Most modern linux distributions sign the kernel and bootloader, but leave the initrd completely unprotected. Secure boot will protect everything up to the kernel, but the initrd is trivially vulnerable to evil maid attacks. This can be fixed using signed Unified Kernel Images (UKI) and systemd-boot.

A UKI contains the kernel, initrd, and command line all in a single PE file. This whole file can be cryptographically signed to prevent tampering. When used with a LUKS encrypted root partition, signed UKIs can prevent evil maid attacks as long as there are no vulnerabilities in the UEFI firmware. UKIs also are a single file in the EFI partition, which makes them easy to manage.

I am not responsible for any bricked computers, loss of important data, wasted time, and/or other problems caused by this article.

Prerequisites

This needs reasonably modern UEFI firmware (most x86_64 CPUs will work) which is in secure boot setup mode. This guide assumes that Fedora is the only installed OS and that it is using the default partition layout. Do not reboot until after the last step, and make sure to have a backup before beginning.

Move the EFI partition Mount Point

It is recommended to mount the EFI partition to /efi.

Create the new mount point.

mkdir /efi

Change the line in /etc/fstab that starts with /boot/efi to start with just /efi, and delete the line that starts with /boot.

Mount the EFI partition at the new mount point.

umount /boot/efi
mount /efi

Tell kernel-install to install new kernels to /efi by adding this to /etc/kernel/install.conf:

BOOT_ROOT=/efi

Remove /boot and Grow /efi

The boot partition is not useful with systemd-boot, so it can safely be deleted. Identify the current root device by running lsblk, I am going to use /dev/sdX as a placeholder.

Back up all of /efi.

cp -a /efi /efi.bak

Unmount the EFI and boot partitions.

umount /efi /boot

Modify the partition table.

fdisk /dev/sdX

Delete (d) partition 1 (the EFI partition) and partition 2 (the boot partition). Create a new partition (n) and accept the default values. Set the partition type (t) to EFI System. Save these changes and quit (w).

Create an empty FAT32 filesystem in the new EFI partition.

mkfs.vfat -F 32 /dev/sdX1

Run lsblk -o NAME,UUID and copy the UUID of the new EFI partition. Open /etc/fstab, find the line that starts with /efi, and change the second field to /dev/disk/by-uuid/DEAD-BEAF (replace DEAD-BEEF with the UUID you just copied).

Mount the new EFI partition.

mount /efi

Restore the contents of the EFI partition.

cp -a /efi.bak/. /efi
rm -r /efi.bak

Uninstall GRUB

Remove GRUB packages and install systemd-boot. Shim is also unnecessary because the firmware will directly trust the local signing key.

rm /etc/dnf/protected.d/{grub2*,shim.conf}
dnf remove grubby grub2-efi grub2-common shim
dnf install systemd-boot-unsigned sdubby

Disable the Rescue Kernel

The rescue kernel does not work with UKI generation or secure boot signing. Fedora already keeps the last 3 installed kernels, so the rescue kernel is not very useful. Disable rescue kernel generation by adding this to /etc/dracut.conf.d/50-no-rescue-image.conf:

dracut_rescue_image=no

Sign the Bootloader

First you have to install some utilities to help sign the bootloader. Install sbctl from the copr package. The RPM package has a scriptlet that will automatically sign EFI binaries when they are updated by dnf, so it is not recommended to manually install sbctl.

dnf copr enable chenxiaolong/sbctl
dnf install sbctl

Create a new signing key and enroll it with the firmware. This will also add the Microsoft signing key which is required for some hardware. Enrolling the key will cause secure boot to exit setup mode, so other operating systems or ISOs will not boot anymore.

sbctl create-keys
sbctl enroll-keys

Copy the private key from /var/lib/sbctl/keys/PK/PK.key to somewhere safe, like a password manager, so you don't lose it. Make sure you keep this key a secret, anyone with the key can install a rootkit on your computer.

Sign and Install systemd-boot

Sign the bootloader binary and register it with sbctl. bootctl will always use the signed binary, even if it is older than the unsigned binary.

sbctl sign -s /usr/lib/systemd/boot/efi/systemd-bootx64.efi -o /usr/lib/systemd/boot/efi/systemd-bootx64.efi.signed

Install systemd-boot into the efi partition.

bootctl install

Run bootctl status to make sure that everything is working correctly.

Sign `fwupd`

Sign fwupd, which is required for firmware updates.

sbctl sign -s /usr/libexec/fwupd/efi/fwupdx64.efi -o /usr/libexec/fwupd/efi/fwupdx64.efi.signed

Tell fwupd to not use GRUB chainloading by adding this to /etc/fwupd/fwupd.conf:

[uefi_capsule]
EnableGrubChainLoad=false
DisableShimForSecureBoot=true

Enable Unified Kernel Images

ukify can generate UKIs locally and is natively supported by kernel-install.

dnf install systemd-ukify

Add these lines to /etc/kernel/install.conf to enable ukify. Dracut will still be used to generate the initrd, so everything else will continue to work the same.

layout=uki
initrd_generator=dracut
uki_generator=ukify

Install the kernel into the EFI partition.

kernel-install add-all

You can reboot your computer now. You should also enable a firmware password, because otherwise attackers can secretly disable secure boot.

Credits

https://swsnr.de/simple-secure-boot-in-fedora/

https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface/Secure_Boot

https://www.freedesktop.org/software/systemd/man/latest/ukify.html

https://www.freedesktop.org/software/systemd/man/253/kernel-install.html