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LUKS TPM2

A small utility script to manage LUKS keyfiles sealed by a TPM 2.0.

This script assumes you will be using a sealed keyfile or a key stored in the TPM during boot to unlock the root file system. It is intended to be used as part of your kernel update process to generate a key sealed against the new kernel's PCR values.

Requirements

This script requires:

Update Process

The script facilitates a variety of kernel update flows. For example, you could set a temporary passphrase interactively during the update:

  1. Kernel is updated
  2. luks-tpm2 temp is called, either manually or via an update hook, and sets a temporary LUKS passphrase
  3. The system is rebooted into the new kernel
  4. Because the TPM PCRs have changed, the old keyfile cannot be unsealed
  5. User enters the temporary passphrase to unlock the disk
  6. luks-tpm2 reset is called, generating a new keyfile sealed by the TPM and removing the temporary passphrase

Alternately, the PCR values of the new kernel can be computed in advance using and external command. The example below uses tpm_futurepcr:

  1. Kernel is updated
  2. luks-tpm2 -c "tpm_futurepcr -L '::pcr::' -o '::output::'" compute is called, either manually or via an update hook, which pre-computes the new kernel PCR values and replaces the existing TPM key with a new, random value
  3. The system is rebooted into the new kernel
  4. Because the pre-computed PCR values match the new kernel, unsealing the key succeeds
  5. The unsealed key is used to unlock the disk normally

LUKS Key Slots

The script requires two LUKS key slots to function: one for the sealed keyfile and one for the temporary passphrase. You are also strongly encouraged to dedicate an additional slot for a recovery passphrase not managed by luks-tpm2.

The default key slot layout is:

  • Slot 0: Recovery passphrase (optional)
  • Slot 1: TPM keyfile
  • Slot 2: Temporary passphrase

Usage

luks-tpm2 [OPTION]... [DEVICE] ACTION

Options

-h         Print help
-v         Print version
-q         Don't prompt for confirmation
-m PATH    Mount point for the tmpfs file system used to store TPM keyfiles
           Default: /root/keyfs
-p PATH    Sealed keyfile path
           .priv will be added to the path for the private section and .pub
           will be added for the public portion
           Default: /boot/keyfile  (/boot/keyfile.priv /boot/keyfile.pub)
-H HEX     The TPM handle of the parent object for the sealed key
           Default: 0x81000001
-K         Prompt for parent key password
-k PATH    Path to a file containing the parent key password
-x HEX     Index of the TPM NVRAM area holding the key
-s NUMBER  Key size in byes
           Default: 32
-f PATH    LUKS keyfile used instead of prompting for LUKS passphraes
-o PATH    Optionally write the new LUKS keyfile sealed in the tpm to this path
-t NUMBER  LUKS slot number for the TPM key
           Default: 1
-r NUMBER  LUKS slot number for temporary reset passphrase
           Default: 2
-L STRING  List of PCR banks used to seal LUKS key
           Default: sha256:0,2,4,7
-l STRING  List of PCR banks used to unseal LUKS key
           Default: <value of -L>
-T STRING  TCTI module used to communicate with the TPM
           Default: device:/dev/tpmrm0
-c COMMAND The command used to precompute PCR values
           Ex: tpm_futurepcr -L '::pcr::' -o '::output::'

Actions

init

Initialize the LUKS TPM key slot, by default in LUKS slot 1. This action will prompt for an existing LUKS passphrase and remove any existing key in slot 1. It will then generate a random key, seal it with the TPM against the current PCR values, and store the sealed key on disk or in NVRAM depending on the options specified.

temp

Set a temporary LUKS passphrase. The TPM will be used to unseal the passphrase for LUKS slot 1, which will be used to set a temporary passphrase in slot 2. The user will be interactively prompted to enter this temporary passphrase.

reset

Prompts the user for the temporary passphrase (if needed) and uses it to set a new passphrase in slot 1. The slot one key is then sealed by the TPM using the current PCR values, and LUKS slot 2 is cleared.

replace

The replace action allows a TPM-sealed LUKS key to be replaced (overwritten) by a new, randomly generated key. By default, LUKS slot 1 will be replaced. This action will not prompt for a passphrase, so the current key must be both "unsealable" by the TPM and a valid LUKS key.

compute

Pre-computes the PCR values for the kernel that will be used on next boot. Use the precomputed values to replace the current LUKS passphrase with a new, random value.

Pre-computing the PCR values is accomplished using an external command specified by the -c option or using the defaults file. The command is executed using the system sh POSIX shell. The command is expected to accept the PCR bank specification used to seal the passphrase, compute the PCR values for the next system boot, and write their binary values to the supplied output path.

Two placeholders can be used in the command string:

  • ::pcr:: will be replaced by the PCR bank specfication
  • ::output:: will be replaced by the output path for the binary PCR values

In addition, the command environment will contain PCR_LIST and OUTPUT_PATH variables that contain the same information as the placeholders.

Default configuration

The script will read default configuration values by sourcing /etc/default/luks-tpm2 if it exists. The location of this file can changed by setting the CONFFILE environment variable. Variables read from the config file will override hard coded defaults, but will not override command line arguments.

LUKS device detection

If the path to a LUKS block device is not provided luks-tpm2 will use the first device with a crypto_LUKS filesystem.

How-to

luks-tpm2 can protect LUKS keys using the TPM in one of two ways:

  • On disk as a pair of "sealed" files that can only be decrypted by the TPM
  • In TPM non-volatile memory (NVRAM)

In either case, the data is only accessible when certain Platform Configuration Registers (PCRs) have not changed. This indicates that the system has not been altered since the data was sealed.

Note that all TPM objects will be created in the owner hierarchy.

Before working with the TPM, consider setting the TPM2TOOLS_TCTI environment variable for your TPM resource manager. For example, to use the in-kernel RM:

$ export TPM2TOOLS_TCTI=device:/dev/tpmrm0

If you want to be able to access the TPM as a normal user, add yourself to the tss group, otherwise you will have to run all the following tpm2_ commands as root.

On-disk

Before storing sealed key files on disk, you must create a parent encryption key on the TPM. In this example, we create a primary RSA key in the owner hierarchy and make it persistent at handle 0x81000001:

$ tpm2_createprimary -c primary.ctx
$ tpm2_evictcontrol -c primary.ctx 0x81000001

Next, call luks-tpm2 with appropriate options:

$ sudo -E luks-tpm2 -p /boot/keyfile -H 0x81000001 /dev/sdaX init

Two sealed files will be generated (in /boot for this example): /boot/keyfile.priv and /boot/keyfile.pub.

Parent key password

The parent encryption key can optionally be created with a password. This password will need to be supplied during operations that require the parent key. The -K option will cause luks-tpm2 to display an interactive password prompt. -k PATH will instead attempt to read the password from a file at PATH.

$ tpm2_createprimary -c primary.ctx -p MyPassword
$ tpm2_evictcontrol -c primary.ctx 0x81000001
$ sudo -E luks-tpm2 -p /boot/keyfile -H 0x81000001 -K /dev/sdaX init

NVRAM

Most TPMs provide a small amount of user-configurable non-volatile memory (NVRAM) that will perisist between reboots. Note that NVRAM often has a limited number of writes, so it may not be a good option if frequent updates are required.

Before initializing NVRAM storage, locate a free index:

$ tpm2_getcap handles-nv-index

And then call luks-tpm2 with appropriate options:

$ sudo -E luks-tpm2 -x 0x1500001 /dev/sdaX init

License and Copyright

Copyright 2018-2023 Corey Hinshaw corey@electrickite.org

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.