linux-kernel - Re: [RFC] capabilities: add capability cgroup controller

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Message-ID: <20160622181713.GA1469@mail.hallyn.com>
Date:	Wed, 22 Jun 2016 13:17:13 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Topi Miettinen <toiwoton@...il.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>, linux-kernel@...r.kernel.org,
	Kees Cook <keescook@...omium.org>
Subject: Re: [RFC] capabilities: add capability cgroup controller

Quoting Topi Miettinen (toiwoton@...il.com):
> On 06/22/16 17:14, Serge E. Hallyn wrote:
> > Quoting Topi Miettinen (toiwoton@...il.com):
> >> On 06/21/16 15:45, Serge E. Hallyn wrote:
> >>> Quoting Topi Miettinen (toiwoton@...il.com):
> >>>> On 06/19/16 20:01, serge@...lyn.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.