Re: [tpmdd-devel] [PATCH RFC 2/2] tpm: refactor tpm2_get_tpm_pt to tpm2_getcap_cmd

From: Nayna
Date: Tue Nov 22 2016 - 04:08:54 EST

On 11/18/2016 09:43 PM, Jarkko Sakkinen wrote:
On Fri, Nov 18, 2016 at 05:42:01PM +0530, Nayna wrote:

On 11/17/2016 11:12 PM, Jarkko Sakkinen wrote:
On Thu, Nov 17, 2016 at 05:20:36PM +0530, Nayna wrote:

I tested this for capability TPM2_CAP_PCRS. It seems TPM2_CAP_PCRS
capability always returns full PCR allocation, and more_data as 0, So, I
think the idea of looping over based on more_data may not work for this

You can always request one value at a time until there's no more.

If you request N values, depending on the hardware, the hardware returns
to you anything from 1 to N values. If you implement a function that
requests N values in the command, you *must* handle the case where
moreData is 1 even if the hardware you are testing that never happens.

That's the reason why I would start with a function that you request one
property of one capability and optimize it in future if it doesn't scale
for some workload.

Do you have a workload where it doesn't scale?

Thanks Jarkko for explaining in detail.

If I understood correctly, the idea is to request for one property at a
time, and if we need multiple properties, then to request for each of them
in a loop. In case of TPM2_CAP_PCRS, property is always zero. This is how I
am calling getcap_cmd for TPM2_CAP_PCRS.

tpm2_getcap_cmd(chip, TPM2_CAP_PCRS, 0, &cap_data, "get active pcr banks");

Output :

[ 17.081665] tpm: cap id to receive value is 2
[ 17.081666] tpm: TPM2_CAP_COMMANDS: more data 1
[ 17.081667] tpm: 2
[ 17.081668] tpm: tpm2_get_active_banks -------> cap is TPM2_CAP_PCRS
[ 17.171665] tpm: cap id to receive value is 5
[ 17.171666] tpm: TPM2_CAP_PCRS: more data 0 ---> more data is zero.
[ 17.171666] tpm: TPM2_CAP_PCRS: more data 0
[ 17.171667] tpm: count pcr banks is 2 ------> count of active pcr banks
information returned

more_data is always zero here, so am not sure how to handle more_data in
this case ?
Since property_id is always zero, I am not able to request for one property
at a time.
and response_buffer returns the details for both active banks.

This is the expected behavior defined in TCG 2.0 Part 3 Commands
Specification (Section 30.2.1):

"TPM_CAP_PCRS â Returns the current allocation of PCR in a
TPML_PCR_SELECTION. The property parameter shall be zero. The TPM will
always respond to this command with the full PCR allocation and moreData
will be NO."

Please let me know, if I am missing something.

Thanks for pointing that. I think you got it right and I had some wrong
assumptions about 'moreData'.

Here's what I propose. Do a non-generic function just for getting CAP_PCRS.
You could call it tpm2_get_pcr_allocation() as you don't want or rather
need to handle all the bells and whistles in that TPM command.

It makes a lot more sense now than having one-size-for-all function.

Thanks Jarkko, Yeah, Sure, I will write it as different non-generic call.
Otherwise, the function works good.
Also, I am thinking now I can write "multi-bank support for extend" on top of master branch itself. Any issues with that ?

Thanks & Regards,
- Nayna