Re: [RFC] capabilities: add capability cgroup controller

Kees Cook
2016-06-22T23:06:40Z

On Wed, Jun 22, 2016 at 11:17 AM, Serge E. Hallyn <serge@hallyn.com> wrote:
> Quoting Topi Miettinen (toiwoton@gmail.com):
>> On 06/22/16 17:14, Serge E. Hallyn wrote:
>> > Quoting Topi Miettinen (toiwoton@gmail.com):
>> >> On 06/21/16 15:45, Serge E. Hallyn wrote:
>> >>> Quoting Topi Miettinen (toiwoton@gmail.com):
>> >>>> On 06/19/16 20:01, serge@hallyn.com wrote:
>> >>>>> apologies for top posting, this phone doesn't support inline)
>> >>>>>
>> >>>>> Where are you preventing less privileged tasks from limiting the caps of a more privileged task? It looks like you are relying on the cgroupfs for that?
>> >>>>
>> >>>> I didn't think that aspect. Some of that could be dealt with by
>> >>>> preventing tasks which don't have CAP_SETPCAP to make other tasks join
>> >>>> or set the bounding set. One problem is that the privileges would not be
>> >>>> checked at cgroup.procs open(2) time but only when writing. In general,
>> >>>> less privileged tasks should not be able to gain new capabilities even
>> >>>> if they were somehow able to join the cgroup and also your case must be
>> >>>> addressed in full.
>> >>>>
>> >>>>>
>> >>>>> Overall I'm not a fan of this for several reasons. Can you tell us precisely what your use case is?
>> >>>>
>> >>>> There are two.
>> >>>>
>> >>>> 1. Capability use tracking at cgroup level. There is no way to know
>> >>>> which capabilities have been used and which could be trimmed. With
>> >>>> cgroup approach, we can also keep track of how subprocesses use
>> >>>> capabilities. Thus the administrator can quickly get a reasonable
>> >>>> estimate of a bounding set just by reading the capability.used file.
>> >>>
>> >>> So to estimate the privileges needed by an application? Note this
>> >>> could also be done with something like systemtap, but that's not as
>> >>> friendly of course.
>> >>>
>> >>
>> >> I've used systemtap to track how a single process uses capabilities, but
>> >> I can imagine that without the cgroup, using it to track several
>> >> subprocesses could be difficult.
>> >>
>> >>> Keeping the tracking part separate from enforcement might be worthwhile.
>> >>> If you wanted to push that part of the patchset, we could keep
>> >>> discussing the enforcement aspect separately.
>> >>>
>> >>
>> >> OK, I'll prepare the tracking part first.
>> >
>> > So this does still have some security concerns, namely leaking information
>> > to a less privileged process about what privs a root owned process used.
>> > That's not on the same level as giving away details about memory mappings,
>> > but could be an issue. Kees (cc'd), do you see that as an issue ?
>> >
>> > thanks,
>> > -serge
>> >
>>
>> Anyone can see the full set of capabilities available to each process
>
> But not the capabilities used. That's much more invasive.
>
>> via /proc/pid/status. But should I for example add a new flag
>> CFTYPE_OWNER_ONLY to limit reading capability.used file to only owner
>> (root) and use it here?
>
> Not sure that it's needed, let's see what Kees says. However if it is,
> then using owner would not suffice, since that's tangential to the
> privilege level of the task.

I don't see a problem exposing the history of used capabilities to
less privileged processes. The only thing I could see that being used
for would be to improve some kind of race against a buggy process
where you know caps get used at a certain time in the code, so
spinning on reading /proc/pid/status might give you a better chance of
timing the race. That seems like a pretty far-out exposure, though. I
imagine instruction counters would give a way finer grained timing
too, so I wouldn't object to this being visible.

-Kees

--
Kees Cook
Chrome OS & Brillo Security