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

[Stefan Berger]

On 06/25/2018 11:05 AM, 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.

Actually that was my interpretation of the specs and from what it looks 
like I was wrong assuming that once PCR_Allocate() was used that 
TPM2_Shutdown(SU_STATE) cannot be used anymore at all. The text is a bit 
ambiguous about it. This command can be sent, but the machine needs to 
be rebooted and with that the TPM 2 reset.

The next issue is that the IBM TSS2 is hard coded for 3 PCR banks and a 
few commands are breaking because of that. Now the solution would be to:

- compile-time disable the SHA512 bank; this will break existing state 
but for as long as it's in preview, I hope this is ok; we cannot easily 
enable SHA512 then in the future.

- swtpm_setup gets a --pcr-banks <PCR banks> option that 
PCR_Allocate()'s the active PCR banks for the swtpm. The default will be 
SHA 1 and SHA 256, which disables the SHA 384 PCR bank; Users can choose 
their banks if they run this command directly. SHA1 and SHA256 seems to 
be a reasonable set of active PCR banks for now.

   Stefan



> 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.
>
> Regards,
>
>    Stefan