systemd-pcrphase.service, systemd-pcrphase-sysinit.service, systemd-pcrphase-initrd.service, systemd-pcrmachine.service, systemd-pcrfs-root.service, systemd-pcrfs@.service, systemd-pcrextend — Measure boot phase into TPM2 PCR 11, machine ID and file system identity into PCR 15
systemd-pcrphase-initrd.service are system services that measure specific strings
into TPM2 PCR 11 during boot at various milestones of the boot process.
systemd-pcrmachine.service is a system service that measures the machine ID
(see machine-id(5)) into
services that measure file system identity information (i.e. mount point, file system type, label and
UUID, partition label and UUID) into PCR 15.
systemd-pcrfs-root.service does so for
the root file system,
systemd-pcrfs@.service is a template unit that measures the
file system indicated by its instance identifier instead.
These services require systemd-stub(7) to be used in a unified kernel image (UKI). They execute no operation when the stub has not been used to invoke the kernel. The stub will measure the invoked kernel and associated vendor resources into PCR 11 before handing control to it; once userspace is invoked these services then will extend TPM2 PCR 11 with certain literal strings indicating phases of the boot process. During a regular boot process PCR 11 is extended with the following strings:
enter-initrd" — early when the initrd initializes, before activating
system extension images for the initrd. It acts as a barrier between the time where the kernel
initializes and where the initrd starts operating and enables system extension images, i.e. code
shipped outside of the UKI. (This extension happens when the
service is started.)
leave-initrd" — when the initrd is about to transition into the host
file system. It acts as barrier between initrd code and host OS code. (This extension happens when the
systemd-pcrphase-initrd.service service is stopped.)
sysinit" — when basic system initialization is complete (which
includes local file systems having been mounted), and the system begins starting regular system
services. (This extension happens when the
service is started.)
ready" — during later boot-up, after remote file systems have been
activated (i.e. after
remote-fs.target), but before users are permitted to log in
systemd-user-sessions.service). It acts as barrier between the time
where unprivileged regular users are still prohibited to log in and where they are allowed to log in.
(This extension happens when the
systemd-pcrphase.service service is started.)
shutdown" — when the system shutdown begins. It acts as barrier
between the time the system is fully up and running and where it is about to shut down. (This extension
happens when the
systemd-pcrphase.service service is stopped.)
final" — at the end of system shutdown. It acts as barrier between
the time the service manager still runs and when it transitions into the final shutdown phase where
service management is not available anymore. (This extension happens when the
service is stopped.)
During a regular system lifecycle, PCR 11 is extended with the strings
shutdown", and "
Specific phases of the boot process may be referenced via the series of strings measured, separated
by colons (the "phase path"). For example, the phase path for the regular system runtime is
enter-initrd:leave-initrd:sysinit:ready", while the one for the initrd is just
enter-initrd". The phase path for the boot phase before the initrd is an empty string;
because that's hard to pass around a single colon ("
:") may be used instead. Note that
the aforementioned six strings are just the default strings and individual systems might measure other
strings at other times, and thus implement different and more fine-grained boot phases to bind policy
By binding policy of TPM2 objects to a specific phase path it is possible to restrict access to them to specific phases of the boot process, for example making it impossible to access the root file system's encryption key after the system transitioned from the initrd into the host root file system.
pre-calculate expected PCR 11 values for specific boot phases (via the
automatically pulled into the initial transaction by
for the root and
will do this for all mounts with the
x-systemd.pcrfs mount option in
/usr/lib/systemd/system-pcrextend executable may also be invoked from the
command line, where it expects the word to extend into PCR 11, as well as the following switches:
Takes the PCR banks to extend the specified word into. If not specified the tool automatically determines all enabled PCR banks and measures the word into all of them.
Takes the index of the PCR to extend. If
--file-system= are specified defaults to 15, otherwise defaults to 11.
Controls which TPM2 device to use. Expects a device node path referring to the TPM2
/dev/tpmrm0). Alternatively the special value "
may be specified, in order to automatically determine the device node of a suitable TPM2 device (of
which there must be exactly one). The special value "
list" may be used to enumerate
all suitable TPM2 devices currently discovered.
If no TPM2 firmware, kernel subsystem, kernel driver or device support is found, exit with exit status 0 (i.e. indicate success). If this is not specified any attempt to measure without a TPM2 device will cause the invocation to fail.
Instead of measuring a word specified on the command line into PCR 11, measure the host's machine ID into PCR 15.
Instead of measuring a word specified on the command line into PCR 11, measure identity information of the specified file system into PCR 15. The parameter must be the path to the established mount point of the file system to measure.
Measurements are logged into an event log file maintained in
/run/log/systemd/tpm2-measure.log, which contains a JSON-SEQ series of objects that follow the
general structure of the TCG Common Event Log
Format (CEL-JSON) event objects (but lack the "
LOCK_EX BSD file lock (flock(2)) on
the log file is acquired while the measurement is made and the file is updated. Thus, applications
that intend to acquire a consistent quote from the TPM with the associated snapshot of the event log
should acquire a
LOCK_SH lock while doing so.