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

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 06/25/2018 11:59 AM, Daniel P. Berrangé wrote:
> 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.

The set of PCR banks a future TPM 2 would be 'manufactured with' would 
be determined by parameters to swtpm_setup. That's when the TPM2 is 
'manufactured' and the certificates are created and written into its 
NVRAM locations. QEMU is not talking to the TPM 2 at this point. So it 
would be parameters passed from libvirt to swtpm_setup that determine 
the set of PCR banks. swtpm itself would get those supplied via command 
line options when invoked by swtpm_setup. If one was to skip over the 
swtpm_setup step, then why not use the swtpm command line options that 
need to be there for swtpm_setup support. Though I think few people will 
use it like that. I would not extend the protocol for this purpose.

    Stefan

>
> Regards,
> Daniel