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

From: Stefan Berger
Subject: Re: [Qemu-devel] Choosing PCR banks for swtpm's TPM 2
Date: Mon, 25 Jun 2018 12:23:17 -0400
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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:

    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

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
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.


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