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

[Stefan Berger]

On 06/25/2018 12:11 PM, Dr. David Alan Gilbert wrote:
> * Stefan Berger (address@hidden) wrote:
> > On 06/25/2018 11:29 AM, Dr. David Alan Gilbert wrote:
> > > * Stefan Berger (address@hidden) wrote:
> > > > On 06/25/2018 11:18 AM, Dr. David Alan Gilbert wrote:
> > > > > * 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?)
> > > > It is a compile-time option...
> > > Hmm, that's a shame - I was hoping you'd be able to switch them at
> > > runtime (or at least hide them?) then you can solve the upgrade problem
> > > by running the new swtpm with a flag telling it to hide the new banks.
> > > I hope the ondisk formats for suspend/resume/migration are descriptive
> > > enough to be able to spot an error if you try and load one configured
> > > differently.4
> > The disk format does detect it and refuses to take the state if either there
> > are too many PCR banks or not enough.
> What happens if there are the right number just the wrong type?

The state would be rejected since they are incompatible.

> > For the initial version of swtpm we would need to define a default set of
> > PCR banks since the TPM 2 code uses compile time options to build in that
> > set of PCR banks.
> You talk of PCR_Allocate() above as a spec-defined command to hide PCRs
> but with the downside of breaking TPM2_Shutdown - could you implement
> something from the commandline without that downside (I don't know how
> PCR banks work).

Like I said, during swtpm_setup TPM 2 manufacturing the set of PCR banks 
would be chosen and that set would be used by that TPM 2 from then on. 
Though this flexibility is not supported by the code today.


> > A future version of swtpm could expose command line options for selecting
> > the PCR banks an instance of swtpm is to run with. libtpms would be compiled
> > with support for all of them and only the chosen subset would be active
> > starting with the initial creation of a particular instance of swtpm.
> Right, that would solve the upgrade half of the problem.

For now I think an initial set of banks to go with would be appropriate.

   Stefan