Re: [tpmdd-devel] [Linux-ima-devel] [PATCH v3 0/6] Updated API for TPM 2.0 PCR extend

Mimi Zohar
2017-07-05T16:09:05Z

On Wed, 2017-07-05 at 11:18 -0400, Ken Goldman wrote:
> On 6/28/2017 1:28 PM, Jarkko Sakkinen wrote:
> > On Mon, Jun 26, 2017 at 08:33:59AM -0400, Mimi Zohar wrote:
> >> On Sat, 2017-06-24 at 11:03 +0200, Jarkko Sakkinen wrote:
> >>> On Wed, Jun 21, 2017 at 04:29:35PM +0200, Roberto Sassu wrote:

> >>> There's absolutely no need to pass digest size like you do BTW as
> it >>> is defined by the standard.
> >>
> >> For algorithms known to the crypto subsystem, that is fine, but for
> >> the unknown TPM crypto algorithms, we would need to somehow query the
> >> TPM for the digest sizes to create the mapping.
> >>
> >> Mimi
> >
> > There's a TPM command to query TPM algorithms.
>
> This is true - one getcap to determine the number of algorithms, then a
> pcr read, then parse the response structures and match the algorithms to
> sizes.
>
> Alternatively, could you create a table mapping the algorithm to the
> size? There are currently 8 approved algorithms, meaning the table is
> 32 bytes, probably less code than the queries.
>
> As for an algorithm appearing in the TPM that's not in the table, it
> takes a year or more for a new algorithm to appear. Is that enough time
> to patch the device driver?
>
> FYI, the 8 algorithms are:
>
> sha1, sha256, sha384, sha512, sm3-256, sha3-256, sha3-384, sha3-512.
>
> I am only aware of sha1, sha256, and sm3-256 being used in production
> hardware TPMs.

New devices aren't being shipped with the most recent kernels.  So
even if the upstream kernel supports the newer crypto algorithms, that
doesn't imply that it is available.

A safer method would be to query the TPM for the digest sizes.

Mimi