Re: IMA: Data included in the key measurement

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Thu, 2019-11-21 at 08:17 -0800, Lakshmi Ramasubramanian wrote:
> Hi Mimi,
> 
>  >>> everything needed for verifying a signature is included in
>  >>> the key measurement.
> 
> Regarding the requirement you had stated above, I would like some 
> clarification.
> 
> When I started this change to measure keys through IMA, the use case
> we had in mind was enabling an attestation service, for instance, to
> verify if the client has only known good (trusted) keys - for
> example, in keyrings such as ".builtin_trusted_keys", ".ima", etc.
> 
> On the client IMA verifies the signature of system binaries using
> keys in the IMA keyring. And, if the config namely 
> CONFIG_IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY is
> enabled, only keys signed by a built-in trusted key can be added to
> the IMA keyring.
> 
> An attestation service can keep a list of public keys of "known good 
> (trusted)" keys for various keyrings, and verify against the
> measurement data provided by the client.
> 
> To achieve the above we decided to include only the public key in
> the key measurement buffer.
> 
> I would like to know what benefit we'd get by including "everything 
> needed for verifying a signature in the key measurement"?  X.509
> itself doesn't buy this isomorphism property, which is why the
> subject key id 
> 
>  From testing point of view, if we have the certificate (like the
> .DER file), we can validate the key measurement data in the IMA log.
> 
> Do you see a need to include more data or the entire cert for the 
> product code?

You're making the assumption that the public key and the certificate
are isomorphic.  That's only true if you trust the issuer (which you
obviously do, since it's you [microsoft]) but nothing in X.509 prevents
the issuer from issuing multiple certificates with the same public key
and different properties.  Even in your use case, I would think
attesting to whether the certificate had expired or not would be
useful.

James