Re: [PATCH 0/4] Trusted Key policy for TPM 2.0

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

On Fri, 2021-05-21 at 16:22 +0100, David Woodhouse wrote:
> On Fri, 2021-05-21 at 07:28 -0700, James Bottomley wrote:
> > On Fri, 2021-05-21 at 14:48 +0100, David Woodhouse wrote:
> > > On Thu, 2021-05-20 at 17:43 -0700, James Bottomley wrote:
> > > > Now that the ASN.1 representation of trusted keys is upstream
> > > > we can add policy to the keys as a sequence of policy
> > > > statements meaning the kernel can now construct and use the
> > > > policy session rather than the user having to do it and pass
> > > > the session down to the kernel.  This makes TPM 2.0 keys with
> > > > policy much easier.
> > > > 
> > > > The format of the policy statements is compatible with the
> > > > openssl_tpm2_engine policy implementation:
> > > > 
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/jejb/openssl_tpm2_engine.git/
> > > > 
> > > > And the seal_tpm2_data command in the above can be used to
> > > > create sealed keys (including with policy statements) for the
> > > > kernel.
> > > 
> > > I'd love to see that format properly defined and documented
> > > instead of just a reference to another implementation.
> > 
> > Well if you want to help me write an RFC, I can try to submit it.
> 
> The xml2rfc tool makes it fairly easy.

XML and easy ... there's two words you don't often see in a sentence
...

> See https://github.com/dwmw2/ietf-cert-best-practice for a template;
> in Appendix B there is an example of specifying an ASN.1 format.

OK, I'll add it to 

https://git.kernel.org/pub/scm/linux/kernel/git/jejb/openssl_tpm2_engine.git/

But I'll be expecting patches ...

> We should probably define not just the ASN.1 format but also a URI
> scheme for referencing objects in NVRAM. A TPMv2 version of 
> https://datatracker.ietf.org/doc/html/draft-mavrogiannopoulos-tpmuri-01
> might be a good idea.

I'm not so sure ... the keys are files not tokens and the pkcs11 URI
doesn't have a file pointer.  I suspect the correct way to represent
them would be to fully represent the key in the URI, which sounds like
a length explosion.

If you really want to use PKCS11 for TPM keys, I've actually got an
engine exporter for any openssl engine key:

https://git.kernel.org/pub/scm/linux/kernel/git/jejb/openssl-pkcs11-export.git/

So you can refer to them with standard PKCS11 URI components instead of
making your own up.  I use it for TPM keys in firefox for instance.

James