This guide aims to show how to modify an EOS installation to use secureboot and TPM.
Prerequisites:
- EOS installation with encrypted root and using UEFI
- TPM 2.0 module
- This guide assumes no dual booting is present. It is possible, but outside the scope of this guide.
- EOS live ISO installation media
Overview:
One can stop following this guide after each stage. Stopping before completing a stage may result in an unbootable machine.
In stage 1 a unified kernel image replaces the grub boot loader. The reason is that a unified kernel image can be easily signed for secureboot and that luks2 support is needed for TPM. Mkinitcpio is used to generate the unified kernel image. Efibootmgr is used to add the boot entry to the motherboard’s uefi firmware. As a bonus the boot process should now be significantly faster.
In stage 2 secureboot is configured using sbctl. New keys are generated and enrolled, microsoft keys are excluded if the hardware allows it, the unified kernel image is signed and secureboot enabled in the uefi setup utility. Sbctl comes with a pacman hook that automatically signes the image when the kernel gets updated.
In stage 3 TPM is configured using sytemd-cryptenroll. For this the luks1 encrypted root partition needs to be converted to luks2. After this stage there will be no password prompt when booting. When using a TPM, unlocking of the root partition is bound to the state of the TPM’s PCR registers. This is a trade-off between security and convenience and makes the machine more vulnerable to cold boot attacks.
Instructions:
/dev/block_deviceY is a placeholder for the root partition (commonly /dev/sda2 or /dev/nvme0n1p2)
Stage 1 unified kernel image
uninstall grub
pacman -Rs grub
delete content of efi system partition
rm -r /boot/efi/EFI
determine luks key slot N associated with keyfile used by grub
cryptsetup open -v --test-passphrase --disable-external-tokens /dev/block_deviceY --key-file /crypto_keyfile.bin
remove luks key slot N
cryptsetup luksKillSlot /dev/block_deviceY N
configure mkinitcpio
remove keyfile from /etc/mkinitcpio.conf
FILES=""
set hooks in /etc/mkinitcpio.conf
HOOKS=(base systemd keyboard autodetect sd-vconsole modconf block sd-encrypt filesystems fsck)
modify /etc/mkinitcpio.d/linux.preset . This instructs mkinitcpio to create a unified kernel image at /boot/efi/archlinux-linux.efi
ALL_config="/etc/mkinitcpio.conf"
ALL_kver="/boot/vmlinuz-linux"
ALL_microcode="/boot/*-ucode.img"
PRESETS=('default')
default_image="/boot/initramfs-linux.img"
default_efi_image="/boot/efi/archlinux-linux.efi"
create /etc/crypttab.initramfs . The UUID can be found with ls -l /dev/disk/by-uuid/ or in /etc/crypttab. Mkinitcpio will write the content of this file to /etc/crypttab in the initramfs. In early userspace this will cause the root partition to be unlocked and made available at /dev/mapper/root
root UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
set kernel commandline in /etc/kernel/cmdline
root=/dev/mapper/root rw quiet
regenerate mkinitcpio presets
mkinitcpio -P
delete the entry in /etc/crypttab . Otherwise the system might attempt to unlock root multiple times.
install efibootmgr
pacman -S efibootmgr
list boot entries
efibootmgr
delete boot entry for grub. XXXX is the id of the boot entry
efibootmgr -B -b XXXX
add boot entry. /dev/block_device is the disk (commonly /dev/sda or /dev/nvme0n1). X is the partition number of the efi system partition (commonly 1)
efibootmgr --disk /dev/block_device --part X --create --label "endeavouros" --loader /archlinux-linux.efi
Stage 2 secureboot
boot into the UEFI setup utility and enable secureboot setup mode. This process is different from device to device. The option should sound similar to
- clear/delete keys/certificates
- reset to setup mode
install sbctl
pacman -S sbctl
verify that setup mode is enabled
sbctl status
Installed: ✗ sbctl is not installed
Setup Mode: ✗ Enabled
Secure Boot: ✗ Disabled
create keys
sbctl create-keys
sbctl enroll-keys uses the TPM eventlog to check if option ROM firmware is part of the bootchain. If it is and microsoft certificates are not enrolled, enabling secureboot may brick the machine.
sbctl enroll-keys
to include microsoft certificates perform
sbctl enroll-keys --microsoft
sign the unified kernel image
sbctl sign -s /boot/efi/archlinux-linux.efi
boot into the UEFI setup utility and enable secureboot. Set a UEFI password.
Stage 3 TPM
boot into an EOS Live ISO. For this, secureboot needs to be temporarily disabled.
convert luks1 to luks2
cryptsetup convert --type luks2 /dev/block_deviceY
re-enable secureboot and boot into the EOS installation
install tpm2-tss
pacman -S tpm2-tss
enroll TPM to luks2 partition. --tpm2-pcrs=0+7 specifies which PCR registers state is bound to the unlocking of the root partition. In this case registers 0 and 7. A Table of PCR Definitions can be found here https://man.archlinux.org/man/systemd-cryptenroll.1 . Rerun this command if you knowingly caused the used PCR states to change.
systemd-cryptenroll --wipe-slot=tpm2 --tpm2-device=auto --tpm2-pcrs=0+7 /dev/block_deviceY
modify /etc/crypttab.initramfs
root UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX - tpm2-device=auto
regenerate mkinitcpio presets
mkinitcpio -P
This created a new unified kernel image. Because this happened outside pacman the sbctl pacman hook did not run and the image needs to be manually signed
sbctl sign-all
Closing Thoughts:
- Only one of many ways of setting up secureboot and TPM is shown here.
- Dracut can replace Mkinitcpio.
- Clevis can replace systemd-cryptenroll.
- A different boot loader can be used.
- Instead of enrolling user generated keys, a signed boot loader such as shim can be used. This might be preferable when dual booting windows but less secure.
- Any questions or improvements to this guide are welcome.
links
efistub
https://wiki.archlinux.org/title/EFISTUB
kernel parameters
https://wiki.archlinux.org/title/Kernel_parameters#Parameter_list
mkinitcpio
https://wiki.archlinux.org/title/Mkinitcpio
dmcrypt
https://wiki.archlinux.org/title/Dm-crypt
dmcrypt system configuration
https://wiki.archlinux.org/title/Dm-crypt/System_configuration
dmcrypt device encryption
https://wiki.archlinux.org/title/Dm-crypt/Device_encryption
TPM
https://wiki.archlinux.org/title/Trusted_Platform_Module
persisten block device naming
https://wiki.archlinux.org/title/Persistent_block_device_naming
unified kernel image
https://wiki.archlinux.org/title/Unified_kernel_image
secureboot
https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface/Secure_Boot
sbctl option rom