[systemd-devel] Enrolling PCR11 does not work as expected

[Mantas Mikulėnas]

On Wed, Jul 5, 2023 at 2:11 PM Felix Rubio <felix at kngnt.org> wrote:

> For what is explained on the the systemd-pcrphase.service(8) and
> comparing it to what I see in the log of the systemd services, there are
> three events in relation to this question:
>
> systemd-pcrphase-initrd.service
> [...]
> [systemd-ask-password-console.service]
> [...]
> systemd-pcrphase-sysinit
> systemd-pcrphase
>
> This means that, indeed, running cryptenroll after the new kernel has
> booted will never provide the correct PCR registry for 11. But then...
> what options do I have? Do I need to choose between having PCRs 7 and
> 14, so that I make sure that SB is up and running and all the certs from
> shim have not changed, or to have only PCR 11 so that I know that the
> UKI has not changed although SB can potentially be even disabled
> (please, correct me if wrong)?
>

I think the idea is to use `systemd-measure` to precompute PCR 11 for a
specific phase, then use the precomputed PCR value instead of the "live"
PCR value when sealing the data.

systemd-cryptenroll does not accept raw PCR values directly (though I use a
separate python script for that); instead it accepts --tpm2-public-key= as
a public key that could be used to *sign* PCR values, and an external
--tpm2-signature= path that'll contain the signed data.

So I believe you're supposed to use systemd-measure to precompute and sign
PCR 11, put the signed file in /boot, and tell systemd-cryptenroll to use
that when unlocking. (Later you only need to re-sign the PCR measurements
in /boot without needing to re-do cryptenroll.)

-- 
Mantas Mikulėnas
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