TPM2 PCR Measurements Made by systemd

Various systemd components issue TPM2 PCR measurements during the boot process, both in UEFI mode and from userspace. The following lists all measurements done, and describes (in case done before ExitBootServices()) how they appear in the TPM2 Event Log, maintained by the PC firmware. Note that the userspace measurements listed below are (by default) only done if a system is booted with systemd-stub — or in other words: systemd’s userspace measurements are linked to systemd’s UEFI-mode measurements, and if the latter are not done the former aren’t made either.

systemd will measure to PCRs 5 (boot-loader-config), 11 (kernel-boot), 12 (kernel-config), 13 (sysexts), 15 (system-identity).

Currently, four components will issue TPM2 PCR measurements:

A userspace measurement event log in a format close to TCG CEL-JSON is maintained in /run/log/systemd/tpm2-measure.log.

Measurements Added in Future

We expect that we’ll add further PCR extensions in future (both in firmware and user mode), which also will be documented here. When executed from firmware mode future additions are expected to be recorded as EV_EVENT_TAG measurements in the event log, in order to make them robustly recognizable. Measurements currently recorded as EV_IPL will continue to be recorded as EV_IPL, for compatibility reasons. However, EV_IPL will not be used for new, additional measurements.

PCR Measurements Made by systemd-boot (UEFI)

PCS 5, EV_EVENT_TAG, “loader.conf”

The content of systemd-boot’s configuration file, loader/loader.conf, is measured as a tagged event.

→ Event Tag 0xf5bc582a

→ Description in the event log record is the file name, loader.conf.

→ Measured hash covers the content of loader.conf as it is read from the ESP.

PCR 12, EV_IPL, “Kernel Command Line”

If the kernel command line was specified explicitly (by the user or in a Boot Loader Specification Type #1 file), the kernel command line passed to the invoked kernel is measured before it is executed. (In case an UKI/Boot Loader Specification Type #2 entry is booted, the built-in kernel command line is implicitly measured as part of the PE sections, because it is embedded in the .cmdline PE section, hence doesn’t need to be measured by systemd-boot; see below for details on PE section measurements done by systemd-stub.)

→ Description in the event log record is the literal kernel command line in UTF-16.

→ Measured hash covers the literal kernel command line in UTF-16 (without any trailing NUL bytes).

PCR Measurements Made by systemd-stub (UEFI)

PCR 11, EV_IPL, “PE Section Name”

A measurement is made for each PE section of the UKI that is defined by the UKI specification, in the canonical order described in the specification.

Happens once for each UKI-defined PE section of the UKI, in the canonical UKI PE section order, as per the UKI specification. For each record a pair of records is written, first one that covers the PE section name (described here), and the second one that covers the PE section data (described below), so that both types of records appear interleaved in the event log.

→ Description in the event log record is the PE section name in UTF-16.

→ Measured hash covers the PE section name in ASCII (including a trailing NUL byte!).

PCR 11, EV_IPL, “PE Section Data”

Happens once for each UKI-defined PE section of the UKI, in the canonical UKI PE section order, as per the UKI specification, see above.

→ Description in the event log record is the PE section name in UTF-16.

→ Measured hash covers the (binary) PE section contents.

PCR 12, EV_IPL, “Kernel Command Line”

Might happen up to three times, for kernel command lines from:

  1. Passed cmdline
  2. System and per-UKI cmdline add-ons (one measurement covering all add-ons combined)
  3. SMBIOS cmdline

→ Description in the event log record is the literal kernel command line in UTF-16.

→ Measured hash covers the literal kernel command line in UTF-16 (without any trailing NUL bytes).

PCR 12, EV_EVENT_TAG, “Devicetrees”

Devicetree addons are measured individually as a tagged event.

→ Event Tag 0x6c46f751

→ Description the addon filename.

→ Measured hash covers the content of the Devicetree.

PCR 12, EV_IPL, “Per-UKI Credentials initrd”

→ Description in the event log record is the constant string “Credentials initrd” in UTF-16.

→ Measured hash covers the per-UKI credentials cpio archive (which is generated on-the-fly by systemd-stub).

PCR 12, EV_IPL, “Global Credentials initrd”

→ Description in the event log record is the constant string “Global credentials initrd” in UTF-16.

→ Measured hash covers the global credentials cpio archive (which is generated on-the-fly by systemd-stub).

PCR 13, EV_IPL, “sysext initrd”

→ Description in the event log record is the constant string “System extension initrd” in UTF-16.

→ Measured hash covers the per-UKI sysext cpio archive (which is generated on-the-fly by systemd-stub).

PCR Measurements Made by systemd-pcrextend (Userspace)

PCR 11, “Boot Phases”

The systemd-pcrphase.service, systemd-pcrphase-initrd.service, systemd-pcrphase-sysinit.service services will measure the boot phase reached during various times of the boot process. Specifically, the strings “enter-initrd”, “leave-initrd”, “sysinit”, “ready”, “shutdown”, “final” are measured, in this order. (These are regular units, and administrators may choose to define additional/different phases.)

→ Measured hash covers the phase string (in UTF-8, without trailing NUL bytes).

PCR 15, “Machine ID”

The systemd-pcrmachine.service service will measure the machine ID (as read from /etc/machine-id) during boot.

→ Measured hash covers the string “machine-id:” suffixed by the machine ID formatted in hexadecimal lowercase characters (in UTF-8, without trailing NUL bytes).

PCR 15, “File System”

The systemd-pcrfs-root.service and systemd-pcrfs@.service services will measure a string identifying a specific file system, typically covering the root file system and /var/ (if it is its own file system).

→ Measured hash covers the string “file-system:” suffixed by a series of six colon-separated strings, identifying the file system type, UUID, label as well as the GPT partition entry UUID, entry type UUID and entry label (in UTF-8, without trailing NUL bytes).

PCR Measurements Made by systemd-cryptsetup (Userspace)

PCR 15, “Volume Key”

The systemd-cryptsetup@.service service will measure a key derived from the LUKS volume key of a specific encrypted volume, typically covering the backing encryption device of the root file system and /var/ (if it is its own file system).

→ Measured hash covers the (binary) result of the HMAC(V,S) calculation where V is the LUKS volume key, and S is the string “cryptsetup:” followed by the LUKS volume name and the UUID of the LUKS superblock.