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Quick Start: NixOS Secure Boot

This document attempts to guide users into setting up UEFI Secure Boot for their NixOS system using a custom key chain. The audience are experienced NixOS users.

This guide has been tested on a Lenovo Thinkpad and is expected to work on other Thinkpads without change. On other systems, certain steps may be different.

⚠ Disclaimers ⚠

Secure Boot for NixOS is still in development and has some sharp edges. There may be cases where you end up with a system that does not boot.

We only recommend this to NixOS users that are comfortable using recovery tools to restore their system or have a backup ready.

Requirements

To be able to setup Secure Boot on your device, NixOS needs to be installed in UEFI mode and systemd-boot must be used as a boot loader.

These prerequisites can be checked via bootctl status:

$ bootctl status
System:
     Firmware: UEFI 2.70 (Lenovo 0.4720)
  Secure Boot: disabled (disabled)
 TPM2 Support: yes
 Boot into FW: supported

Current Boot Loader:
      Product: systemd-boot 251.7
...

In the bootctl output, the firmware needs to be UEFI and the current boot loader needs to be systemd-boot. If this is the case, you are all set to continue.

Part 1: Preparing Your System

In the first part, we will prepare everything on the software side of things. At the end of this part, you will have your own Secure Boot keys and a NixOS that has signed boot binaries.

Finding the UEFI System Partition (ESP)

The UEFI boot process revolves around a special partition on the disk. This partition is called ESP, the (U)EFI System Partition. This partition is by convention mounted at /boot on NixOS and the rest of this document assumes this.

You can verify that /boot is the ESP by looking for ESP: in bootctl status output.

Creating Your Keys

To create Secure Boot keys, we use sbctl, a popular Secure Boot Key Manager. sbctl is available in Nixpkgs as pkgs.sbctl.

Once you have installed sbctl (or entered a Nix shell), creating your Secure Boot keys requires this command:

$ sudo sbctl create-keys
[sudo] password for julian:
Created Owner UUID 8ec4b2c3-dc7f-4362-b9a3-0cc17e5a34cd
Creating secure boot keys...✓
Secure boot keys created!

This takes a couple of seconds. When it is done, your Secure Boot keys are located in /etc/secureboot. sbctl sets the permissions of the secret key so that only root can read it.

Switching to bootspec

lzbt currently doesn't handle non-bootspec generations well (#55). As such, we need to switch to bootspec and get rid of all previous generations before we can continue.

Bootspec is currently available as preview in nixpkgs unstable. To enable bootspec, set boot.bootspec.enable = true; in your system configuration, rebuild and reboot.

When everything is working, you can garbage collect your old non-bootspec generations: nix-collect-garbage -d.

🔪 Sharp edge: 🔪 This will leave old boot entries lying around in the ESP. systemd-boot will display these during boot. This can be confusing during boot. After you made a backup of your ESP, you may delete these entries in /boot/loader/entries.

Configuring NixOS (with Flakes)

Below is a fragment of a NixOS configuration that enables the Secure Boot stack.

nixosConfigurations = {
  yourHost = nixpkgs.lib.nixosSystem {
    system = "x86_64-linux";

    modules = [
      # ... other modules ...

      lanzaboote.nixosModules.lanzaboote

      ({ config, pkgs, lib, ... }: {
        # This should already be here from switching to bootspec earlier.
        # It's not required anymore, but also doesn't do any harm.
        boot.bootspec.enable = true;

        environment.systemPackages = [
          # For debugging and troubleshooting Secure Boot.
          pkgs.sbctl
        ];

        # Lanzaboote currently replaces the systemd-boot module.
		# This setting is usually set to true in configuration.nix
		# generated at installation time. So we force it to false
		# for now.
        boot.loader.systemd-boot.enable = lib.mkForce false;

        boot.lanzaboote = {
          enable = true;
          pkiBundle = "/etc/secureboot";
        };
      })
    ];
  };

After you rebuild your system, check sbctl verify output:

$ sudo sbctl verify
Verifying file database and EFI images in /boot...
✓ /boot/EFI/BOOT/BOOTX64.EFI is signed
✓ /boot/EFI/Linux/nixos-generation-355.efi is signed
✓ /boot/EFI/Linux/nixos-generation-356.efi is signed
✓ /boot/EFI/nixos/0n01vj3mq06pc31i2yhxndvhv4kwl2vp-linux-6.1.3-bzImage.efi is signed
✓ /boot/EFI/systemd/systemd-bootx64.efi is signed

🔪 Sharp edge: 🔪 In case something is not signed in the sbctl verify output, you have hit a bug (#39). You have to fix this to avoid ending up with an unbootable system (#58). The way to solve this is deleting the unsigned files indicated by sbctl and switching to the configuration again. This will copy and sign the missing files.

Part 2: Enabling Secure Boot

Now that NixOS is ready for Secure Boot, we will setup the firmware. At the end of this section, Secure Boot will be enabled on your system and your firmware will only boot binaries that are signed with your keys.

These instructions are specific to Thinkpads and may need to be adapted on other systems.

Entering Secure Boot Setup Mode

The UEFI firmware allows enrolling Secure Boot keys when it is in Setup Mode.

On a Thinkpad enter the BIOS menu using the "Reboot into Firmware" entry in the systemd-boot boot menu. Once you are in the BIOS menu:

Select the "Security" tab.
Select the "Secure Boot" entry.
Set "Secure Boot" to enabled.
Select "Reset to Setup Mode".
Select "Clear All Secure Boot Keys".

When you are done, press F10 to save and exit.

You can see these steps as a video here.

Enrolling Keys

Once you've booted your system into NixOS again, you have to enroll your keys to activate Secure Boot. We include Microsoft keys here to avoid boot issues.

$ sudo sbctl enroll-keys --microsoft
Enrolling keys to EFI variables...
With vendor keys from microsoft...✓
Enrolled keys to the EFI variables!

You can now reboot your system. After you've booted, Secure Boot is activated:

$ bootctl status
System:
      Firmware: UEFI 2.70 (Lenovo 0.4720)
 Firmware Arch: x64
   Secure Boot: enabled (user)
  TPM2 Support: yes
  Boot into FW: supported

That's all! 🥳

Disabling Secure Boot and Lanzaboote

When you want to get back to a system without the Secure Boot stack, first disable Secure Boot in your firmware settings. Then you can disable the Lanzaboote related settings in the NixOS configuration and rebuild.

You may need to clean up the EFI/Linux directory in the ESP manually to get rid of stale boot entries. Please backup your ESP, before you delete any files in case something goes wrong.

Alternatives

The ArchLinux wiki contains alternatives to handling your keys, in case sbctl is not flexible enough.

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