[tpm2] Choosing PCR banks for swtpm's TPM 2

[tpm2] Choosing PCR banks for swtpm's TPM 2

[Stefan Berger]

[-- Attachment #1: Type: text/plain, Size: 1700 bytes --]

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.

Regards,

    Stefan

Re: [tpm2] Choosing PCR banks for swtpm's TPM 2

[Stefan Berger]

[-- Attachment #1: Type: text/plain, Size: 2840 bytes --]

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
>
>