dotfiles/guides/secure_boot.md

Secure boot + TPM unlocking

This guide assumes you already have a working Arch Linux system, set up by following the installation.md guide. That means you have an encrypted root partition, and that your computer has a TPM module.

This should mean that currently, every time you boot, you have to enter the LUKS password to decrypt your root partition. This can be pretty annoying though, and this guide aims to fix that, without compromising security.

Once finished, this will basically store the decryption key to your root partition in the TPM module, and so on boot, you'll be able to do so without manually entering your password. This is still perfectly secure, assuming you have a good login screen (in my case, I just use the default linux tty, which I trust fully), and you don't allow anyone to modify the kernel command line (which won't be possible due to secure boot, you'd have to re-sign the images to do so).

So, if the laptop gets stolen, and the drive is pulled out, it's contents will be LUKS encrypted, so the security here is the same. If the attacker boots up the system on the laptop, they will get past encryption, however they'll only see a login screen, and they'd have to also get past linux authentication to actually get anywhere, making this approach safe enough.

Setup Unified Kernel Image (UKI)

A Unified Kernel Image is a single executable (.efi file), which can be booted directly from UEFI firmware, or be automatically sourced by boot loaders with no extra configuration.

A UKI will include:

• a UEFI stub loader like (systemd-stub)
• the kernel command line
• microcode
• an initramfs image
• a kernel image
• a splash screen

To set up secure boot, this is a requirement, as it's something that can be signed and represents an immutable executable used for booting into your system.

This is good, because with a standalone bootloader, you would be allowed you to edit the kernel parameters, or even change the kernel image by editing the configuration inside of the (unencrypted) EFI partition. This is obviously dangerous, and we don't want to allow this.

Define kernel command line

Since UKI contains the kernel command line, we will need to define it so that when the image is being built, it can pick it up.

This is a crucial step especially when you have encryption set up, as without it, the kernel wouldn't know what root partition to use.

To set this up, we will use /etc/kernel/cmdline.

This is how I setup my kernel arguments (If you're unsure what arguments you need, just check your current systemd-boot configuration, if you followed installation.md, you will have it in: /efi/loader/entries/arch-hyprland.conf, all of the options= line contain kernel command line args):

echo "rw loglevel=3" > /etc/kernel/cmdline
echo "cryptdevice=LABEL=ARCH_LUKS:cryptroot:allow-discards" >> /etc/kernel/cmdline
echo "root=/dev/mapper/cryptroot rootflags=subvol=/@" >> /etc/kernel/cmdline

Note that you shouldn't be specifying the cryptdevice or root kernel parameters if you're using systemd initramfs, rather than udev one (you do still need rootflags to select the btrfs subvolume though, unless the root partition is your default subvolume). (If you haven't messed with it, you will be using udev initramfs).

Modify the linux preset for mkinitcpio to build UKIs

Now open /etc/mkinitcpio.d/linux.preset, where you'll want to:

• Uncomment ALL_config
• Comment default_image
• Uncomment default_uki (unified kernel image)
• Uncomment default_options
• Comment fallback_image
• Uncomment fallback_uki

Recreate /efi

First, we'll need to unmount /mnt/boot, which is currently bind mounted to /efi/EFI/arch-1. This is because we'll no longer be storing the kernel, initramfs nor the microcode in the EFI partition at all. The EFI partition will now only contain the UKI, the rest can be left in /boot, which will now be a part of the root partition, not mounted anywhere.

umount /boot
vim /etc/fstab  # remove the bind mount entry for /boot

Now, we will remove everything in the EFI partition, and start from scratch (this will erase the current systemd-boot configuration, you may want to back it up (/efi/loader/loader.conf and /efi/loader/entries/))

rm -rf /efi/*

Then, we will create the /efi/EFI/Linux directory, which will contain our UKIs. (You can change this location in /etc/mkinitcpio.d/linux.preset if you wish to use some other directory in the EFI partition, however it is recommended that you stick with Linux).

mkdir -p /efi/EFI/Linux

Finally, we will reinstall the kernel and microcode, populating /boot (now on the root partition).

This will also trigger a initramfs rebuild, which will now create the UKI image based on the linux.preset file.

pacman -S linux amd-ucode

Boot Manager

This step is optional, because the Unified Kernel Images can actually be booted into directly from the UEFI, if you wish to do that, you can run the following to add them as entries in the UEFI boot menu:

pacman -S efibootmgr
efibootmgr --create --disk /dev/disk/nvme0n1 --part 1 --label "Arch Linux (Hyprland)" --loader 'EFI\Linux\arch-linux.efi' --unicode
efibootmgr -c -d /dev/disk/nvme0n1 -p 1 -L "Arch Linux (Hyprland) Fallback" -l 'EFI\Linux\arch-linux-fallback.efi' -u
pacman -R systemd-boot

You can also specify additional kernel parameters / override the default ones in the UKI, by simply adding a string as a last positional argument to the efibootmgr command, allowing you to create entires with different kernel command lines easily.

Doing the above is technically safer than going with a boot manager, as it cuts out the middle-man entirely, however it can sometimes be nice to have boot manager, as it can show you a nice boot menu, and allow you to modify the kernel parameters, or add entries for different operating systems very easily, without having to rely on the specific implementation of the boot menu in your UEFI firmware (which might take really long to open, or just generally not provide that good/clean experience). Because of that, I like to instead still install the systemd-boot. To do so, we can just install normally with:

bootctl install --esp-path=/efi

We can now reboot. Systemd-boot will pick up any UKI images in /efi/EFI/Linux automatically (this path is hard-coded), even without any entry configurations.

That said, if you do wish to do so, you can still add an explicit entry for your configuration in /efi/loader/entries/arch-hyprland.conf:

title Arch Linux (Hyprland)
sort-key 0
efi /EFI/Linux/arch-linux.efi
# If you wish, you can also specify kernel options here, it will
# append/override those in the UKI image
#options rootflags=subvol=/@
#options rw loglevel=3

Although do note that if your UKI image is stored in /efi/EFI/Linux, because systemd-boot picks it up automatically, you will see the entry twice, so you'll likely want to change the target directory for the UKIs (in /etc/mkinitcpio.d/linux.preset) to something else.

I however wouldn't recommend this approach, and I instead just let systemd-boot autodetect the images, unless you need something specific.

If everything went well, you should see a new systemd based initramfs, from where you'll be prompted for the LUKS2 password.

Secure Boot

Now that we're booting with UKIs, we have images that that we'll be able to sign for secure boot, hence only allowing us to boot from those images.

However before we can set signing up, we will need to create new signing keys, and upload them into secure boot.

Enter Setup mode

To allow us to upload new signing keys into secure boot, we will need to enter "setup mode". This should be possible by going to the Secure Boot category in your UEFI settings, and clicking on Delete/Clear certificates, or there could even just be a "Setup Mode" option directly.

Once enabled, save the changes and boot back into Arch linux.

pacman -S sbctl
sbctl status

Make sure that sbctl reports that Setup Mode is Enabled.

Create Secure Boot keys

We can now create our new signing keys for secure boot. These keys will be stored in /usr/share/secureboot (so in our encrypted root partition). Once created, we will add (enroll) these keys into the UEFI firmware (only possible when in setup mode)

sbctl create-keys
#  the -m adds microsoft vendor key, required for most HW. Not using it could brick your device.
sbctl enroll-keys -m

Note: If you see messages about immutable files, run chattr -i [file] for all of the listed immutable files, then re-run enroll-keys command. (Linux kernel will sometimes mark the runtime EFI files as immutable for security - to prevent bricking the device with just rm -rf /*, or similar stupid commands, however since we trust that sbctl will work and won't do anything malicious, we can just remove the immutable flag, and re-running will now work).

Sign the bootloader and Unified Kernel Images

Finally then, we can sign the .efi executables that we'd like to use:

sbctl sign -s -o /usr/lib/systemd/boot/efi/systemd-bootx64.efi.signed /usr/lib/systemd/boot/efi/systemd-bootx64.efi
sbctl sign -s /efi/EFI/BOOT/BOOTX64.EFI
sbctl sign -s /efi/EFI/systemd/systemd-bootx64.efi
sbctl sign -s /efi/EFI/Linux/arch-linux.efi
sbctl sign -s /efi/EFI/Linux/arch-linux-fallback.efi

(If you're not using systemd-boot, only sign the UKI images in /efi/EFI/Linux)

The -s flag means save: The files will be automatically re-signed when we update the kernel (via a sbctl pacman hook). To make sure that this is the case, we can run pacman -S linux and check that messages about image signing appear.

When done, we can make sure that everything that needed to be signed really was signed with:

sbctl verify

Reboot and enable secure boot

We should now be ready to enable secure boot, as our .efi images were signed, and the signing key was uploaded.

sbctl status

If you see Secure Boot marked as Enabled, it worked!

Set up TPM unlocking

We'll now set up the TPM module to store a LUKS encryption key for our root partition, which it can release if certain conditions are met (I'll talk about the specific conditions a few sections later). This will allow us to set it up in such a way, that allows automatic unlocking without having to enter the password at boot.

This is safe, because set up correctly, TPM will only release the password to unlock the drive if there wasn't any editing done to the way the system was booted up, in which case we should always end up at a lockscreen after the bootup, which will be our line of defense against attackers, rather than it being the encryption password itself.

Do make sure that if you go this route, your lockscreen doesn't have any vulnerabilities and can't be easily bypassed. In my case, I'm using the default linux account login screen, which I do trust is safe enough to keep others without password out. I also have PAM set up in such a way that after 3 failed attempts, the account will get locked for 10 minutes, which should prevent any brute-force attempts (this is actually the default).

Since TPM is a module integrated in the CPU or the motherboard, so if someone took out the physical drive with the encrypted data, they would still need to have a LUKS decryption key to actually be able to read the contents of the root partition.

Make sure you have a TPM v2 module

pacman -S tpm2-tss tpm2-tools

Verify that your system does actually have a TPM v2 module

systemd-cryptenroll --tpm2-device=list

Make sure that there is a device listed.

Switch from udev initramfs to systemd

By default, your initramfs will be using udev, however you can instead use systemd, which has a bunch of extra capabilities, such as being able to pick up the key from TPM2 chip.

Open /etc/mkinitcpio.conf and find a line that starts with HOOKS=

• Change udev to systemd
• Change keymap consolefont to sd-vconsole
• Add sd-encrypt before block, and remove encrypt
• If you were using mkinitcpio-numlock, also remove numlock, it doesn't work with systemd

(As an alternative to mkinitcpio-numlock, there is systemd-numlockontty, which creates a systemd service that enables numlock in TTYs after booting (you'll need to enable it), this however doesn't happen in initramfs directly, only aftwerwars. This shouldn't be too annoying though, as we'lll no longer have to be entering the encryption password, which is the only reason we'd need numlock in initramfs anyway.)

Additionally, with systemd initramfs, you shouldn't be specifying root nor cryptdevice kernel arguments, as systemd can actually pick those up automatically (they'll be discovered by systemd-cryptsetup-generator and auto-mounted from initramfs via systemd-gpt-auto-generator). We will however still need the rootflags argument for selecting the btrfs subvolume (unless your default subvolume is the root partition subvolume).

So, let's edit our kernel parameters:

echo "rw loglevel=3" > /etc/kernel/cmdline  # overwrite the existing cmdline
echo "rootflags=subvol=/@" >> /etc/kernel/cmdline

You'll also need to modify the /etc/fstab, as systemd will not use the /dev/mapper/cryptroot name, but rather you'll have a /dev/gpt-auto-root (there'll also be /dev/gpt-auto-root-luks, which is the encrypted partition). If you absolutely insist on using a mapper device, you can also use /dev/mapper/root though, which is what systemd will actually call it.

vim /etc/fstab

We can now regenerate the initramfs with: pacman -S linux (we could also do mkinitcpio -P, however that won't trigger the pacman hook which auto-signs our final UKI images, so we'd have to re-sign them with sbctl manually) and reboot to check if it worked.

Choosing PCRs

PCR stands for Platform Configuration Register, and all TPM v2 modules have a bunch of these registers, which hold hashes about the system's state. These registers are read-only, and their value is set by the TPM module itself.

The data held by the TPM module (our LUKS encryption key) can then only be accessed when all of the selected PCR registers contain the expected values. You can find a list of the PCR registers on Arch Wiki.

You can look at the current values of these registers with this command:

systemd-analyze pcrs

For our purposes, we will choose these:

• PCR0: Hash of the UEFI firmware executable code (may change if you update UEFI)
• PCR7: Secure boot state - contains the certificates used to validate each boot application
• PCR12: Overridden kernel command line, credentials

Important

If you're using systemd-boot (instead of booting directly from the UKI images), it is very important that we choose all 3, including PCR12, as many tutorials only recommend 0 and 7, which would however lead to a security hole, where an attacker would be able to remove the drive with the (unencrypted) EFI partition, and modify the systemd-boot loader config (loaders/loader.conf), adding editor=yes, and the put the drive back in.

This wouldn't violate secure boot, as the .efi image files were unchanged, and are still signed, so the attacker would be able to boot into the systemd-boot menu, from where they could edit the boot entry for our UKI and modify the kernel parameters (yes, even though UKIs contain the kernel command line inside of them, systemd-boot can still edit those arguments if editor=yes).

From there, the attacker could simply add a kernel argument like init=/bin/bash, which would bypass systemd as the init system and instead make the kernel run bash executable as the PID=1 (init) program. This would mean you would get directly into bash console that is running as root, without any need to enter a password.

However, with PCR12, this is prevented, as it detects that the kernel cmdline was overridden, and so the TPM module wouldn't release the key.

The nice thing about also selecting PCR12 is that it will even allow us to securely keep editor=yes in our loader.conf, for easy debugging, as all that will happen if we do edit the kernel command line will be that the TPM module will not release the credentials, and so the initramfs will just ask us to enter the password manually.

Generate recovery key

The following command will generate a new LUKS key and automatically add it to the encrypted root. You will be prompted for a LUKS password to this device.

This step is optional, as all it does is adding another LUKS keyslot with a generated recovery key, so that in case TPM wouldn't unlock the drive, you can use this key instead.

You're expected to delete your own key, which is assumed to be less secure than what this will generate. We will do this after the next step.

If you instead wish to keep your own key working, feel free to skip this step, and the key removal step later.

systemd-cryptenroll /dev/gpt-auto-root-luks --recovery-key

Enroll the key into TPM

The following command will enroll a new key into the TPM module and add it as a new keyslot of the specified LUKS2 encrypted device.

We also specify --tpm2-pcrs=0+7+12, which selects the PCR registers that we decided on above.

Note: If you already had something in the tpm2 moudle, you will want to add --wipe-slot=tpm2 too.

You will be prompted for a LUKS password to this device (you can still enter your original key, you don't need to use the recovery one, as we haven't deleted the original one yet).

systemd-cryptenroll --tpm2-device=auto --tpm2-pcrs=0+7+12 /dev/gpt-auto-root-luks

This will enroll the TPM2 token token as a key slot 2 for the encrypted drive.

If you're extra paranoid, you can also provide --tpm2-with-pin=yes, to prompt for a PIN code on each boot.

To check that it worked, you can use:

cryptsetup luksDump /dev/gpt-auto-root-luks

Make sure that there is an additional LUKS key slot.

Remove original key

This is an optional step, only follow it if you have generated and properly stored a recovery key.

Warning: Make absolutely certain that the recovery key does in fact work before doing this, otherwise, you may get locked out! You can test your recovery key with:

cryptsetup luksOpen /dev/gpt-auto-root-luks crypttemp # enter the recovery key
cryptsetup luksClose crypttemp

If this worked, proceed to:

cryptsetup luksRemoveKey /dev/disk/by-label/ARCH_LUKS  # Enter your key to be deleted

Reboot

After a reboot, the system should now get unlocked automatically without prompting for the password.

Remove key from TPM

In case you'd ever want to remove the LUKS key from TPM, you can do so simply with:

csystemd-cryptenroll --wipe-slot=tpm2

This will actually also remove the LUKS key from the /dev/gpt-auto-root-luks device.