I’ve been using Archlinux for a while. One painpoint for me is that there is no push-button end-to-end data protection solution like Bitlocker on Windows or FileVault on MAC. I’ve used solutions like LUKS2 with TPM2, but they are not as seamless as Bitlocker. I have to enter the password every time the kernel is updated since the TPM measurement updated.

Recently, I learned that systemd-measure is introduced in systemd from version 252. In short, it provides the ability to measure the PCR of a unified kernel image (UKI) before they are actually booted. I played around with it, and below is my config. Hopefully it works for you if you are using Archlinux.

Environment before the setup

  • An LUKS2 encrypted root partition with sd-encrpyt hook, see here
  • A properly configured secureboot PKI system. I personally use sbctl

How it works

The systemd-cryptenroll allows to config a keyslot, which, ANY PCR values signed by a specific pubkey can unlock the volume. The systemd-measure can measure the PCR values of a UKI, and the measurement is signed by the keypair and inserted into the UKI. Thus, the initrd can unlock the volume since a signature of the current PCR measurements is provided to the TPM to release the master key to the volume.

Steps

  1. Generate a keypair to sign the PCR measurement. 
    openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out /etc/systemd/tpm2-pcr-private-key.pem
openssl rsa -pubout -in /etc/systemd/tpm2-pcr-private-key.pem -out /etc/systemd/tpm2-pcr-public-key.pem

    The path here /etc/systemd/tpm2-pcr-private-key.pem has specific meaning documented in the man page

    If this option is not specified but it is attempted to unlock a LUKS2 volume with a signed TPM2 PCR enrollment a suitable signature file tpm2-pcr-signature.json is searched for in /etc/systemd/, /run/systemd/, /usr/lib/systemd/ (in this order).

  2. Install systemd-ukify 
    pacman -S systemd-ukify
  3. Draft a config file per the man page 
    [UKI]
Linux=/boot/vmlinuz-linux
Initrd=/boot/intel-ucode.img /boot/initramfs-linux.img
Cmdline=@/etc/kernel/cmdline
OSRelease=@/etc/os-release
PCRBanks=sha256
SecureBootSigningTool=sbsign
SecureBootPrivateKey=/usr/share/secureboot/keys/db/db.key
SecureBootCertificate=/usr/share/secureboot/keys/db/db.pem
[PCRSignature:default]
PCRPrivateKey=/etc/systemd/tpm2-pcr-private-key.pem
PCRPublicKey=/etc/systemd/tpm2-pcr-public-key.pem

    Please note that, I used sbctl’s db key for secureboot signature. I used the previously generated keypair for PCR measurement signature.

  4. Run ukify to generate the UKI 
    ukify -c /etc/ukify.conf build --output /boot/EFI/Linux/ukify-linux.efi

    Per man page of ukify, it will call the systemd-measure, precalculate the PCR values, and sign the measurement with the keypair. The signature is stored in .pcrsig. According to the man page of the systemd-stub:

    A “.pcrsig” section with a set of cryptographic signatures for the expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a specific key.
    The output file /boot/EFI/Linux/ukify-linux.efi is also deliberately chosen so that systemd-boot can automatically pick it up.

  5. Create a new LUKS2 slot with the pubkey of the keypair 
    systemd-cryptenroll --tpm2-device=auto --tpm2-public-key=/etc/systemd/tpm2-pcr-public-key.pem \
/dev/disk/by-uuid/$DISKUUID

    Now that the LUKS2 volume can be unlocked by any PCR measurement signed by the keypair. We already got the PCR measurement signed by the keypair in the UKI. Thus, the initrd can unlock the volume.

Please note, I did not create pacman hooks to recreate the UKI after Linux kernel or ucode updated, please do it yourself.

Conclusion

In conclusion, implementing seamless Linux disk encryption with systemd-measure and related tools offers a robust solution for protecting your data. While the setup process involves several steps, the resulting configuration provides a secure and user-friendly experience.

By leveraging systemd-cryptenroll and ukify, we’ve established a system where the Unified Kernel Image’s measurements, signed by a secure keypair, enable automatic unlocking of the LUKS2-encrypted volume. This approach reduces the need for manual intervention during kernel updates, enhancing the overall convenience of disk encryption on Arch Linux.

Remember to adapt the configurations to your specific environment and consider additional security measures, such as updating the UKI after kernel or microcode updates.

With these steps, you can enjoy the benefits of seamless Linux disk encryption, bringing us closer to the ease of use provided by similar solutions on other operating systems.

Happy encrypting!