pcrlock: add support for unlocking a root fs with a pcrlock file

[poettering]

We store the pcrlock policy JSON file in /var/. We need it to unlock LUKS devices that are locked to pcrlock. That means we have to copy the pcrlock policy file somewhere better to be able to unlock the root fs itself (where /var/ typically resides), to avoid the cyclic dependency.

Let's address this by wrapping the policy file in a credential, and writting it to /loader/credentials in the ESP. Then sd-stub will pick it up, and pass it to the initrd where we can consume it.

We disable encryption/authentication for the credential, since we need to access it to unlock TPM-locked objects, and hence cannot lock it to TPM itself. We already had a codepath for that in place, to cover for TPM-less systems, but refused to enable the codepath if we detected a TPM present (simple in order to refuse unauthenticated credentials). Hence we relax the rules here a bit and allow the data to be consumed even with disabled encryption/authentication on explicit request.

All in all behaviour is fully automatic.

(As preparation this converts a good chunk of the tpm2 codebase to use struct iovec for maintaining "blobs" of arbitrary data, instead of manual pairs of void* and size_t.)

[github-actions]

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[poettering]

force pushed a new version with some points addressed, but see discussions above.