Re: [Qemu-devel] Choosing PCR banks for swtpm's TPM 2

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 06/25/2018 11:25 AM, Daniel P. Berrangé wrote:
> On Mon, Jun 25, 2018 at 11:05:55AM -0400, Stefan Berger wrote:
>> Hi!
>>
>>   I am sending this email to solicit input on the choice of the PCR banks to
>> enable for swtpm's TPM 2. I have currently enabled 4 PCR banks for
>> SHA{1,256,384,512}. The downside of this is that running the TPM 2 with so
>> many PCR banks has a performance impact when the Linux integrity measurement
>> architecture is used and has to extend measurements into all PCR banks,
>> which Linux does already.
>>
>> TPM 2 has the PCR_Allocate() command for a user to select the PCR banks to
>> use. This command allows to make some PCR banks invisible. The change has to
>> be done through the firmware and has the downside that the TPM2 does not
>> support TPM2_Shutdown(SU_STATE) after this command was used. This prevents
>> suspend/resume from working properly. So, it seems that one shouldn't have
>> to use this command, which in turn means the number of PCR banks should be
>> small.
>>
>> Another complication with the swtpm is the upgrade path. Suspended VMs will
>> expect that the PCR banks that were available before the suspend will be
>> available after the resume and a possible swtpm upgrade. This in turn means
>> that the PCR banks should be chosen now and we'll have to stick with them.
> Anything that has a risk of needing to change between versions would need
> to be tied into the machine type in some way.

You mean a machine type like q35? I am not sure how it would be tied 
into QEMU since the swtpm command line options are chosen more or less 
independently of the ones from QEMU.

     Stefan