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Unified Kernel Support Phase 1

This is a proposed Change for Fedora Linux.
This document represents a proposed Change. As part of the Changes process, proposals are publicly announced in order to receive community feedback. This proposal will only be implemented if approved by the Fedora Engineering Steering Committee.

Summary

Add support for unified kernels images to Fedora.

Owner

Current status

  • Targeted release: Fedora Linux 38
  • Last updated: 2022-12-20
  • FESCo issue: <will be assigned by the Wrangler>
  • Tracker bug: <will be assigned by the Wrangler>
  • Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

The goal is to move away from initrd images being generated on the installed machine. They are generated while building the kernel package instead, then shipped as part of a unified kernel image.

A unified kernel image is an all-in-one efi binary containing kernel, initrd, cmdline and signature. The secure boot signature covers everything, specifically the initrd is included which is not the case when the initrd gets loaded as separate file from /boot.

Main motivation for this move is to make the distro more robust and more secure.

Switching the whole distro over to unified kernels quickly is not realistic though. Too many features are depending on the current workflow with a host-specific initrd (and host-specific kernel command line), which is fundamentally incompatible with unified kernels where everybody will have the same initrd and command line. Thats why there is 'Phase 1' in title, so we can have more Phases in future releases 😃

A host-specific initrd / command line is needed today for:

  • features needing optional dracut modules (initrd rebuild needed to enable them).
  • configuration / secrets baked into the initrd (booting from iscsi for example).
  • configuration being specified on the kernel command line.

Phase 1 goals (high priority):

  • Ship a unified kernel image as (optional) kernel sub-rpm. Users can opt-in to use that kernel by installing the sub-rpm. Initial focus is on booting virtual machines where we have a relatively small and well defined set of drivers / features needed. Supporting modern physical machines with standard setup (i.e. boot from local sata/nvme storage) too should be easy.
  • Update kernel install scripts so unified kernels are installed and updated properly.
  • Add bootloader support for unified kernel images. Add unified kernel bls support to grub2, or support using systemd-boot, or both.

Phase 1 goals (lower priority, might move to Phase 2):

  • Add proper discoverable partitions support to installers (anaconda, image builder, ...).
    • Temporary workaround possible: set types using sfdisk in %post script.
    • When using btrfs: configure 'root' subvolume as default volume.
  • Add proper systemd-boot support to installers.
    • Temporary workaround possible: run 'bootctl install' in %post script.
  • Better measurement and remote attestation support.
    • store kernel + initrd hashes somewhere (kernel-hashes.rpm ?) to allow pre-calculate TPM PCR values.
    • avoid using grub2 (measures every config file line executed which is next to impossible to pre-calculate).
  • Switch cloud images to use unified kernels.

Phase 2/3 goals (longer-term stuff which is not realistic to complete for F38).

  • Move away from using the kernel command line for configuration.
  • Move away from storing secrets in the initrd.
  • Handle dracut optional modules in a different way.

systemd has some building blocks which can be used, although none of them are used by fedora today. systemd credentials can be used for secrets (also for configuration). The unified kernel stub can load credentials from the ESP.

The unified kernel stub can also load extensions from the ESP, which can possibly be used to replace optional dracut modules.

Feedback

Benefit to Fedora

  • Better secure boot support (specifically the initrd is covered by the signature).
  • Better confidential computing support (measurements are much more useful if we know what hashes to expect for the initrd).
  • More robust boot process (generating the initrd on the installed system is fragile, root cause for kernel bugs reported is simply a broken initrd sometimes).

Scope

  • Policies and guidelines: N/A (not needed for this Change)
  • Trademark approval: N/A (not needed for this Change)
  • Alignment with Objectives:

Upgrade/compatibility impact

None (using unified kernel is opt-in for Phase 1).

How To Test

Try on a existing (uefi) system:

  • make sure you are running fedora 37 or rawhide.
  • make sure your root filesystem has type "Linux root (x86-64)" (use fdisk -l to check).
    • should that not be the case use the fdisk tag command ('t') to change the partition type.
  • dnf copr enable kraxel/unified.kernel
  • dnf update "grub2*"
  • dnf install kernel-unified-virt
  • reboot

You should find two entries in the grub2 boot menu, one for classing kernel with separate initrd and one for the unified kernel image. Both should boot fine.

The https://gitlab.com/kraxel/fedora-uki project has kickstart files and helper scripts for generating virtual machine images.

Prebuilt virtual machine images are available from https://www.kraxel.org/fedora-uki/.

User Experience

Dependencies

Contingency Plan

  • Contingency mechanism:
    • Probably none (unified kernel images are opt-in for Phase 1).
    • In case we tried switching the cloud images to unified kernels: revert the kickstart config changes.
  • Contingency deadline: N/A (not a System Wide Change)
  • Blocks release? No

Documentation

N/A (not a System Wide Change)

Release Notes

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