ZFSbootmenu with Full Disk Encryption Guide

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As described in the general encryption guide, Libreboot allows for full disk encryption including the boot partition. Just as with the general guide, this explanation will demonstrate how to create a partition with moderate encryption for GRUB as well as a root partition with strong encryption. The major differences between the encryption method described in the general guide and this guide are:

ZFSbootmenu works by placing modified versions of the operating system kernel where they can be loaded by the system’s bootloader. ZFSbootmenu provides installation guides for various major distros in their official docs. You should follow those docs for installation, only noting the differences necessary for full disk encryption described below. The only differences between this guide and the docs are:

Creating Encrypted Partition for GRUB

The following section is mostly identical to the main encryption guide except for the naming conventions of the partition in question. When using ZFSbootmenu, the OS kernel/initramfs will reside on the root partion in the /boot directory; not on a separate boot partition. The partition created in this section is only used to load the ZFSbootmenu kernel/initramfs itself and is therefore referred to as the ‘pre-boot environment’ (pbe) partition.

Step 1: Create a LUKS2 formatted device with the PBKDF2 algorithm. You can play around with the iteration count. A higher iteration is more secure but will take GRUB a very long time to decrypt. The debian encrypted boot guide recommends a count of 500,000 which will still take GRUB a very long time (around 25 seconds) but is faster than the default 1,000,000. Use whatever count makes you feel comfortable. I’ll use an arbitrarily low count. You’ll also want to use a different password than you intend to use for your root partition. We don’t want someone to be able to get our root key by brute-forcing our less secure boot key.

sudo cryptsetup luksFormat /dev/sda1 --type luks2 --pbkdf pbkdf2 --pbkdf-force-iterations 200000

Step 2: Format and mount the new LUKS2 device.

sudo cryptsetup luksOpen /dev/sda1 pbe
sudo mkfs.ext4 -L boot /dev/mapper/pbe
sudo mkdir -p /boot/pbe
sudo mount /dev/mapper/boot /boot/pbe

Note: If you wish to change the passphrase for the boot partition in the future then you’ll need to pass the same arguments to cryptsetup as when you created it. If you don’t pass any special arguments, the key will be changed to the distro’s default encryption and grub won’t be able to decrypt it. The command to use is:

cryptsetup luksChangeKey /dev/sda1 --type luks2 --pbkdf pbkdf2 --pbkdf-force-iterations 200000

Configure ZFSbootmenu

The official ZFSbootmenu docs will provide the most up-to-date information. The only differences from the official documentation relevant here are that anything related to syslinux can be ignored and the configuration must be tailored to create only a single kernel/initramfs set. Note that you should follow the MBR/syslinux guide for your distro if you are using the ZFSbootmenu guides.

Here is an example configuration:

> vim /etc/zfsbootmenu/config.yaml

  ManageImages: true
  BootMountPoint: /boot/pbe
  DracutConfDir: /etc/zfsbootmenu/dracut.conf.d
  PreHooksDir: /etc/zfsbootmenu/generate-zbm.pre.d
  PostHooksDir: /etc/zfsbootmenu/generate-zbm.post.d
  InitCPIOConfig: /etc/zfsbootmenu/mkinitcpio.conf
  ImageDir: /boot/pbe/zfsbootmenu
  Versions: false
  Enabled: true
    Config: /boot/syslinux/syslinux.cfg
    Enabled: false
  ImageDir: /boot/pbe
  Versions: false
  Enabled: false
  CommandLine: ro quiet loglevel=4

Final Steps

Refer to the general guide on how to set up fstab/crypttab to mount the pre-boot environment on boot. Replace references to boot with pbe if copying commands from the guide. For example: make sure the partition is mounted at /boot/pbe rather than just /boot.

Ensure that your OS kernel/initramfs is generated with LUKS support. LUKS support is generally automatically enabled in the kernel upon installing cryptsetup.

Create a simulated grub configuration to point Libreboot’s GRUB to ZFSbootmenu. Libreboot will search for and source a grub configuration file on boot/decryption automatically. Do not actually install GRUB. Simply create a file on the partition created for GRUB at /boot/pbe/grub/grub.cfg which points to the ZFSbootmenu kernel/initramfs.

mkdir -p /boot/pbe/grub
> vim /boot/pbe/grub/grub.cfg

linux /zfsbootmenu/vmlinuz-* loglevel=4
initrd /zfsbootmenu/initramfs-*

Markdown file for this page: https://libreboot.org/docs/linux/zfsbootmenu.md

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