From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Serge E. Hallyn" <serge@hallyn.com>
Subject: Re: [PATCH] capabilities: Ambient capability set V1
Date: Thu, 26 Feb 2015 14:55:12 -0600
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To: Andy Lutomirski <luto@amacapital.net>
Cc: "Serge E. Hallyn" <serge@hallyn.com>, Christoph Lameter <cl@linux.com>, Serge Hallyn <serge.hallyn@ubuntu.com>, Serge Hallyn <serge.hallyn@canonical.com>, Aaron Jones <aaronmdjones@gmail.com>, Ted Ts'o <tytso@mit.edu>, LSM List <linux-security-module@vger.kernel.org>, Andrew Morton <akpm@linuxfoundation.org>, "Andrew G. Morgan" <morgan@kernel.org>, Mimi Zohar <zohar@linux.vnet.ibm.com>, Austin S Hemmelgarn <ahferroin7@gmail.com>, Markku Savela <msa@moth.iki.fi>, Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>, Michael Kerrisk <mtk.manpages@gmail.com>, Jonathan Corbet <corbet@lwn.net>
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On Thu, Feb 26, 2015 at 12:51:57PM -0800, Andy Lutomirski wrote:
> On Thu, Feb 26, 2015 at 12:34 PM, Serge E. Hallyn <serge@hallyn.com> wrote:
> > On Thu, Feb 26, 2015 at 02:13:00PM -0600, Christoph Lameter wrote:
> >> On Thu, 26 Feb 2015, Serge E. Hallyn wrote:
> >>
> >> > Andrew Morgan was against that.  What if we changed
> >> >
> >> > pE' = pP' & (fE | pA)
> >> >
> >> > to
> >> >
> >> >     if (pA)
> >> >             pE' = pP' & fE
> >> >     else
> >> >             pE' = pP'
> >> >
> >>
> >> Same problem as before. The ambient bits will not be set in pE'.
> >
> > And what if I weren't scatterbrained and we did
> >
> >         if (pA)
> >                 pE' = pP'
> >         else
> >                 pE' = pP' & fE
> >
> > All pP' bits would be set in pE'.
> 
> That seems reasonable to me, except for my paranoia:
> 
> What if there's a program with CAP_DAC_OVERRIDE in fP and fE set to
> the empty set (i.e. the magic effective bit cleared), and the program
> relies on that.  A malicious user has CAP_NET_BIND and sets pA =
> CAP_NET_BIND.  Boom!
> 
> If we changed that to if (pA') and zeroed pA if fP is non-empty then
> this problem goes away.

Hm, the problem is that then the empty pA is inherited by children.
I do see that any program with fP set should probably run with only
what it requested.  Would

         if (pA && is_empty(fP))
                 pE' = pP'
         else
                 pE' = pP' & fE

help?  Or are you worried about a program with fP set which then
executes other programs?