283 lines
8.9 KiB
Markdown
283 lines
8.9 KiB
Markdown
# Quick Start: NixOS Secure Boot
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This document attempts to guide users into setting up UEFI Secure Boot
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for their NixOS system using a custom key chain. The audience are
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experienced NixOS users.
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This guide has been tested on a Lenovo Thinkpad and is expected to
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work on other Thinkpads without change. On other systems, certain
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steps may be different.
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## ⚠ Disclaimers ⚠
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Secure Boot for NixOS is still in development and has some sharp
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edges. There may be cases where you end up with a system that does not
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boot.
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**We only recommend this to NixOS users that are comfortable using
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recovery tools to restore their system or have a backup ready.**
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## Functional Requirements
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To be able to setup Secure Boot on your device, NixOS needs to be
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installed in UEFI mode and
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[`systemd-boot`](https://www.freedesktop.org/wiki/Software/systemd/systemd-boot/)
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must be used as a boot loader.
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These prerequisites can be checked via `bootctl status`:
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```console
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$ bootctl status
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System:
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Firmware: UEFI 2.70 (Lenovo 0.4720)
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Secure Boot: disabled (disabled)
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TPM2 Support: yes
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Boot into FW: supported
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Current Boot Loader:
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Product: systemd-boot 251.7
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...
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```
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In the `bootctl` output, the firmware needs to be `UEFI` and the
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current boot loader needs to be `systemd-boot`. If this is the case,
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you are all set to continue.
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## Security Requirements
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These requirements are _optional_ for a development system. Feel free
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to skip them, if you just want to hack on Secure Boot support.
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To provide any security your system needs to defend against an
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attacker turning UEFI Secure Boot off or being able to sign binaries
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with the keys we are going to generate.
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The easiest way to achieve this is to:
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1. Enable a BIOS password in your system.
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2. Use full disk encryption.
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**The topic of security around Secure Boot is complex. We are only
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scratching the surface here and a comprehensive guide is out of
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scope.**
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## Part 1: Preparing Your System
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In the first part, we will prepare everything on the software side of
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things. At the end of this part, you will have your own Secure Boot
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keys and a NixOS that has signed boot binaries.
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### Finding the UEFI System Partition (ESP)
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The UEFI boot process revolves around a special partition on the
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disk. This partition is called _ESP_, the (U)EFI System
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Partition. This partition is by convention mounted at `/boot` on NixOS
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and the rest of this document assumes this.
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You can verify that `/boot` is the ESP by looking for `ESP:` in
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`bootctl status` output.
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### Creating Your Keys
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To create Secure Boot keys, we use `sbctl`, a popular Secure Boot Key
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Manager. `sbctl` is available in
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[Nixpkgs](https://github.com/NixOS/nixpkgs) as `pkgs.sbctl`.
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Once you have installed sbctl (or entered a Nix shell), creating your
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Secure Boot keys requires this command:
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```console
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$ sudo sbctl create-keys
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[sudo] password for julian:
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Created Owner UUID 8ec4b2c3-dc7f-4362-b9a3-0cc17e5a34cd
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Creating secure boot keys...✓
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Secure boot keys created!
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```
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This takes a couple of seconds. When it is done, your Secure Boot keys
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are located in `/etc/secureboot`. `sbctl` sets the permissions of the
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secret key so that only root can read it.
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### Switching to bootspec
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`lzbt` currently doesn't handle
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non-[bootspec](https://github.com/grahamc/rfcs/blob/bootspec/rfcs/0125-bootspec.md)
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generations well
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([#55](https://github.com/nix-community/lanzaboote/issues/55)). As
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such, we need to switch to bootspec and get rid of all previous
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generations before we can continue.
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Bootspec is currently available as preview in nixpkgs unstable. To
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enable bootspec, set `boot.bootspec.enable = true;` in your system
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configuration, rebuild and reboot.
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When everything is working, you can garbage collect your old
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non-bootspec generations: `nix-collect-garbage -d`.
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🔪 **Sharp edge:** 🔪 This will leave old boot entries lying around in
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the ESP. `systemd-boot` will display these during boot. This can be
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confusing during boot. **After you made a backup of your ESP**, you
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may delete these entries in `/boot/loader/entries`.
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### Configuring NixOS (with Flakes)
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Below is a fragment of a NixOS configuration that enables the Secure
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Boot stack.
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```nix
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{
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description = "A SecureBoot-enabled NixOS configurations";
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inputs = {
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nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
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lanzaboote = {
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url = "github:nix-community/lanzaboote";
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# Optional but recommended to limit the size of your system closure.
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inputs.nixpkgs.follows = "nixpkgs";
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};
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};
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outputs = { self, nixpkgs, lanzaboote, ...}: {
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nixosConfigurations = {
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yourHost = nixpkgs.lib.nixosSystem {
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system = "x86_64-linux";
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modules = [
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# This is not a complete NixOS configuration and you need to reference
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# your normal configuration here.
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lanzaboote.nixosModules.lanzaboote
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({ pkgs, lib, ... }: {
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# This should already be here from switching to bootspec earlier.
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# It's not required anymore, but also doesn't do any harm.
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boot.bootspec.enable = true;
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environment.systemPackages = [
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# For debugging and troubleshooting Secure Boot.
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pkgs.sbctl
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];
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# Lanzaboote currently replaces the systemd-boot module.
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# This setting is usually set to true in configuration.nix
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# generated at installation time. So we force it to false
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# for now.
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boot.loader.systemd-boot.enable = lib.mkForce false;
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boot.lanzaboote = {
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enable = true;
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pkiBundle = "/etc/secureboot";
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};
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})
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];
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};
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};
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};
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}
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```
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After you rebuild your system, check `sbctl verify` output:
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```console
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$ sudo sbctl verify
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Verifying file database and EFI images in /boot...
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✓ /boot/EFI/BOOT/BOOTX64.EFI is signed
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✓ /boot/EFI/Linux/nixos-generation-355.efi is signed
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✓ /boot/EFI/Linux/nixos-generation-356.efi is signed
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✗ /boot/EFI/nixos/0n01vj3mq06pc31i2yhxndvhv4kwl2vp-linux-6.1.3-bzImage.efi is not signed
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✓ /boot/EFI/systemd/systemd-bootx64.efi is signed
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```
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It is expected that the files ending with `bzImage.efi` are _not_
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signed.
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🔪 **Sharp edge:** 🔪 In case any of the `nixos-generation-*.efi`
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files are not signed, you have hit a bug
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([#39](https://github.com/nix-community/lanzaboote/issues/39)). This
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issue will prevent the system from booting successfully when Secure
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Boot is enabled. The way to solve this is **deleting** the unsigned
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files indicated by `sbctl` and switching to the configuration
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again. This will copy and sign the missing files.
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## Part 2: Enabling Secure Boot
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Now that NixOS is ready for Secure Boot, we will setup the
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firmware. At the end of this section, Secure Boot will be enabled on
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your system and your firmware will only boot binaries that are signed
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with your keys.
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These instructions are specific to Thinkpads and may need to be
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adapted on other systems.
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### Entering Secure Boot Setup Mode
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The UEFI firmware allows enrolling Secure Boot keys when it is in
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_Setup Mode_.
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On a Thinkpad enter the BIOS menu using the "Reboot into Firmware"
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entry in the systemd-boot boot menu. Once you are in the BIOS menu:
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1. Select the "Security" tab.
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2. Select the "Secure Boot" entry.
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3. Set "Secure Boot" to enabled.
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4. Select "Reset to Setup Mode".
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5. Select "Clear All Secure Boot Keys".
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When you are done, press F10 to save and exit.
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You can see these steps as a video [here](https://www.youtube.com/watch?v=aLuCAh7UzzQ).
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### Enrolling Keys
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Once you've booted your system into NixOS again, you have to enroll
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your keys to activate Secure Boot. We include Microsoft keys here to
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avoid boot issues.
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```console
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$ sudo sbctl enroll-keys --microsoft
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Enrolling keys to EFI variables...
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With vendor keys from microsoft...✓
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Enrolled keys to the EFI variables!
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```
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You can now reboot your system. After you've booted, Secure Boot is
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activated:
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```console
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$ bootctl status
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System:
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Firmware: UEFI 2.70 (Lenovo 0.4720)
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Firmware Arch: x64
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Secure Boot: enabled (user)
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TPM2 Support: yes
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Boot into FW: supported
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```
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That's all! 🥳
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## Troubleshooting
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If your system doesn't boot with Secure Boot enabled, the most likely
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issue is that Lanzaboote could not verify a cryptographic hash. To
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recover from this, disable Secure Boot in your firmware
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settings. Please file a bug, if you hit this issue.
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## Disabling Secure Boot and Lanzaboote
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When you want to permanently get back to a system without the Secure
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Boot stack, **first** disable Secure Boot in your firmware
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settings. Then you can disable the Lanzaboote related settings in the
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NixOS configuration and rebuild.
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You may need to clean up the `EFI/Linux` directory in the ESP manually
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to get rid of stale boot entries. **Please backup your ESP, before you
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delete any files** in case something goes wrong.
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## Alternatives
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The [ArchLinux wiki](https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface/Secure_Boot)
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contains alternatives to handling your keys, in case `sbctl` is not
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flexible enough.
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