Secure Boot

Secure Boot usually refers to a platform firmware capability to verify the boot components and ensure that only your own operating system to boot.

Secure Boot has multiple implementations, the most known one is UEFI Secure Boot, which relies on the UEFI platform firmware, but others implementations can exist on embedded systems.

There are at present two ways to enable Secure Boot for NixOS. The first is by using Limine. This has the advantage of being entirely in tree in nixpkgs.

Secondly, one can use the Lanzaboote project.

It is recommended to enable a BIOS password and full disc encryption to prevent attacks against UEFI and Secure Boot.

Limine

Limine Secure Boot support is principally controlled by the module options.

To enable Secure Boot, first switch to Limine as your bootloader. You may find the existing documentation for both Limine and Bootloader helpful.

Then

Add sbctl to you systems packages. It must be installed on a system level.

Generate keys with <core>sbctl create-keys

Enable Secure Boot Setup Mode. This usually involves entering the BIOS and selecting an option to do so. On some BIOS implementations, there is not an explicit option to do so, but it will enter setup mode if one clears all Secure Boot keys.

Now, run sbctl enroll-keys -m -f. to actually enroll your keys.

You can now rebuild with boot.loader.limine.secureBoot.enable</true> set to true, and disable Secure Boot setup mode.

If this has been successful, you can check using bootctl status

$ bootctl status
systemd-boot not installed in ESP.
System:
      Firmware: n/a (n/a)
 Firmware Arch: x64
   Secure Boot: enabled (user)
...

Lanzaboote

Lanzaboote has two components: lzbt and stub. 
lzbt is the command line that signs and installs the boot files on the ESP.  
stub is a UEFI application that loads the kernel and initrd from the ESP, it's different from systemd-stub, see below to see precise differences.


  ⚠︎
Warning: Lanzaboote is still in development and requires some prerequisites and precautions. Currently it's only available for nixos-unstable. For more information, please see the GitHub repository or the Quick Start guide.

Requirements

The Secure Boot implementation of Lanzaboote requires a system installed in UEFI mode together with systemd-boot enabled.  This can be checked by running 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
...

Setup

Follow the instructions in the Quick Start guide.

Key management

At the time of writing, Lanzaboote offers only local storage of the keyring, otherwise, it is not possible to rebuild the system and sign the new resulting files.
In the future, Lanzaboote will offer two new signature backends: remote signing (an HTTP server which receives signature requests and answers with signatures) and PKCS#11-based signing (that is, bringing an HSM-like device, e.g. YubiKey, NitroKey, etc.).


  ⚠︎
Warning: Key management is a hard problem which is out of scope for Lanzaboote project, many recipes exist and there is no single perfect solution. Taking the time to learn how to key manage and figure out the right level of threat protection is crucial to achieve an effective boot protection.

Differences with `systemd-stub`

systemd and distribution upstream have an existing solution called `systemd-stub` but this is not a realistic solution for NixOS as there's too many generations on a system.
Using `systemd-stub`, a kernel and an initrd has to be duplicated for each generation, using Lanzaboote's stub, a kernel and initrd can be deduplicated without compromising on the security.
Tracking the feature parity with `systemd-stub` can be done in this issue: https://github.com/nix-community/lanzaboote/issues/94.