[betaby]

I did. It says 'From here it’s easy to manually use the TPM to unlock the disk with the Clevis tooling and mount the root volume for hacking (it takes a few tries sometimes, but it gets there in the end)'

However screenshot says 'Unsealing failing'. Yet Ubuntu-lv is mounted. I don't follow how Luks password was guessed for it.

[bri3d]

There wasn't a password necessary, the TPM was an unlocking mechanism.

Secure Boot with TPM-backed disk encryption works off of a series of numbered hashes. The idea of TPM based FDE is that the machine will use Secure Boot to boot only a software chain that the end-user trusts not to contain authentication bypasses. In Secure Boot, the EFI firmware provides hashes of each stage in the boot chain to the TPM, and the TPM only unlocks the full-disk encryption key (really the key encryption key, since the TPM isn't fast enough to actually decrypt the disk) slot if each stage / configuration is valid.

This issue breaks that chain. In some sense it's an illustration of this system being silly conceptually, but it is a real issue IMO.

[j0057]

No LUKS password was guessed, clevis-disk-unlock command in the last screenshot used the TPM to provide a key to a LUKS keyslot for getting at the actual decryption key to decrypt the disk. The TPM should have had information about the boot state to be able to refuse to provide the key, but didn't.