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

[Dr. David Alan Gilbert]

* Stefan Berger (address@hidden) 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.
> 
> That said, my suggestion would be to enable only PCR banks for SHA256 for
> 'now' and SHA512 for the future. Having two PCR banks should enable decent
> performance. If someone wants to have better performance he will have to go
> through the firmware to select the PCR banks at the expense of loosing
> suspend/resume support.
> 
> The change of PCR banks for the current 4 PCR banks will break the state of
> all swtpms.
> 
> If you have suggestions, please let me know.

Is this something that has to be set at compile time or could it be
something chosen at run time (as options to the swtpm command line?)

Dave
> Regards,
> 
>    Stefan
> 
> 
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK