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

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Date:	Thu, 26 Feb 2015 14:55:12 -0600
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Christoph Lameter <cl@...ux.com>,
	Serge Hallyn <serge.hallyn@...ntu.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 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@...lyn.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?
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