Arch Linux

jwodder

Member

Registered: 2026-05-14

Posts: 19

[SOLVED] Will this TPM setup work?

I'm putting together a plan for a future Arch installation, and I'd like people with more knowledge & experience to audit my steps for setting up TPM to automatically decrypt a root LUKS partition on boot.

It's my understanding that binding to just PCR 7 when enrolling in TPM results in a TPM setup that can be bypassed (See https://oddlama.org/blog/bypassing-disk … m2-unlock/), and the wiki's suggestion of also binding to PCR 15 can apparently be bypassed as well.  Thus, I'm planning to also bind to PCR 11, but the wiki doesn't do the best job covering this, so I'd like to get my steps checked.

In addition to setting up TPM securely, the goal here includes doing it in such a way that no manual re-enrollment is required after a kernel upgrade.

Scenario

The steps in dm-crypt/Encrypting an entire system: LUKS on a partition with TPM2 and Secure Boot have been followed, up to but not including "Enrolling the TPM".  This includes using sbctl to sign the UKI and boot loader.

The TPM Setup

For these steps, assume that the encrypted LUKS partition (which will be decrypted and mounted as the filesystem root) is /dev/sda2.

1. Edit the kernel parameters in /etc/kernel/cmdline to include:

   rd.luks.options=tpm2-measure-pcr=yes,tpm2-device=auto

2. Install ukify, qrencode, and tpm2-tss

3. Create a high-entropy recovery key for the LUKS partition by running:

   sudo systemd-cryptenroll --recovery-key /dev/sda2

   This will print out a QR code that can be scanned to obtain the key.

4. Create the keys that will be used to sign the Unified Kernel Image for PCR 11:

   cd /etc/systemd
   sudo ukify genkey \
       --pcr-private-key=tpm2-pcr-private-key.pem \
       --pcr-public-key=tpm2-pcr-public-key.pem
   sudo ukify genkey \
       --pcr-private-key=tpm2-pcr-private-key-initrd.pem \
       --pcr-public-key=tpm2-pcr-public-key-initrd.pem

5. Configure ukify by creating the file /etc/kernel/uki.conf with the contents:

   [PCRSignature:all]
   PCRPrivateKey=/etc/systemd/tpm2-pcr-private-key.pem
   PCRPublicKey=/etc/systemd/tpm2-pcr-public-key.pem
   
   [PCRSignature:initrd]
   Phases=enter-initrd
   PCRPrivateKey=/etc/systemd/tpm2-pcr-private-key-initrd.pem
   PCRPublicKey=/etc/systemd/tpm2-pcr-public-key-initrd.pem

   I am assuming that I don't need to configure signing of the kernel in a "UKI" section here, as sbctl should take care of that via its mkinitcpio post-hook.

6. Finally, enroll:

   sudo systemd-cryptenroll \
       --wipe-slot=0 \
       --tpm2-device=auto \
       --tpm2-pcrs=7+15:sha256=0000000000000000000000000000000000000000000000000000000000000000 \
       --tpm2-public-key=/etc/systemd/tpm2-pcr-public-key-initrd.pem \
       --tpm2-public-key-pcrs=11 \
       /dev/sda2

Questions

• Will this properly enroll the LUKS key in the TPM so that the root partition is automatically unlocked on boot?

• Will this continue to work after a kernel upgrade or initramfs regeneration using mkinitcpio without having to manually run systemd-cryptenroll?

• Do I need the "PCRSignature:all" section and its keys, or can I get rid of them?

• Can I safely add the "rd.luks.options" kernel parameters to /etc/kernel/cmdline before setting up TPM?  Assume there is one or more reboot in between.

• Does the use of ukify (which uses systemd-measure, which seems to assume systemd-stub is in use) limit my boot loader options in any way, or does this only work with systemd-boot?

Last edited by jwodder (2026-06-30 13:01:22)

twig

Member

Registered: 2025-09-25

Posts: 9

Re: [SOLVED] Will this TPM setup work?

Hello,

I am not very experienced with PCR 11, but your questions intrigued me, so I replicated your setup with QEMU. To your questions:
1) Yes, the root partition will be automatically unlocked on boot.
2) Yes, everything will be automatically signed after a kernel upgrade or initramfs regeneration using mkinitcpio. Since you're installing systemd-ukify, mkinitcpio will use it to handle the UKI creation and PCR 11 with your uki.conf.
3) You don't need PCRSignature:all at all. You can only sign for the enter-initrd phase because you only need TPM to unlock the disk at boot, not after.
4) Yes, there is no issue with that. Until the TPM is enrolled, it will just fall back to asking for your standard password on those reboots.
5) Yes, other bootloaders should work fine. I tried a bit with GRUB, chainloading the unified .efi image directly, and it worked. I personally use EFI stub, though systemd-boot is probably more straightforward to use in this case.

I learned a lot of things in the process, thanks!

Last edited by twig (2026-06-29 15:48:27)

jwodder

Member

Registered: 2026-05-14

Posts: 19

Re: [SOLVED] Will this TPM setup work?

Thanks for looking into this!  I hope your results are reproducible.