What's the difference when booting off a USB drive? That's been my goto in the past when I forgot my login password; does the TPM only unlock boot devices?
replyGenerally you'll have your drive only unlock against certain PCRs and their values. It depends on which PCRs you select and then how exactly they are measured.
replyE.g. systemd measures basically everything that is part of the boot process (kernel, kernel cli, initrd, ...[1]) into different PCRs, so if any of those are different they result in differen PCR values and won't unlock the boot device (depending on which PCRs you decided to encrypt against). I forgot what excatly it measures, but I remember that some PCRs also get measured during the switch_root operation from initrd -> rootfs which can be used to make something only unlock in the initrd.
The TPM holds the decryption keys and will unlock as long as all checks pass. Booting off the previously registered drive/kernel being one of them.
replyIf this fails you can always manually input the decryption key and reregister with the TPM. The whole point of this setup is you can't just use a bootable USB to reset the devices password.