I'm a little
> >> befuddled as to what next steps I should consider (and honestly, I'm
> >> not totally sure I really grok your concern here, particularly what
> >> you mean with "dangrous cgroups").
> >>
> >> So is going back to the CAP_CGROUP_MIGRATE approach (to properly
> >> separate "sufficiently" from "insufficiently privileged") better?
> >>
> >> Or something closer to the original method Android used of each cgroup
> >> having an allow_attach() check which could determine what is
> >> sufficiently privledged for the respective level of danger the cgroup
> >> might poise?
> >>
> >> Or just stepping back, what method would you imagine to be reasonable
> >> to allow a specified task to migrate other tasks between cgroups
> >> without it having to be root/suid?
> >
> > Any suggested feedback here?
> 
> I really don't know.  The cgroupfs interface is a bit unfortunate in
> that it doesn't really express the constraints.  To safely migrate a
> task, ISTM you ought to have some form of privilege over the task
> *and* some form of privilege over the cgroup.

Agreed.  The problem is that the privilege required should depend on
the controller (I guess).  For memory and cpuset, CAP_SYS_NICE seems
right.  Perhaps CAP_SYS_RESOURCE would be needed for some..  but then,
as I look through the lists (capabilities(7) and the list of controllers),
it seems like CAP_SYS_NICE works for everything.  What else would we need?
Maybe CAP_NET_ADMIN for net_cls and net_prio?  CAP_SYS_RESOURCE|CAP_SYS_ADMIN
for pids?

>   cgroupfs only handles
> the latter.

If we need different checks for different controllers, we can add
checks to cgroupfs.

> CAP_CGROUP_MIGRATE ought to be okay.  Or maybe cgroupfs needs to gain
> a concept of "dangerous" cgroups and further restrict them and
> CAP_SYS_RESOURCE should be fine for non-dangerous cgroups?  I think I
> favor the latter, but it might be nice to hear from Tejun first.
> 
> --Andy

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