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

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Date:	Tue, 24 Feb 2015 09:47:15 -0600
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	Serge Hallyn <serge.hallyn@...ntu.com>,
	Christoph Lameter <cl@...ux.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Andy Lutomirski <luto@...capital.net>,
	Aaron Jones <aaronmdjones@...il.com>, Ted Ts'o <tytso@....edu>,
	linux-security-module@...r.kernel.org, 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-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 11:19:29PM -0600, Serge E. Hallyn wrote:
> On Mon, Feb 23, 2015 at 06:15:53PM +0000, Serge Hallyn wrote:
> > Quoting Christoph Lameter (cl@...ux.com):
> > > On Mon, 23 Feb 2015, Serge E. Hallyn wrote:
> > > 
> > > > > I do not see a problem with dropping privilege since the ambient set
> > > > > is supposed to be preserved across a drop of priviledge.
> > > >
> > > > Because you're tricking the program into thinking it has dropped
> > > > the privilege, when in fact it has not.
> > > 
> > > So the cap was dropped from the cap perm set but it is still active
> > > in the ambient set?
> > 
> > Right, and the legacy program doesn't know to check the new set.
> 
> we've been assuming the ambient set must be like fP.  is there any
> reason why it doesn't suffice for them to be or'ed with fI instead at
> exec?  then the bits would need to ne in pI. this might sufice for
> Christoph's use case, as pI will generally not change.  and for programs
> that really care, they can check pI.

The other way to look at it then is that it's basically as though the
privileged task (which has CAP_SETFCAP) could've just added fI=full to
all binaries on the filesystem;  instead it's using the ambient set
so that the risk from fI=full is contained to its own process tree.
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