Re: [tpmdd-devel] [PATCH v2 0/2] tpm: enhance TPM 2.0 extend function to support multiple PCR banks

[Jarkko Sakkinen]

On Tue, Jan 03, 2017 at 07:27:49AM -0500, Mimi Zohar wrote:
> On Tue, 2017-01-03 at 00:15 +0200, Jarkko Sakkinen wrote:
> > On Fri, Dec 30, 2016 at 02:02:28PM -0500, Nayna Jain wrote:
> > > IMA extends its hash measurements in the TPM PCRs, based on policy.
> > > The existing in-kernel TPM extend function extends only the SHA1
> > > PCR bank. TPM 2.0 defines multiple PCR banks, to support different
> > > hash algorithms. The TCG TPM 2.0 Specification[1] recommends
> > > extending all active PCR banks to prevent malicious users from
> > > setting unused PCR banks with fake measurements and quoting them.
> > > This patch set adds support for extending all active PCR banks,
> > > as recommended.
> 
> For this reason, the change is needed whether we're extending the SHA1
> bank or any of the other TPM 2.0 banks.
> 
> > > The first patch implements the TPM 2.0 capability to retrieve
> > > the list of active PCR banks.
> > > 
> > > The second patch modifies the tpm_pcr_extend() and tpm2_pcr_extend()
> > > interface to support extending multiple PCR banks. The existing
> > > tpm_pcr_extend() interface expects only a SHA1 digest. Hence, to
> > > extend all active PCR banks with differing digest sizes for TPM 2.0,
> > > the SHA1 digest is padded with 0's as needed.
> > > 
> > > This approach is taken to maintain backwards compatibility for IMA
> > > in order to continue working with both TPM 1.2 and TPM 2.0 without
> > > any changes and still comply with TCG TPM 2.0 Specification[1].
> > 
> > What is the plan to improve IMA so that it can use better hash
> > algorithms? For me this zero padding sounds like a hack.
> 
> In one case, we'll be padding the SHA1 hash, while in the other cases we
> would be truncating the hash.   Unfortunately, the need to extend
> multiple banks doesn't go away when IMA supports larger digests.
> 
> Nayna, could you remove this [unnecessary] paragraph?

OK, I think I got it. I'll try to test these patches before further
reviewing. Thank you.

> Mimi

/Jarkko