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NAME | SYNOPSIS | DESCRIPTION | COMMANDS | OPTIONS | EXAMPLES | EXIT STATUS | SEE ALSO | NOTES | COLOPHON |
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SYSTEMD-MEASURE(1) systemd-measure SYSTEMD-MEASURE(1)
systemd-measure - Pre-calculate and sign expected TPM2 PCR values
for booted unified kernel images
/usr/lib/systemd/systemd-measure [OPTIONS...]
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.
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=, --ucode=, --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.
The following options are understood:
--linux=PATH, --osrel=PATH, --cmdline=PATH, --initrd=PATH,
--ucode=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, --certificate=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.
--certificate= can be used to specify an X.509 certificate as
an alternative to --public-key= since v256.
Added in version 252.
--private-key=PATH/URI, --private-key-source=TYPE[:NAME],
--certificate=PATH
As an alternative to --public-key= for the sign command,
these switches can be used to sign with an hardware token.
The private key option can take a path or a URI that will be
passed to the OpenSSL engine or provider, as specified by
--private-key-source= as a type:name tuple, such as
engine:pkcs11. The specified OpenSSL signing engine or
provider will be used to sign.
Added in version 256.
--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.
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.
On success, 0 is returned, a non-zero failure code otherwise.
systemd(1), systemd-stub(7), ukify(1), systemd-creds(1),
systemd-cryptsetup@.service(8), systemd-pcrphase.service(8)
1. Unified Kernel Image (UKI)
https://uapi-group.org/specifications/specs/unified_kernel_image/
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have
a bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 257~devel SYSTEMD-MEASURE(1)
Pages that refer to this page: systemd-creds(1), systemd-cryptenroll(1), ukify(1), crypttab(5), systemd.directives(7), systemd.index(7), systemd-stub(7), systemd-pcrphase.service(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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