[patrakov]

Hello, this is a security auditor. Could you please confirm items on the following checklist?

1. A BIOS password exists, with at least 8 characters, not based on a single dictionary word or keyboard-run sequence, and not easily guessable in other ways.

2. Booting an OS from any non-default device requires entering the BIOS password.

3. GRUB entry for bringing up the firmware configuration does not bypass the password.

4. GRUB itself has a password defined, with similar password strength requirements.

5. Editing the kernel command line or accessing the raw GRUB command prompt requires the password and, likewise, cannot be used to boot kernel/initrd pairs from external media.

[gradschool]

I don't trust bios passwords. There's probably some jumper on the board that bypasses them or a previously planted hardware keylogger. I always boot off separately stored immutable rescue media whenever the machine has been out of my custody and checksum the whole boot device. Your move.

[patrakov]

Sorry, this is not a valid answer - you can checksum the boot device as much as you like, but I have not seen any procedure that ensures that you know the correct checksum. What could have helped you is not just a checksum, but a signature used with dm-verity.

What I do on my laptop is:

* BIOS password, of course. Note that this also prevents the attacker from resetting or turning off Secure Boot.

* Secure Boot enabled, with my keys only (no Microsoft keys).

* No GRUB, I use systemd-boot (part of systemd) and UKIs signed with my own key. Although, as I don't dual-boot, this might be an unneeded step. In any case, with Secure Boot enabled, systemd-boot does not allow editing kernel command line arguments at all and so cannot trick my UKI into doing anything else than what it is supposed to do.

* The main SSD partition is encrypted (with the passphrase that I have to type).

* The USB storage driver is not in the initramfs, so the storage device found by the initramfs is guaranteed to be my internal SSD.

* The Secure Boot keys are stored inside that encrypted partition and are used to dynamically sign new sd-boot releases and UKIs. I guarantee that I don't sign anything except UKIs that ask for my SSD password, and any shell-out possibility is treated as a bug.

* There is a separate set of keys for signing the custom rescue media, which is also a big UKI.

This way, the attacker cannot boot anything other than my UKI (first, because of the BIOS password, and second, because Secure Boot won't allow anything else), cannot obtain a shell by running something before I enter the password, and, therefore, cannot clone or overwrite my disk.

Note that this setup has also been criticized as insecure ("you don't use TPM, so you can't be secure, it's a theorem"), but I don't understand this argument.

As for the hardware keyloggers, you are, of course, right.

[NekkoDroid]

> * The USB storage driver is not in the initramfs, so the storage device found by the initramfs is guaranteed to be my internal SSD.

Do note, if you are using systemd-gpt-auto-generator with the DPS[1] it only searches for a root partition on the drive with the booted from kernel[2] (and any other DPS partitions are searched for from the drive with the root partition, so if you somehow specify a different drive than the one with the boot loader it will search on that different drive)

[1] https://uapi-group.org/specifications/specs/discoverable_par...

[2] https://www.freedesktop.org/software/systemd/man/latest/syst...