Previously, I set up Secure Boot with custom keys and an encrypted root disk on my homeserver in 2022. Now, four years later I received a warning during a pacman update making me aware that some parts of the previous setup are now obsolete:

==> WARNING: /etc/crypttab.initramfs is deprecated. Merge it into /etc/crypttab and add a x-initrd.attach option to the devices that need to be unlocked in the initramfs.

That warning made me check the whole setup again and adapt two parts of the configuration:

Unifying the configuration of encrypted devices

Beforehand, the configuration of encrypted block devices was split over two files:

  • /etc/crypttab for devices mounted after the boot process has finished like external drives, and

  • /etc/crypttab.initramfs for the encrypted root disk that should be mounted early during the boot process.

    # /etc/crypttab.initramfs

cryptlvm  UUID=bf8dc6fd-533f-48f2-881b-107f24b61c01 - luks,tpm2-device=/dev/tpmrm0

In the meantime, the option x-initrd.attach was introduced with systemd version 245 in 2020. That option marks devices in /etc/crypttab for mount during early boot and detachment in late shutdown. With it, /etc/crypttab.initramfs becomes obsolete and entries for encrypted disks, which should be mounted in the initramfs can be added to the main /etc/crypttab. So the entry for my root disk becomes

# /etc/crypttab

cryptlvm  UUID=bf8dc6fd-533f-48f2-881b-107f24b61c01 - luks,tpm2-device=/dev/tpmrm0,x-initrd.attach

Migrating to sbctl

In the previous post, I recommended sbupdate. As of 2023 the repository for the project has been archived on GitHub as the tool is now obsolete. Nowadays sbctl includes pacman hooks to sign updated binaries and images automatically.

Create a unified kernel image with mkinitcpio

Previously, sbupdate generated a unified kernel image with systemd-ukify. Now, mkinitcpio will be configured to handle the UKI generation instead. First, create /etc/cmdline.d/root.conf with the root drive

# /etc/cmdline.d/root.conf

root=/dev/CryptedArch/root

Then edit /etc/mkinitcpio.d/linux-lts.preset and make sure a single line

default_uki="/boot/EFI/Arch/linux-lts-signed.efi"

exists. The path for the unified kernel image must match the image loaded by the bootloader or configured in an UEFI boot entry.

In my case, the kernel is booted directly from the UEFI and I double-checked the existing boot entry:

$ efibootmgr -u
BootCurrent: 0000
Timeout: 1 seconds
BootOrder: 0000
Boot0000* Arch Linux    HD(1,GPT,b95b2877-7029-48b0-937b-9aaa2091c558,0x800,0x400000)/\EFI\Arch\linux-lts-signed.efi

After that, run mkinitcpio again and see if the unified kernel image is created successfully:

$ mkinitcpio -P
[...]
==> Creating unified kernel image: '/boot/EFI/Arch/linux-lts-signed.efi'
  -> Using ukify to build UKI
Using config file: /usr/lib/kernel/uki.conf
Wrote unsigned /boot/EFI/Arch/linux-lts-signed.efi
==> Unified kernel image generation successful

Import existing keys to sbctl

After the installation of sbctl, the kernel is still not signed automatically. The installation of a new kernel will output

Secureboot key directory doesn’t exist, not signing!

during the post hooks.

For sbctl to work, the existing keys first need be imported. First, transfer the existing platform key, key exchange key and database key to the system using a secure method like scp. Then import them into sbctl’s structure with:

sbctl import-keys --db-cert db.crt --db-key db.key
sbctl import-keys --kek-cert KEK.crt --kek-key KEK.key
sbctl import-keys --pk-cert PK.crt --pk-key PK.key

Afterwards, make sure sbctl is ready to work by running sbctl setup --print-state --json. The output should look like

{
  "installed": true,
  "landlock": true
}

to indicate a working installation.

Lastly, reinstall the kernel and check if the post hook runs successfully.

# pacman -Syu linux-lts
[...]
==> Initcpio image generation successful
==> Running post hooks
  -> Running post hook: [sbctl]
Signing /boot/vmlinuz-linux-lts
✓ Signed /boot/vmlinuz-linux-lts
==> Post processing done
(4/4) Signing EFI binaries...
Generating EFI bundles....

Cleaning up

Finally it’s time to clean stuff up. Remove the sbupdate-git package and clean up configuration files.

pacman -R sbupdate-git
rm -f /etc/sbupdate.conf.pacsave
rm -rf /etc/efi-keys
# I build AUR packages in `/opt/aur`
rm -rf /opt/aur/sbupdate-git

Furthermore, I checked the permissions of the stored key files:

ll /var/lib/sbctl/
total 4.0K
-rw-r--r-- 1 root root    0 Sep 18 23:24 bundles.json
-rw-r--r-- 1 root root    0 Sep 18 23:24 files.json
drwxr-xr-x 5 root root 4.0K Sep 18 23:29 keys

That’s bad - the directory containing my Secure Boot keys is world-readable. Let’s fix that and make the directory as well as its subdirectories and the key files readable only by root.

chmod -R 500 /var/lib/sbctl/keys
chmod 400 /var/lib/sbctl/keys/*/*