Trusted Platform Module
Trusted Platform Module (TPM) is an international standard for a secure cryptoprocessor, which is a dedicated microprocessor designed to secure hardware by integrating cryptographic keys into devices.
In practice a TPM can be used for various different security applications such as secure boot, key storage and random number generation.
TPM is naturally supported only on devices that have TPM hardware support. If your hardware has TPM support but it is not showing up, it might need to be enabled in the BIOS settings.
Versions
There are two very different TPM specifications: 2.0 and 1.2, which also use different software stacks. This article only describes TPM 2.0, for the older TPM 1.2 see /1.2.
TPM 2.0 allows direct access via /dev/tpm0 (one client at a time), kernel-managed access via /dev/tpmrm0, or managed access through the tpm2-abrmd resource manager daemon. According to a systemd project member, using tpm2-abrmd is no longer recommended. There are two choices of userspace tools, tpm2-tools by Intel and ibm-tssAUR by IBM.
TPM 2.0 requires UEFI boot; BIOS or Legacy boot systems can only use TPM 1.2.
Some TPM chips can be switched between 2.0 and 1.2 through a firmware upgrade (which can be done only a limited number of times).
Usage
Many informative resources to learn how to configure and make use of TPM 2.0 services in daily applications are available from the tpm2-software community.
Checking support
A TPM 2.0 chip has been a requirement for computers certified to run Windows 10 since 2016-07-28.[1] Linux has support for TPM 2.0 since version 3.20[2] and should not require any other steps to be enabled on a default Arch install.
Two ways to verify whether TPM 2.0 is setup without specific software:
- checking the logs, e.g., by running
journalctl -k --grep=tpmas root - read the value of
/sys/class/tpm/tpm0/device/description[3] or/sys/class/tpm/tpm0/tpm_version_major
Data-at-rest encryption with LUKS
There are two methods for unlocking a LUKS volume using a TPM. You can use Clevis or #systemd-cryptenroll.
Using either method, an encrypted volume or volumes may be unlocked using keys stored in a TPM, either automatically at boot or manually at a later time. Using a TPM for this purpose ensures that your drives will not unlock unless certain conditions are met, such as your firmware not having been modified and Secure Boot not having been disabled (see #Accessing PCR registers).
- This means that access to data is protected in case only the encrypted disk is stolen, but not in case the whole PC gets stolen.
- Be aware that this method makes you more vulnerable to cold boot attacks, because even if your computer has been powered off for a long time (ensuring the memory is completely cleared), an attacker could simply turn it on and wait for the TPM to load the key automatically. This may be a concern for high-value targets.
systemd-cryptenroll
See systemd-cryptenroll#Trusted Platform Module.
SSH
For TPM sealed SSH keys, there are two options:
- ssh-tpm-agent — ssh-agent compatible agent using TPM backed keys.
- https://github.com/Foxboron/ssh-tpm-agent || ssh-tpm-agent
- See Store ssh keys inside the TPM: ssh-tpm-agent.
- tpm2-pkcs11 — PKCS#11 interface for Trusted Platform Module 2.0 hardware.
- https://github.com/tpm2-software/tpm2-pkcs11 || tpm2-pkcs11
- See SSH configuration and Using a TPM for SSH authentication (2020-01).
GnuPG
GnuPG, since version 2.3, supports moving compatible keys into the TPM. See Using a TPM with GnuPG 2.3 for the instructions.
Other good examples of TPM 2.0 usage
- Configuring Secure Boot + TPM 2 (2018-06, Debian)
- Using the TPM - It's Not Rocket Science (Anymore) - Johannes Holland & Peter Huewe (2020-11, Youtube): examples for OpenSSL with tpm2-tss-engine
Accessing PCR registers
Platform Configuration Registers (PCR) contain hashes that can be read at any time but can only be written via the extend operation, which depends on the previous hash value, thus making a sort of blockchain. They are intended to be used for platform hardware and software integrity checking between boots (e.g. protection against Evil Maid attack). They can be used to unlock encryption keys and proving that the correct OS was booted.
The TCG PC Client Specific Platform Firmware Profile Specification defines the registers in use, and The Linux TPM PCR Registry assigns Linux system components using them.
The registers are:
| PCR | Use | Notes |
|---|---|---|
| PCR0 | Core System Firmware executable code (aka Firmware) | May change if you upgrade your UEFI |
| PCR1 | Core System Firmware data (aka UEFI settings) | |
| PCR2 | Extended or pluggable executable code | |
| PCR3 | Extended or pluggable firmware data | Set during Boot Device Select UEFI boot phase |
| PCR4 | Boot Manager Code and Boot Attempts | Measures the boot manager and the devices that the firmware tried to boot from |
| PCR5 | Boot Manager Configuration and Data | Can measure configuration of boot loaders; includes the GPT Partition Table |
| PCR6 | Resume from S4 and S5 Power State Events | |
| PCR7 | Secure Boot State | Contains the full contents of PK/KEK/db, as well as the specific certificates used to validate each boot application[4] |
| PCR81 | Hash of the kernel command line | Supported by grub and systemd-boot |
| PCR91 | Hash of the initrd and EFI Load Options | Linux measures the initrd and EFI Load Options, essentially the kernel cmdline options. |
| PCR101 | Reserved for Future Use | |
| PCR111 | Hash of the Unified kernel image | see systemd-stub(7) |
| PCR121 | Overridden kernel command line, Credentials | see systemd-stub(7) |
| PCR131 | System Extensions | see systemd-stub(7) |
| PCR141 | shim's MokList, MokListX, and MokSBState. | [5] |
| PCR151 | Unused | |
| PCR161 | Debug | May be used and reset at any time. May be absent from an official firmware release. |
| PCR23 | Application Support | The OS can set and reset this PCR. |
- Use case defined by the OS and might change between various Linux distros and Windows devices.
On Windows, BitLocker uses PCR8-11 (Legacy) or PCR11-14 (UEFI) for its own purposes. Documentation from tianocore[6].
tpm2-totp facilitates this check with a human observer and dedicated trusted device.
The current PCR values can be listed with systemd-analyze(1):
$ systemd-analyze pcrs
Or, alternatively with tpm2_pcrread(1) from tpm2-tools:
# tpm2_pcrread
Troubleshooting
TPM2 LUKS2 unlocking still asking for password
If you followed the instruction described above for automatically unlocking luks2 devices with enrolled keys in a TPM2 hardware module, but still receive a prompt to input a password during the initramfs boot stage. You may need to early load the kernel module (you can obtain its name with systemd-cryptenroll --tpm2-device=list) that is responsible for handling your specific TPM2 module.
See also
- Gentoo:Trusted Platform Module
- TPM-JS testing tool: source - live web version.