linux-kernel - Re: [RFC 1/1] ima: digital signature verification using asymmetric keys

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Date:	Mon, 28 Jan 2013 15:22:41 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>,
	dhowells@...hat.com, jmorris@...ei.org,
	linux-security-module@...r.kernel.org,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/1] ima: digital signature verification using asymmetric
 keys

On Mon, Jan 28, 2013 at 03:15:49PM -0500, Mimi Zohar wrote:
> On Mon, 2013-01-28 at 13:56 -0500, Vivek Goyal wrote:
> > On Mon, Jan 28, 2013 at 05:20:20PM +0200, Kasatkin, Dmitry wrote:
> > 
> > [..]
> > > > Ok. I am hoping that it will be more than the kernel command line we
> > > > support. In the sense that for digital signatures one needs to parse
> > > > the signature, look at what hash algorithm has been used  and then
> > > > collect the hash accordingly. It is little different then IMA requirement
> > > > of calculating one pre-determine hash for all files.
> > > 
> > > Yes... It is obvious. It's coming.
> > > But in general, signer should be aware of requirements and limitation
> > > of the platform.
> > > It is not really a problem...
> > 
> > One more question. I specified "ima_appraise_tcb" on kernel command line
> > and I had an unbootable system. It refused to run "init" as it was not
> > labeled/signed. Is there any policy/way where it appraises only signed
> > files and does not refuse to open/execute unsigned ones.
> 
> The policy defines what needs to be measured/appraised, not the other
> way around.  There's nothing preventing you from defining and loading a
> different policy, one to your liking, before pivoting root.

Hi Mimi,

By policy you mean ima rules here? So I can either enable default rules
(tcb default rules for appraisal and measurement) by using kernel command
line options or dynamically configure my own rules using /sysfs interface?

If yes, AFAIK, existing inputtable policies do not allow this selective
mode where we do appraisal only on signed executable. That means I shall
have to extend the way policies can be specified so that one specify
that appraise only signed files?

Also given the fact that we allow loading policy from initramfs, root
can rebuild initramfs and change the policy which takes effect over next
reboot. So in priciple this works only when we are trying to impose some
policy on non-root users?

Thanks
Vivek
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