Julian Stecklina
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README.md
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README.md
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[](https://www.rust-lang.org/)
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🚧🚧🚧 **This is not ready for non-developer usage.** 🚧🚧🚧
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This repository contains tooling for [UEFI Secure
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Boot](https://en.wikipedia.org/wiki/UEFI#Secure_Boot) on
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[NixOS](https://nixos.org/). The goal is to make Secure Boot available
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from [nixpkgs](https://github.com/NixOS/nixpkgs) for any platform that
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supports UEFI.
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This repository contains experimental tooling for Secure Boot on
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[NixOS](https://nixos.org/).
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## ⚡ Quickstart ⚡
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## 🪛 To Do 🪛
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If you want to try this out, head over [here](./docs/QUICK_START) for
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instructions.
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There is a bunch of work to do. Please coordinate in the [Matrix
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room](https://matrix.to/#/#nixos-secure-boot:ukvly.org), if you want
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to take something up:
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## 🪛 Get Involved 🪛
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- Overview documentation about the approach
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- Document a experimental setup for developers on how to use this repository
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- Coordinate with bootspec RFC stakeholders to communicate a experience report on the bootspec usage
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- Cleaning up flakes.nix for AArch64
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- Upstream nixpkgs work
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- Lanzatool
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- Lanzaboote (needs unstable Rust!)
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- NixOS boot loader installation etc.
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- Unit testing for Lanzatool
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- Investigating how this can fit into systemd-boot theory about sysexts for initrds while keeping NixOS semantics
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- Threat modelling explanations: "bring your own PKI", "share your PKI with MSFT CA", "bring rhboot shim with MOK", etc.
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- Ensuring 99 % of the paths are "happy paths" : protecting user against bricking their machines, identifying sources of risk, communicating intent and detecting risks
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- Experimenting with `fwupd` / Green Checkmark in GNOME Device Security
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- https://github.com/fwupd/fwupd/issues/5284
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- Experimenting with TPM2 measurements
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- Support bootspec with no initrd
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- Studying the initrd secrets feature in NixOS wrt SecureBoot & TPM2
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- ...
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There is still a bunch of work to do before this work can be
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upstreamed into [nixpkgs](https://github.com/NixOS/nixpkgs). Please
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coordinate in the [Matrix
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room](https://matrix.to/#/#nixos-secure-boot:ukvly.org) or check the
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[issues](https://github.com/nix-community/lanzaboote/issues), if you
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want to take something up.
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## High-Level Boot Flow
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## Overview
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```mermaid
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flowchart LR
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systemd[systemd-boot]
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lanzaboote[lanzaboote]
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kernel[Linux Kernel]
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### Secure Boot
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systemd --> lanzaboote
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lanzaboote --> kernel
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```
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The goal of UEFI Secure Boot is to allow only trusted operating
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systems to boot on a system. This can be used to defend against
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certain classes of attacks that compromise the boot flow of a
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system. For example, an attacker will have difficulty replacing the
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Linux kernel that boots a system when Secure Boot is active.
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## lanzatool
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UEFI Secure Boot works by digitally signing all drivers, bootloaders,
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the Linux kernel and its initrd. This establishes a chain of trust
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where one trusted component only hands off control to the next part of
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the boot flow when the integrity of the chain is cryptographically
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validated.
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`lanzatool` is a Linux command line application that takes a
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[bootspec](https://github.com/NixOS/rfcs/pull/125) document and
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installs the boot files into the UEFI
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[ESP](https://en.wikipedia.org/wiki/EFI_system_partition).
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### Caveats
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There are some additional steps that are required to make UEFI Secure
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Boot effective:
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To make systemd-boot recognize a new boot target, `lanzatool` builds a
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[UKI](https://wiki.archlinux.org/title/Unified_kernel_image) image. To
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avoid having to embed kernel and initrd, we use a custom stub
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`lanzaboote` (see below) that loads kernel and initrd from the ESP.
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- There must be a BIOS password or a similar restriction that prevents
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unauthorized changes to the Secure Boot policy.
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- The booted system must have some form of integrity protection.
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- The firmware must be kept up-to-date.
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Remaining items to implement are:
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These steps will not be covered here.
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- Migrations from non-SecureBoot machine (old generation files) ;
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- Alternative Nix stores paths ;
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- Key rotation support ;
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- Bootspec (abuse) cleanups ;
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- Automatic synchronization policies for changing PKI (rotating keys, re-enrolling them, etc.) ;
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- NixOS specialisations support ;
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- Automatic removal of unused files relative to the `configurationLimit` option ;
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- `os-release` patch so `systemd-boot` shows pretty names with generation number
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### Lanzatool
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## lanzaboote
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At the moment, boot loaders, kernels and initrds on NixOS are signed
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on the current system. These then need to be prepared as [Unified
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Kernel Images
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(UKI)](https://uapi-group.org/specifications/specs/boot_loader_specification/#type-2-efi-unified-kernel-images) and placed on the [EFI System Partition (ESP)](https://en.wikipedia.org/wiki/EFI_system_partition).
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`lanzaboote` is the stub that `lanzatool` uses to form an UKI. It
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loads a Linux kernel and initrd without breaking the Secure Boot chain
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of trust. Instead of rolling our own crypto, `lanzaboote` re-uses the
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signature verification that is built-in to UEFI.
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`lanzatool` is a Linux command line application that takes care of
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this flow. It takes a [NixOS
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bootspec](https://github.com/NixOS/rfcs/pull/125) document, signs the
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relevant files, creates a UKI using lanzaboote (see below) and
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installs the UKI along with other required files to the
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ESP. `lanzatool` is also aware of multiple NixOS genertions and will
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sign all configurations that should be bootable.
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### Lanzaboote
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Remaining items to implement are:
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When the Linux kernel and initrd are packed into a UKI, they need an
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UEFI application stub. This role is typically filled by
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[`systemd-stub`](https://www.freedesktop.org/software/systemd/man/systemd-stub.html).
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- TPM measurements like `systemd-stub` does
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- Better error management
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The downside of `systemd-stub` is that it requires the kernel and
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initrd to be packed into the UKI, which makes it pretty large. As we
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need one UKI per NixOS configuration, systems with many configurations
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quickly run out of the limited disk space in the ESP.
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## Relevant Nixpkgs Work
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`lanzaboote` is a UEFI stub that solves the same problem as
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`systemd-stub`, but allows kernel and initrd to be stored separately
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on the ESP. The chain of trust is maintained by validating the
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signature on the Linux kernel and embedding a cryptographic hash of
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the initrd into the signed UKI.
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This project depends on upstream nixpkgs work:
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`lanzaboote` lives in `rust/lanzaboote`.
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- https://github.com/NixOS/nixpkgs/pull/191665
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- https://github.com/DeterminateSystems/bootspec-secureboot/
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- https://github.com/DeterminateSystems/bootspec
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## State of Upstreaming to Nixpkgs
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You can find everything integrated as PoC
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[here](https://github.com/NixOS/nixpkgs/pull/202497).
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Secure Boot is available as an Nixpkgs out-of-tree feature using the
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[bootspec feature preview](https://github.com/NixOS/rfcs/pull/125). It
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works with current nixpkgs-unstable.
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## Funding
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