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sbupdate

CircleCI

This tool allows you to sign Arch Linux kernels using your own Secure Boot keys.

Installation

You should be familiar with the process of creating, installing and using custom Secure Boot keys. See:

After you have generated your custom keys, proceed with setup:

  • Install sbupdate-git from AUR
  • Place your custom keys in /etc/efi-keys
  • Configure /etc/sbupdate.conf (see Configuration)
  • Run sudo sbupdate for first-time image generation

For each kernel /boot/vmlinuz-<NAME> a signed UEFI image will be generated, by default in /boot/EFI/Arch/<NAME>-signed.efi. Additional images can be generated if multiple initramfs are configured. Now you can add these images to your UEFI firmware or boot manager configuration.

After the initial setup, signed images will be (re)generated automatically when you install or update kernels using Pacman.

Note that the kernel command line, initramfs and boot splash will be embedded in the signed UEFI image.

Configuration

The following settings are available:

  • Command line and multiple initramfs1 for each specified kernel
  • A list of additional boot files to sign
  • Locations of the key, ESP and output directories
  • Boot splash image

Edit the file /etc/sbupdate.conf to change the settings. ⚠️ Note: you must set your default kernel command line in the CMDLINE_DEFAULT variable.

1 Intel and AMD microcode updates are handled automatically.

Direct booting vs. boot manager

The generated images are UEFI executables and can be directly booted by UEFI firmware. Therefore, a separate boot manager such as systemd-boot is technically not required. This is similar to Linux EFISTUB.

Booting directly from firmware is arguably more secure, but may also be harder to set up and use. See Using UEFI directly in the above article, with the exception that the kernel command line does not need to be specified in this case.

Alternatively, you can use a boot manager. In this case you need to add the generated UEFI images to the boot manager configuration. For systemd-boot, the basic entry format is

title Arch Linux <NAME>
efi   /EFI/Arch/<NAME>-signed.efi

You also need to sign your boot manager's own UEFI executables with your custom keys. Add corresponding filenames to the EXTRA_SIGN array in /etc/sbupdate.conf, for example (systemd-boot):

EXTRA_SIGN=('/boot/EFI/BOOT/BOOTX64.EFI' '/boot/EFI/systemd/systemd-bootx64.efi')

and re-run the tool if needed. You should remember to run the tool every time you update your boot manager's files (e. g., after sudo bootctl update).

⚠️ Note: when booting with Secure Boot disabled, options passed from an EFI shell (even empty) may override the built-in command line in the combined image, and the boot may fail. See #4.

ESP mount point

Typically ESP is mounted on /boot and contains also the original, unsigned files such as the Linux kernel image and initramfs. You may choose to mount ESP on a different directory (for example, /efi) and keep /boot itself on the secure root file system. This way ESP will only contain signed images which cannot be tampered with.

See Configuration to change the ESP directory.

Note that if you use a boot manager such as systemd-boot, then its files still need to be on the ESP before they are signed. It is customary to sign these files right after they have been installed on the ESP. Direct booting is recommended for increased security.

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