Provided by: systemd_255.4-1ubuntu8.11_amd64 
NAME
systemd-measure - Pre-calculate and sign expected TPM2 PCR values for booted unified kernel images
SYNOPSIS
/usr/lib/systemd/systemd-measure [OPTIONS...]
DESCRIPTION
Note: this command is experimental for now. While it is likely to become a regular component of systemd,
it might still change in behaviour and interface.
systemd-measure is a tool that may be used to pre-calculate and sign the expected TPM2 PCR 11 values that
should be seen when a Linux Unified Kernel Image (UKI)[1] based on systemd-stub(7) is booted up. It
accepts paths to the ELF kernel image file, initrd image file, devicetree file, kernel command line file,
os-release(5) file, boot splash file, and TPM2 PCR PEM public key file that make up the unified kernel
image, and determines the PCR values expected to be in place after booting the image. Calculation starts
with a zero-initialized PCR 11, and is executed in a fashion compatible with what systemd-stub does at
boot. The result may optionally be signed cryptographically, to allow TPM2 policies that can only be
unlocked if a certain set of kernels is booted, for which such a PCR signature can be provided.
It usually doesn't make sense to call this tool directly when constructing a UKI. Instead, ukify(1)
should be used; it will invoke systemd-measure and take care of embedding the resulting measurements into
the UKI.
COMMANDS
The following commands are understood:
status
This is the default command if none is specified. This queries the local system's TPM2 PCR 11+12+13
values and displays them. The data is written in a similar format as the calculate command below, and
may be used to quickly compare expectation with reality.
Added in version 252.
calculate
Pre-calculate the expected values seen in PCR register 11 after boot-up of a unified kernel image
consisting of the components specified with --linux=, --osrel=, --cmdline=, --initrd=, --splash=,
--dtb=, --uname=, --sbat=, --pcrpkey= see below. Only --linux= is mandatory. (Alternatively, specify
--current to use the current values of PCR register 11 instead.)
Added in version 252.
sign
As with the calculate command, pre-calculate the expected value seen in TPM2 PCR register 11 after
boot-up of a unified kernel image. Then, cryptographically sign the resulting values with the
private/public key pair (RSA) configured via --private-key= and --public-key=. This will write a JSON
object to standard output that contains signatures for all specified PCR banks (see the --bank=
option below), which may be used to unlock encrypted credentials (see systemd-creds(1)) or LUKS
volumes (see systemd-cryptsetup@.service(8)). This allows binding secrets to a set of kernels for
which such PCR 11 signatures can be provided.
Note that a TPM2 device must be available for this signing to take place, even though the result is
not tied to any TPM2 device or its state.
Added in version 252.
OPTIONS
The following options are understood:
--linux=PATH, --osrel=PATH, --cmdline=PATH, --initrd=PATH, --splash=PATH, --dtb=PATH, --uname=PATH,
--sbat=PATH, --pcrpkey=PATH
When used with the calculate or sign verb, configures the files to read the unified kernel image
components from. Each option corresponds with the equally named section in the unified kernel PE
file. The --linux= switch expects the path to the ELF kernel file that the unified PE kernel will
wrap. All switches except --linux= are optional. Each option may be used at most once.
Added in version 252.
--current
When used with the calculate or sign verb, takes the PCR 11 values currently in effect for the system
(which should typically reflect the hashes of the currently booted kernel). This can be used in place
of --linux= and the other switches listed above.
Added in version 252.
--bank=DIGEST
Controls the PCR banks to pre-calculate the PCR values for – in case calculate or sign is invoked –,
or the banks to show in the status output. May be used more then once to specify multiple banks. If
not specified, defaults to the four banks "sha1", "sha256", "sha384", "sha512".
Added in version 252.
--private-key=PATH, --public-key=PATH
These switches take paths to a pair of PEM encoded RSA key files, for use with the sign command.
Note the difference between the --pcrpkey= and --public-key= switches. The former selects the data to
include in the ".pcrpkey" PE section of the unified kernel image, the latter picks the public key of
the key pair used to sign the resulting PCR 11 values. The former is the key that the booted system
will likely use to lock disk and credential encryption to, the latter is the key used for unlocking
such resources again. Hence, typically the same PEM key should be supplied in both cases.
If the --public-key= is not specified but --private-key= is specified the public key is automatically
derived from the private key.
Added in version 252.
--tpm2-device=PATH
Controls which TPM2 device to use. Expects a device node path referring to the TPM2 chip (e.g.
/dev/tpmrm0). Alternatively the special value "auto" 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.
Added in version 252.
--phase=PHASE
Controls which boot phases to calculate expected PCR 11 values for. This takes a series of
colon-separated strings that encode boot "paths" for entering a specific phase of the boot process.
Each of the specified strings is measured by the systemd-pcrphase-initrd.service,
systemd-pcrphase-sysinit.service, and systemd-pcrphase.service(8) into PCR 11 during different
milestones of the boot process. This switch may be specified multiple times to calculate PCR values
for multiple boot phases at once. If not used defaults to "enter-initrd",
"enter-initrd:leave-initrd", "enter-initrd:leave-initrd:sysinit",
"enter-initrd:leave-initrd:sysinit:ready", i.e. calculates expected PCR values for the boot phase in
the initrd, during early boot, during later boot, and during system runtime, but excluding the phases
before the initrd or when shutting down. This setting is honoured both by calculate and sign. When
used with the latter it's particularly useful for generating PCR signatures that can only be used for
unlocking resources during specific parts of the boot process.
For further details about PCR boot phases, see systemd-pcrphase.service(8).
Added in version 252.
--append=PATH
When generating a PCR JSON signature (via the sign command), combine it with a previously generated
PCR JSON signature, and output it as one. The specified path must refer to a regular file that
contains a valid JSON PCR signature object. The specified file is not modified. It will be read
first, then the newly generated signature appended to it, and the resulting object is written to
standard output. Use this to generate a single JSON object consisting from signatures made with a
number of signing keys (for example, to have one key per boot phase). The command will suppress
duplicates: if a specific signature is already included in a JSON signature object it is not added a
second time.
Added in version 253.
--json=MODE
Shows output formatted as JSON. Expects one of "short" (for the shortest possible output without any
redundant whitespace or line breaks), "pretty" (for a pretty version of the same, with indentation
and line breaks) or "off" (to turn off JSON output, the default).
--no-pager
Do not pipe output into a pager.
-h, --help
Print a short help text and exit.
--version
Print a short version string and exit.
EXAMPLES
Example 1. Generate a unified kernel image, and calculate the expected TPM PCR 11 value
$ ukify --output=vmlinux.efi \
--os-release=@os-release.txt \
--cmdline=@cmdline.txt \
--splash=splash.bmp \
--devicetree=devicetree.dtb \
--measure \
vmlinux initrd.cpio
11:sha1=d775a7b4482450ac77e03ee19bda90bd792d6ec7
11:sha256=bc6170f9ce28eb051ab465cd62be8cf63985276766cf9faf527ffefb66f45651
11:sha384=1cf67dff4757e61e5...7f49ad720be02fd07263e1f93061243aec599d1ee4b4
11:sha512=8e79acd3ddbbc8282...0c3e8ec0c714821032038f525f744960bcd082d937da
ukify(1) internally calls systemd-measure. The output with hashes is from systemd-measure.
Example 2. Generate a private/public key pair, a unified kernel image, and a TPM PCR 11 signature for it,
and embed the signature and the public key in the image
$ openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out tpm2-pcr-private.pem
..+.+++++++++......+.........+......+.......+....+.....+.+...+..........
$ openssl rsa -pubout -in tpm2-pcr-private.pem -out tpm2-pcr-public.pem
# systemd-measure sign \
--linux=vmlinux \
--osrel=os-release.txt \
--cmdline=cmdline.txt \
--initrd=initrd.cpio \
--splash=splash.bmp \
--dtb=devicetree.dtb \
--pcrpkey=tpm2-pcr-public.pem \
--bank=sha1 \
--bank=sha256 \
--private-key=tpm2-pcr-private.pem \
--public-key=tpm2-pcr-public.pem >tpm2-pcr-signature.json
# ukify --output=vmlinuz.efi \
--os-release=@os-release.txt \
--cmdline=@cmdline.txt \
--splash=splash.bmp \
--devicetree=devicetree.dtb \
--pcr-private-key=tpm2-pcr-private.pem \
--pcr-public-key=tpm2-pcr-public.pem \
--pcr-banks=sha1,sha256 \
vmlinux initrd.cpio
Later on, enroll the signed PCR policy on a LUKS volume:
# systemd-cryptenroll --tpm2-device=auto \
--tpm2-public-key=tpm2-pcr-public.pem \
--tpm2-signature=tpm2-pcr-signature.json \
/dev/sda5
And then unlock the device with the signature:
# systemd-cryptsetup attach \
volume5 /dev/sda5 - \
tpm2-device=auto,tpm2-signature=/path/to/tpm2-pcr-signature.json
Note that when the generated unified kernel image vmlinux.efi is booted, the signature and public key
files will be placed at locations systemd-cryptenroll and systemd-cryptsetup will look for anyway, and
thus these paths do not actually need to be specified.
Example 3. Introduce a second public key, signing the same kernel PCR measurements, but only for the
initrd boot phase
This example extends the previous one, but we now introduce a second signing key that is only used to
sign PCR policies restricted to the initrd boot phase. This can be used to lock down root volumes in a
way that they can only be unlocked before the transition to the host system. Thus we have two classes of
secrets or credentials: one that can be unlocked during the entire runtime, and the other that can only
be used in the initrd.
$ openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out tpm2-pcr-private.pem
.+........+.+........+.......+...+...+........+....+......+..+..........
$ openssl rsa -pubout -in tpm2-pcr-private.pem -out tpm2-pcr-public.pem
$ openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out tpm2-pcr-initrd-private.pem
..+.......++........+........+......+........+....+.....+.+..+..........
$ openssl rsa -pubout -in tpm2-pcr-initrd-private.pem -out tpm2-pcr-initrd-public.pem
# ukify --output vmlinux-1.2.3.efi \
--os-release=@os-release.txt \
--cmdline=@cmdline.txt \
--splash=splash.bmp \
--devicetree=devicetree.dtb \
--pcr-private-key=tpm2-pcr-private.pem \
--pcr-public-key=tpm2-pcr-public.pem \
--phases=enter-initrd,enter-initrd:leave-initrd,enter-initrd:leave-initrd:sysinit,enter-initrd:leave-initrd:sysinit:ready \
--pcr-banks=sha1,sha256 \
--pcr-private-key=tpm2-pcr-initrd-private.pem \
--pcr-public-key=tpm2-pcr-initrd-public.pem \
--phases=enter-initrd \
vmlinux-1.2.3 initrd.cpio \
--uname=1.2.3
+ /usr/lib/systemd/systemd-measure sign --linux=vmlinux-1.2.3 \
--osrel=os-release.txt --cmdline=cmdline.txt --dtb=devicetree.dtb \
--splash=splash.bmp --initrd=initrd.cpio --bank=sha1 --bank=sha256 \
--private-key=tpm2-pcr-private.pem --public-key=tpm2-pcr-public.pem \
--phase=enter-initrd --phase=enter-initrd:leave-initrd \
--phase=enter-initrd:leave-initrd:sysinit \
--phase=enter-initrd:leave-initrd:sysinit:ready
+ /usr/lib/systemd/systemd-measure sign --linux=vmlinux-1.2.3 \
--osrel=os-release.txt --cmdline=cmdline.txt --dtb=devicetree.dtb \
--splash=splash.bmp --initrd=initrd.cpio --bank=sha1 --bank=sha256 \
--private-key=tpm2-pcr-initrd-private.pem \
--public-key=tpm2-pcr-initrd-public.pem \
--phase=enter-initrd
Wrote unsigned vmlinux-1.2.3.efi
ukify prints out both invocations of systemd-measure as informative output (the lines starting with "+");
this allows us to see how systemd-measure is called. It then merges the output of both invocations into
the ".pcrsig" section. systemd-measure may also do this merge itself using the --append= option.
Note that in this example the ".pcrpkey" PE section contains the key specified by the first
--pcr-private-key= option, covering all boot phases. The ".pcrpkey" section is used in the default
policies of systemd-cryptenroll and systemd-creds. To use the stricter policy bound to
tpm-pcr-initrd-public.pem, specify --tpm2-public-key= on the command line of those tools.
EXIT STATUS
On success, 0 is returned, a non-zero failure code otherwise.
SEE ALSO
systemd(1), systemd-stub(7), ukify(1), systemd-creds(1), systemd-cryptsetup@.service(8), systemd-
pcrphase.service(8)
NOTES
1. Unified Kernel Image (UKI)
https://uapi-group.org/specifications/specs/unified_kernel_image/
systemd 255 SYSTEMD-MEASURE(1)