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.

See UAPI.7 Linux TPM PCR Registry for an overview of PCRs.

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.

NvPCR Measurements

Since the PCR number space is very small, systemd userspace supports additional PCRs implemented via TPM2 NV Indexes (here called NvPCRs, even though they are no less volatile than classic PCRs), using the TPM2_NT_EXTEND type. These mostly behave like real PCRs, but we can allocate them relatively freely from the NV index handle space.

The NV index range to use for this is configurable at build time, so that downstreams have some flexibility to change this if they want. This uses the 0x01d10200 NV index as base by default. To abstract the actual nvindex number away there’s a naming concept, so that nvindexes are referenced by name string rather than number.

NvPCRs are defined in little JSON snippets in /usr/lib/nvpcr/*.nvpcr, that match up index number and name, as well as pick a hash algorithm.

There’s one complication: these NV indexes (like any NV indexes) can be deleted by anyone with access to the TPM, and then be recreated. This could be used to reset the NvPCRs to zero during runtime, which defeats the whole point of them. Our way out: we measure a secret as first thing after creation into the NvPCRs. (Or actually, we measure a per-NvPCR secret we derive from a system secret via an HMAC of the NvPCR name and the NV index handle). This “anchoring” secret is stored in /run/ + /var/lib/ + ESP/XBOOTLDR (the latter encrypted as credential, locked to the TPM), to make it available at the whole runtime of the OS. It’s only accessible to privileged processes with access to the TPM. Due to this, any process with access to the TPM and read access to any of the storage locations of the anchor secret is considered part of the TCB, as they are able to replay the NvPCR with their own content at will, so due care must be employed when designing a system that uses this feature.

PCR Measurements Made by systemd-boot (UEFI)

PCR 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 UAPI.5 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 is the addon filename.

→ Measured hash covers the content of the DeviceTree.

PCR 12, EV_EVENT_TAG, initrd addons

Initrd addons are measured individually as a tagged event.

→ Event Tag 0x49dffe0f

→ Description is the addon filename.

→ Measured hash covers the contents of the initrd.

PCR 12, EV_EVENT_TAG, ucode addons

Ucode addons are measured individually as a tagged event.

→ Event Tag 0xdac08e1a

→ Description is the addon filename.

→ Measured hash covers the contents of the ucode initrd.

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-tpm2-setup (Userspace)

PCR 9, NvPCR Initializations

The systemd-tpm2-setup.service service initializes any NvPCRs defined via *.nvpcr files. For each initialized NvPCR it will measure an event into PCR 9.

→ Measured hash covers the string nvpcr-init:, suffixed by the NvPCR name, suffixed by :0x, suffixed by the NV Index handle (formatted in hexadecimal), suffixed by a colon, suffixed by the hash function used, in lowercase (i.e. sha256 or so), suffixed by a colon, and finally suffixed by the state of the NvPCR after its initialization with the anchor measurement, in hexadecimal. Example: nvpcr-init:hardware:0x1d10200:sha256:de3857f637c61e82f02e3722e1b207585fe9711045d863238904be8db10683f2

PCR/NvPCR 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).

NvPCR hardware (base+0), product UUID

The systemd-pcrproduct.service service will measure the product UUID (as available from SMBIOS or Devicetree) of the host system, once at boot.

→ Measured hash covers the string “product-id:” suffixed by the product UUID formatted in hexadecimal lowercase characters, without separators. If no product UUID of the local system could be determined the string “product-id:missing” is measured instead. Example string: product-id:4691595be6a345f1833cc75fab63e475.

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 9, NvPCR initialization separator

After completion of systemd-tpm2-setup.service (which initializes all NvPCRs and measures their initial state) at arly boot the systemd-pcrnvdone.service service will measure a separator event into PCR 9, isolating the early-boot NvPCR initializations from any later additions.

→ Measured hash covers the string nvpcr-separator.

PCR/NvPCR 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.

NvPCR cryptsetup (base+1), LUKS unlock mechanism/key slot

The systemd-cryptsetup@.service service will measure information about the used LUKS keyslot, and in particular include the used unlock mechanism (pkcs11, tpm2, fido2, …) in it.

→ Measured hash covers the string “cryptsetup-keyslot:”, suffixed by the DM volume name, a “:” separator, the UUID of the LUKS superblock, a “:” separator, a brief string identifying the unlock mechanism, a “:” separator, and finally the LUKS slot number used. Example string: cryptsetup-keyslot:root:1e023a55-60f9-4b6b-9b80-67438dc5f065:tpm2:1