linux-kernel - Re: [PATCH] capabilities: Ambient capability set V1

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Date:	Mon, 23 Feb 2015 10:45:57 -0600
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Serge Hallyn <serge.hallyn@...ntu.com>,
	Christoph Lameter <cl@...ux.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Aaron Jones <aaronmdjones@...il.com>, Ted Ts'o <tytso@....edu>,
	LSM List <linux-security-module@...r.kernel.org>,
	Andrew Morton <akpm@...uxfoundation.org>,
	"Andrew G. Morgan" <morgan@...nel.org>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Markku Savela <msa@...h.iki.fi>,
	Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] capabilities: Ambient capability set V1

On Mon, Feb 23, 2015 at 08:33:58AM -0800, Andy Lutomirski wrote:
> On Mon, Feb 23, 2015 at 8:16 AM, Serge Hallyn <serge.hallyn@...ntu.com> wrote:
> > Quoting Christoph Lameter (cl@...ux.com):
> >> Ok 4.0-rc1 is out and this patch has been sitting here for a couple of
> >> weeks without comment after an intensive discussion about the RFCs.
> >>
> >> Since there were no objections: Is there any chance to get this into -next
> >> somehow?
> >
> > Andrew Morgan and Andy Lutomirski appear to have a similar concern
> > but competing ideas on how to address them.  We need them to agree
> > on an approach.
> >
> > The core concern for amorgan is that an unprivileged user not be
> > able to cause a privileged program to run in a way that it fails to
> > drop privilege before running unprivileged-user-provided code.
> >
> > Andy Lutomirski's concern is simply that code which is currently
> > doing the right thing to drop privilege not be run in a way that
> > it thinks it is dropping privilege, but in fact is not.
> >
> 
> I share both concerns.
> 
> > (Please correct me where I've mis-spoken or misunderstood)
> >
> > Since your desire is precisely for a mode where dropping privilege
> > works as usual, but exec then re-gains some or all of that privilege,
> > we need to either agree on a way to enter that mode that ordinary
> > use caes can't be tricked into using, or find a way for legacy
> > users to be tpiped off as to what's going on (without having to be
> > re-written)
> 
> Is there really a need to drop privilege and then regain it or is it
> sufficient to keep the privilege permitted (and perhaps ambient, too)
> and just to have execve not drop it for you?  I assume the latter.

Well right, any perceived security benefit of the temporary drop would
seem to be easily debunked (just run shell for exec /bin/sh to get
around it)

So this is more of a desire, I suspect, for regular programs which
drop privilege to still be usable in this environment.

I think this may be a decent place for a compromise.  Attempts to
drop privilege when ambient caps are set return EPERM.

-serge
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