On 11/7/2018 4:41 AM, Roberto Sassu wrote:

On 11/7/2018 7:14 AM, Nayna Jain wrote:

In the TPM Commands specification (section 30.2.1), I found:

TPM_CAP_PCRS – Returns the current allocation of PCR in a
TPML_PCR_SELECTION.

You mentioned:

#TPM_RC_SIZE response code when count is greater
than the possible number of banks

but TPML_PCR_SELECTION is provided by the TPM.

Roberto

[snip]

quoted

As per my understanding, the count in the TPML_PCR_SELECTION represent 
the number of possible banks and not the number of active banks.
TCG Structures Spec for TPM 2.0 - Table 102 mentions this as 
explanation of #TPM_RC_SIZE.

FYI: This was clarified in the TCG's TPM work group today.  TPM_CAP_PCRS 
returns:

The TPML_PCR_SELECTION must include a TPMS_PCR_SELECTION for each PCR 
bank in which there is at least one allocated PCR. The 
TPML_PCR_SELECTION may return a TPMS_PCR_SELECTION for each implemented 
PCR bank.  The TPML_PCR_SELECTION may return a TPMS_PCR_SELECTION for 
each implemented hash algorithm.

Also:

The TPM doesn't use the term "active banks"

Allocated = a bank that has at least one PCR bit set in the selection 
bitmap.

Supported or implemented banks = the number of PCR banks that can be 
allocated, based on the TPM hardware.

Hash algorithms = The hash algorithms supported by the TPM

For example, the TPM may support 3 hash algorithms and 2 PCR banks, and 
have 1 bank allocated.