Secure Boot

modules/secureboot.nix adds lanzaboote (pinned to a release tag in flake.nix): systemd-boot's signed stub and signed unified kernel images, with your own keys in /var/lib/sbctl. The desktop host configuration enables lanzaboote; Secure Boot still needs its keys enrolled in firmware and enabled in the firmware menu. sbctl is installed either way.

The disks are already LUKS-encrypted; Secure Boot adds that only boot files you signed will run, so nobody can swap the kernel or initrd on the unencrypted /boot to capture the passphrase.

The cachyos kernel doesn't force lockdown under Secure Boot (LOCK_DOWN_KERNEL_FORCE_NONE, no MODULE_SIG_FORCE), so the unsigned NVIDIA modules keep loading.

Turning it on

1. Keys. If keys are not already present, run sudo sbctl create-keys (into /var/lib/sbctl, root-only). Switching with the option on but no keys fails at the bootloader step (the running generation keeps booting).
2. Sign. Set dots.secureBoot.enable = true;, then nh os switch.
3. Check. sudo sbctl verify: EFI/Boot/bootx64.efi, EFI/systemd/systemd-bootx64.efi and every EFI/Linux/nixos-generation-*.efi must show as signed. EFI/nixos/kernel-*.efi stays unsigned on purpose: the signed generation stubs hash-check the kernel and initrd before booting them. Leftovers from plain systemd-boot (*bzImage.efi, EFI/systemd/systemd-boot-fallbackx64.efi) can be deleted.
4. Setup Mode. systemctl reboot --firmware-setup, then in the firmware's Secure Boot menu reset or delete the keys so it enters Setup Mode (on ASUS boards: Key Management → Clear Secure Boot keys). Keep Secure Boot off for now.
5. Enroll. Back in NixOS: sudo sbctl enroll-keys --microsoft. Keep --microsoft: the RTX 3060's option ROM is Microsoft-signed, and without those keys the card can show nothing before Linux starts.
6. Enable. systemctl reboot --firmware-setup, turn Secure Boot on, boot. bootctl status should say Secure Boot: enabled (user).

TPM unlock

With Secure Boot on, the TPM can unlock the root disk at boot without the passphrase. The key is sealed to two PCRs (TPM measurement registers):

• PCR 7, the Secure Boot state and keys. If Secure Boot is turned off, the keys change, or something signed only by Microsoft boots (a live USB), it won't unseal.
• PCR 15 = 0. With tpm2-measure-pcr=yes the initrd extends PCR 15 as soon as it opens a volume. That stops a decoy volume (same UUID, attacker's passphrase) from being opened first and then used to unseal the real key.

Because the disk opens by itself, root's password is locked (users.users.root.hashedPassword = "!" in the host): a boot that drops to emergency mode refuses to open a shell instead of handing one to whoever is at the keyboard. Recover a machine that won't boot from the installer ISO; sudo is unaffected.

The passphrase keyslot stays as the fallback. A firmware update or a change to the Secure Boot settings changes PCR 7: you get the passphrase prompt, then re-enroll.

1. Find the root container. hosts/nixos/disk.nix names it: today's install is /dev/disk/by-uuid/bd950db0-dc83-432a-bec6-0bc1a72bd76e; after a dots-install reinstall (useDisko = true) it is /dev/disk/by-partlabel/disk-main-root. Use those stable paths: the kernel names (nvme0n1/nvme1n1) swap between boots on this board, and the other NVMe holds Windows. The swap partition next to it is re-keyed randomly every boot and has nothing to enroll.
2. Initrd. dots.secureBoot.tpmUnlock (set in hosts/nixos/disk.nix, per layout) lists the boot.initrd.luks.devices names the initrd tries the TPM for. It needs dots.secureBoot.enable = true;. With no TPM keyslot yet, boot still asks for the passphrase.
3. Enroll. After Secure Boot is enabled in firmware, enroll the TPM key on the root container ($root = the path from step 1):

   sudo systemd-cryptenroll "$root" --tpm2-device=auto \
     --tpm2-pcrs=7+15:sha256=0000000000000000000000000000000000000000000000000000000000000000
   

   Add --tpm2-with-pin=yes to still type a short PIN at boot; the TPM rate-limits guesses.
4. Reboot. Root should unlock without a prompt.

Re-enroll after a PCR 7 change, or after a reinstall (a new LUKS header has no TPM slot): sudo systemd-cryptenroll "$root" --wipe-slot=tpm2, then step 3. Check the path with lsblk -f before wiping or enrolling any slot. To drop it, wipe the slot and empty the list.

Backing out

Turn Secure Boot off in the firmware: everything still boots. To drop lanzaboote too, set the option back to false and switch.

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_{Source: docs/secure-boot.md}