trustedboot

The utility installs a Disk encryption key in the TPM and unencrypts the disk automatically during boot. The settings for trusted boot differ for the various Linux distros and releases. This version is tested for Ubuntu 20.04 LTS. It requires TPM-Tools 4.x as the parameters completely changed from 3.x

Usage

1. Install Ubuntu 20.04 with Full Disk Encryption.
2. Execute install.sh
3. Reboot the machine, you will still be asked for your encryption passphrase
4. Update the TPM PCR policy with update-luks-tpm.sh
5. The next reboot is done automatically

Result

• Grub is still installed, but not used (as a fallback)
• The initial key for Disk Encryption is still valid (fallback for updates)
• Initramfs is updated with the script to ask the TPM for the passphrase
• A unified kernel is generated (kernel + command line parms + initramfs) as one large EFI blob
• Therefore during Boot only PCRs 0-7 are written (GRUB uses 8-9)

Details / manual installation

Ideas taken from

• https://threat.tevora.com/secure-boot-tpm-2/
• https://medium.com/@pawitp/full-disk-encryption-on-arch-linux-backed-by-tpm-2-0-c0892cab9704
• https://medium.com/@pawitp/its-certainly-annoying-that-tpm2-tools-like-to-change-their-command-line-parameters-d5d0f4351206

I used the PCRs 0,1,4,5,7 as policy for unlocking disk encryption. The PCRs 2,3 and 6 have the same hash value and are therefore assumed as empty. For additional security reasons, one may consider including these registers as well (to prevent e.g. Option ROM DMA attacks). Furthermore I use the RNG on the TPM for secret generation and use SHA256 and ECC instead of SHA1 and RSA.

All of the below instructions should be executed as root:

1. Install required tools apt install binutils tpm2-tools
2. Create a new secret for disk encryption: tpm2_getrandom 32 -o /root/secret.bin
3. Add the key to disk encryption cryptsetup luksAddKey /dev/nvme0n1p3 /root/secret.bin
4. Install the tpm-script to initramfs cp -vf ./tpm2-hook.sh /etc/initramfs-tools/hooks/
5. The entry in /etc/crypttab should look like this: dm_crypt-0 UUID=<uuid> none luks,discard,initramfs,keyscript=/usr/sbin/passphrase-from-tpm.sh
6. Update initramfs update-initramfs -u -k all
7. Create the Kernel Command Line echo "/vmlinuz-5.4.0-39-generic root=/dev/mapper/ubuntu--vg-ubuntu--lv ro" > /boot/kernel-command-line.txt
8. Create unified Kernel

mkdir -p /boot/efi/EFI/Linux
objcopy \
  --add-section .osrel="/usr/lib/os-release" --change-section-vma .osrel=0x20000 \
  --add-section .cmdline="/boot/kernel-command-line.txt" --change-section-vma .cmdline=0x30000 \
  --add-section .linux="/boot/vmlinuz-5.4.0-39-generic" --change-section-vma .linux=0x40000 \
  --add-section .initrd="/boot/initrd.img-5.4.0-39-generic" --change-section-vma .initrd=0x3000000 \
  "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" "/boot/efi/EFI/Linux/Linux.efi"

7. Create a EFI Boot entry for the new unified kernel efibootmgr --create --disk /dev/nvme0n1 --part 1 --label "ubuntu unified" --loader "\EFI\BOOT\Linux\Linux.efi" --verbose
8. Reboot the machine
9. Store the secret key in the TPM and use the now valid PCRs as policy

tpm2_evictcontrol -C o -c 0x81000000 #evict an old passphrase before writing the new one
tpm2_createpolicy --policy-pcr -l sha256:0,1,4,5,7 -L /root/policy.digest
tpm2_createprimary -C e -g sha256 -G ecc256 -c /root/primary.context
tpm2_create -g sha256 -u /root/obj.pub -r /root/obj.priv -C /root/primary.context -L /root/policy.digest -a "noda|adminwithpolicy|fixedparent|fixedtpm" -i /root/secret.bin
tpm2_load -C /root/primary.context -u /root/obj.pub -r /root/obj.priv -c /root/load.context
tpm2_evictcontrol -C o -c /root/load.context 0x81000000
# tpm2_unseal -c 0x81000000 -p pcr:sha1:0,1,4,5,7 -o /root/test.bin #proof that the persistence worked
rm -f /root/load.context /root/obj.priv /root/obj.pub /root/policy.digest /root/primary.context

10. The next reboot should work automatically