My laptop is still running the same Arch Linux install from January this year, including full disk encryption with TPM2 and fido2 keys. With multiple unlock options, it usually defaults to just the TPM with a PIN, so that's what I enter every time I boot the machine.
Last night, I was updating my machine, and also decided I'd check for firmware updates:
1fwupdmgr refresh
2fwupdmgr update
It said it needed a reboot, so I let it do that. Upon rebooting, I entered the TPM2 PIN as usual, but was then prompted for the disk password... Which was when I realized I don't remember what the password was.
15 minutes of trying passwords later, I had almost given up hope, and dug out the live usb I still had. I was going to try unlocking the partition:
1cryptsetup open /dev/nvme0n1p2 root
This time, I was prompted for verification from my fido2 key, and I saw a way forward. I could wipe the slot with the passphrase I forgot and add a new passphrase by authenticating with a token.
1cryptsetup luksKillSlot /dev/nvme0n1p2 0 --token-only
2cryptsetup luksAddKey /dev/nvme0n1p2 --key-slot 0 --token-only
This allowed me reboot into my normal environment.
As I was reviewing the config for TPMs using systemd-cryptsetup,
it finally clicked what happened:
the TPM key for unlocking the disk was bound to PCR7,
which includes the EFI firmware in its measurement,
firmware which I had updated.
So by updating the firmware,
the PCR value changed, invalidating the key,
requiring a separate unlock method to proceed
(which I had conveniently forgotten).
To recover, I wiped the slot with the TPM, and reenrolled it:
1systemd-cryptenroll --wipe-slot 1 /dev/nvme0n1p2
2systemd-cryptenroll --tpm2-device auto --tpm2-with-pin true
What I wasn't sure of was why it wouldn't prompt for fido2 tokens on boot.