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
