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


From: Daniel P . Berrangé
Subject: Re: [Qemu-devel] Choosing PCR banks for swtpm's TPM 2
Date: Mon, 25 Jun 2018 16:59:33 +0100
User-agent: Mutt/1.10.0 (2018-05-17)

On Mon, Jun 25, 2018 at 11:56:24AM -0400, Stefan Berger wrote:
> On 06/25/2018 11:25 AM, Daniel P. Berrangé wrote:
> > 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.
> 
> You mean a machine type like q35? I am not sure how it would be tied into
> QEMU since the swtpm command line options are chosen more or less
> independently of the ones from QEMU.

Yes, each QEMU release introduces a new versioned machine type eg
  q35-2.10,  q35-2.11, q35-2.12, q35-3.0

If anything in QEMU changes which impacts live migraiton/save/restore/etc
then we tie it to the versioned machine type. so q35-3.0 would get the
new default value, and all previous machine types keep the old default
value.

For this to be possible with externally launched swtpm though, would
require some way for QEMU to talk to swtpm to tell it what default
to use for this. I don't know enough about swtpm to have an idea how
practical this is or not.

Regards,
Daniel
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