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) 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:
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.
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.)
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.
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.
--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.
--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".
--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.
--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.
--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).
--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.
--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¶--version¶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.cpioLater 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/sda5And then unlock the device with the signature:
# systemd-cryptsetup attach \
volume5 /dev/sda5 - \
tpm2-device=auto,tpm2-signature=/path/to/tpm2-pcr-signature.jsonNote 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.