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Secure Boot

Secure Boot is a security feature found in the UEFI standard, designed to add a layer of protection to the pre-boot process: by maintaining a cryptographically signed list of binaries authorized or forbidden to run at boot, it helps in improving the confidence that the machine core boot components (boot manager, kernel, initramfs) have not been tampered with.

ATTENTION: Using Secure Boot goes hand in hand with using disk encryption. This has security implications: if the storage device the keys are stored on is not encrypted, the keys can be read and used by anybody to sign bootable images with, thereby defeating the purpose of using Secure Boot at all.

Preparations

To determine the current state of Secure Boot execute:

bootctl status

The output looks something like this:

System:
      Firmware: UEFI 2.70 (American Megatrends 5.17)
 Firmware Arch: x64
   Secure Boot: enabled (user)
  TPM2 Support: yes
  Measured UKI: yes
  Boot into FW: supported

...

In order to proceed you need to set your firmware's Secure Boot mode into "setup" mode to proceed. This can usually be achieved by wiping the key store of the firmware. Refer to your mainboard's user manual on how to do this.

Installation

For the most straight-forward Secure Boot toolchain install sbctl:

pacman -S sbctl

It tremendously simplifies generating Secure Boot keys, loading keys into firmware and signing kernel images.

Generating keys

SEE ALSO: The Meaning of all the UEFI Keys

Secure Boot implementations use these keys:

Key Type Description
Platform Key (PK) Top-level key
Key Exchange Key (KEK) Keys used to sign Signatures Database and Forbidden Signatures Database updates
Signature Database (db) Contains keys and/or hashes of allowed EFI binaries
Forbidden Signatures Database (dbx) Contains keys and/or hashes of denylisted EFI binaries

To generate new keys and store them under /usr/share/secureboot/keys/:

sbctl create-keys

Unified Kernel Image

A unified kernel image (UKI) combines an EFI stub image, CPU microcode, kernel command line and an initramfs into a single file that can be read and executed by the machines UEFI firmware. It also makes it easier to sign for secure boot as there will be only a single file to sign.

Starting with v31 mkinitcpio is able to create UKIs out-of-the-box. The maintainers of sbctl also recommend using the system's initramfs generation tool instead of sbctl bundle.

To make mkinitcpio generate UKIs, edit the appropriate .preset file for your kernel in /etc/mkinitcpio.d/:

  • uncomment the default_uki and fallback_uki lines
  • point the file path to somewhere on your EFI System Partition (e.g. /efi, /boot or /boot/efi)

NOTE: mkinitcpio automatically sources /etc/kernel/cmdline for the included kernel command line arguments. If you want the fallback image to receive a different set of kernel command line arguments, specify a different file path in fallback_options with the --cmdline argument. It also sources drop-in files under /etc/cmdline.d/ during UKI generation. However, the latter won't allow you to pass different command line arguments for the default and fallback image.

NOTE: Placing the UKI under /efi/EFI/Linux/ allows systemd-boot to automatically detect images and list them without having to specifically create boot entries for them.

# mkinitcpio preset file for the 'linux' package

#ALL_config="/etc/mkinitcpio.conf"
ALL_kver="/boot/vmlinuz-linux"

PRESETS=('default' 'fallback')

#default_config="/etc/mkinitcpio.conf"
default_image="/boot/initramfs-linux.img"
default_uki="/efi/EFI/Linux/arch-linux.efi"                                # NEW
#default_options="--splash /usr/share/systemd/bootctl/splash-arch.bmp"

#fallback_config="/etc/mkinitcpio.conf"
fallback_image="/boot/initramfs-linux-fallback.img"
fallback_uki="/efi/EFI/Linux/arch-linux-fallback.efi"                      # NEW
fallback_options="-S autodetect --cmdline /etc/kernel/cmdline_fallback"    # NEW

Kernel Command Line Parameters

As mkinitcpio sources command line parameters from a specific file by default, saving them to that file further streamlines the generation process.

First create the directory and open a new file in there:

mkdir /etc/kernel
nano /etc/kernel/cmdline

The parameters to include depend on the kind of initramfs used. You can use any of the persistent block device naming schemes to pass the device. You also need to specify a mapper name under which the decrypted root file system should be made available for mounting.

You can obtain the block device identifier for the LUKS container, e.g. its UUID, with blkid (using /dev/sda1 as an example):

NOTE: Pressing Ctrl + T inside nano allows you to paste the result of a command at the current cursor position.

blkid -s UUID -o value /dev/sda1

Continue to specify additional kernel command line parameters you need. At minimum it should look like this:

  • busybox: 
    cryptdevice=UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX:cryptroot root=/dev/mapper/cryptroot rw
  • systemd: 
    rd.luks.name=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX=cryptroot root=/dev/mapper/cryptroot rw

TIP: You can further simplify this by using a systemd-based initramfs. Create a file named /etc/crypttab.initramfs and specify your encrypted devices in there (same syntax as regular /etc/crypttab, see crypttab(5)):

# <name>    <device>                                     <passphrase>    <options>
cryptroot   UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX                    discard

This allows you to omit any rd.luks parameters, which leaves you with a kernel command line that looks like this:

root=/dev/mapper/cryptroot rw

ATTENTION: Keep the specialties of your chosen root file system in mind, e.g. if using btrfs you also need to supply the subvolume that should be mounted: rootflags=subvol=@.

NOTE: By default, dm-crypt does not allow TRIM for SSDs for security reasons (information leak). To override this behavior:

  • busybox: append :allow-discards after the device mapper name
  • systemd: do one of the following
    • add rd.luks.options=discard as an additional kernel command line parameter
    • specify the discard option in /etc/crypttab.initramfs in the options field

Enroll keys in firmware

WARNING: Replacing the platform keys with your own can end up bricking your machine, making it impossible to get into the UEFI/BIOS settings to rectify the situation. This is due to the fact that some device firmware (OpROMs, e.g. GPU firmware), that gets executed during boot, may be signed using Microsoft's keys. Run sbctl enroll-keys --microsoft if you're unsure if this applies to you (enrolling Microsoft's Secure Boot keys alongside your own custom ones) or include the TPM Event Log with sbctl enroll-keys --tpm-eventlog (if your machine has a TPM and you don't need or want Microsoft's keys) to prevent bricking your machine.

ATTENTION: Make sure your firmware's Secure Boot mode is set to setup mode! You can do this by going into your firmware settings and wiping the factory default keys. Additionally, keep an eye out for any setting that auto-restores the default keys on system start.

TIP: If you plan to dual-boot Windows, run sbctl enroll-keys --microsoft to enroll Microsoft's Secure Boot keys along with your own custom keys.

To enroll your keys, simply:

sbctl enroll-keys

Automated signing of UKIs

Next, add the images to the list of files to be signed (one at a time):

sbctl sign --save /efi/EFI/Linux/arch-linux.efi
sbctl sign --save /efi/EFI/Linux/arch-linux-fallback.efi

The sbctl package comes with a pacman hook to execute sbctl sign-all -g on kernel upgrades or installs. The UKIs are ready to be booted directly by the UEFI firmware (EFISTUB booting) or via a bootloader like grub, systemd-boot or rEFInd.

ATTENTION: Currently, the sbctl package also provides a post mkinitcpio hook which runs sbctl after every kernel build. This means with the default linux kernel installed, sbctl will run at least three times, twice for each time mkinitcpio runs during pacman package upgrades and once after pacman finishes. The usefulness of the hook has been disputed. A patch has been submitted.

For the time being, comment out the line calling sbctl sign-all -g in the hook file: /usr/lib/initcpio/post/sbctl

Signing the Bootloader

NOTE: This is the manual method. If you also want to automate the bootloader update process, skip to the section below.

If you plan on using a boot loader, you will also need to add its *.efi executable(s) to the sbctl database, e.g. systemd-boot:

sbctl sign --save /efi/EFI/BOOT/BOOTX64.EFI
sbctl sign --save /efi/EFI/systemd/systemd-bootx64.efi

Upon system upgrades, pacman will call sbctl to re-sign the files listed in sbctl's database.

Automate systemd-boot updates and signing

systemd comes with a systemd-boot-update.service unit file to automate updating the bootloader whenever systemd is updated. However, it only updates the bootloader after a reboot, by which time sbctl has already run the signing process. This would necessitate manual intervention.

Recent versions of bootctl look for a .efi.signed file before a regular .efi file when copying bootloader files during install and update operations. So to integrate better with the auto-update functionality of systemd-boot-update.service, the bootloader needs to be signed ahead of time.

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

This will add the source and target file paths to sbctl's database. The pacman hook included with sbctl will trigger whenever a file in usr/lib/**/efi/*.efi* changes, which will be the case when systemd is updated and a new version of the unsigned bootloader is written to disk at /usr/lib/systemd/boot/efi/systemd-bootx64.efi.

Finally, enable the systemd-boot-update.service unit:

systemctl enable systemd-boot-update

Now when systemd is updated the signed version of the systemd-bootx64.efi booloader will be copied to the ESP after a reboot, completely automating the bootloader update and signing process!

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