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

[Daniel P . Berrangé]

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.

> 
> 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.

Regards,
Daniel
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