In a "properly" hardened setup, you would protect the GRUB command line with ... yet another password! :) https://help.ubuntu.com/community/Grub2/Passwords
But see my sibling comment about the possibilities of trivially bypassing Secure Boot.
But see my sibling comment about the possibilities of trivially bypassing Secure Boot.
"Errors in creating a password-protected GRUB 2 menu may result in an unbootable system. To restore a system with broken passwords, access and edit the GRUB 2 configuration files using the LiveCD or another OS."
However, it didn't occur to me that you can just press "e" at the grub prompt, then modify the command line without physical access. "Trivial" indeed.
My current Secure Boot configuration only allows booting a signed GRUB EFI image which contains the configuration. Modifying it on disk would invalidate the signature, causing Secure Boot would fail. My `/boot` isn't encrypted, but each file that GRUB accesses (eg. the initrd image, vmlinux, background.png...) also has a `.sig` file and GRUB refuses to load any unsigned (or invalid) resources. This means that GRUB doesn't need a password to get into the initrd, and I can just enter one password from in there.
Next, I'm considering tying user data decryption to login and allowing the root system to be unlocked by the TPM. It seems like a good compromise to me, as I don't keep persistent data on `/` anyways. The host SSH key will be there, but still protected by the TPM and the above chain.
Edit: This would be better with aggressive measured boot parameters-- I don't care about losing `/` to a tempermental TPM, and that SSH host key is otherwise somewhat vulnerable. I'll have to learn more about measured boot and PCR.
The root FS is then encrypted using clevis to lock to the TPM PCRs (only). I use PCRs 0,2,4,7 for this. So the laptop will boot to a login screen without needing a password.
My home directory is separately encrypted and gets unlocked with the login password using pam_zfs_key. It works pretty well and I'm happy with the security for my threat model (casual theft is really my main concern).
I am very aware that my home directory stays unlocked unless I actually power down the machine though.