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

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Mon, 2013-01-28 at 15:22 -0500, Vivek Goyal wrote:
> 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?

We've just added the ability of defining the method for appraising a
file and defining rules in terms of the filesystem UUID.  Extending the
IMA policy shouldn't be a problem, but I'm not sure how you would go
about adding support for only appraising files with digital signatures.

> 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?

The assumption has always been that the initramfs would be measured, for
trusted boot, and appraised, for secure boot, before being executed.

Mimi