Re: [PATCH] capabilities: Ambient capability set V1

From: "Serge E. Hallyn" <serge@hallyn.com>
To: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Serge Hallyn <serge.hallyn@ubuntu.com>,
	Christoph Lameter <cl@linux.com>,
	Serge Hallyn <serge.hallyn@canonical.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Aaron Jones <aaronmdjones@gmail.com>, "Ted Ts'o" <tytso@mit.edu>,
	linux-security-module@vger.kernel.org, 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-api@vger.kernel.org,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH] capabilities: Ambient capability set V1
Date: Tue, 24 Feb 2015 09:47:15 -0600	[thread overview]
Message-ID: <20150224154715.GA20682@mail.hallyn.com> (raw)
In-Reply-To: <20150224051928.GA14755@mail.hallyn.com>

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@linux.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.