Provided by: systemd_252.5-2ubuntu3_amd64 bug

NAME

       systemd-stub, sd-stub, linuxx64.efi.stub, linuxia32.efi.stub, linuxaa64.efi.stub - A
       simple UEFI kernel boot stub

SYNOPSIS

       /usr/lib/systemd/boot/efi/linuxx64.efi.stub

       /usr/lib/systemd/boot/efi/linuxia32.efi.stub

       /usr/lib/systemd/boot/efi/linuxaa64.efi.stub

       ESP/.../foo.efi.extra.d/*.cred

       ESP/.../foo.efi.extra.d/*.raw

       ESP/loader/credentials/*.cred

DESCRIPTION

       systemd-stub (stored in per-architecture files linuxx64.efi.stub, linuxia32.efi.stub,
       linuxaa64.efi.stub on disk) is a simple UEFI boot stub. An UEFI boot stub is attached to a
       Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
       environment before transitioning into the Linux kernel environment. The UEFI boot stub
       ensures a Linux kernel is executable as regular UEFI binary, and is able to do various
       preparations before switching the system into the Linux world.

       The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI
       PE binary itself. This allows combining various resources inside a single PE binary image
       (usually called "Unified Kernel Image", or "UKI" for short), which may then be signed via
       UEFI SecureBoot as a whole, covering all individual resources at once. Specifically it may
       include:

       •   The ELF Linux kernel images will be looked for in the ".linux" PE section of the
           executed image.

       •   OS release information, i.e. the os-release(5) file of the OS the kernel belongs to,
           in the ".osrel" PE section.

       •   The initrd will be loaded from the ".initrd" PE section.

       •   A compiled binary DeviceTree will be looked for in the ".dtb" PE section.

       •   The kernel command line to pass to the invoked kernel will be looked for in the
           ".cmdline" PE section.

       •   A boot splash (in Windows .BMP format) to show on screen before invoking the kernel
           will be looked for in the ".splash" PE section.

       •   A set of cryptographic signatures for expected TPM2 PCR values when this kernel is
           booted, in JSON format, in the ".pcrsig" section. This is useful for implementing TPM2
           policies that bind disk encryption and similar to kernels that are signed by a
           specific key.

       •   A public key in PEM format matching this TPM2 PCR signature data in the ".pcrpkey"
           section.

       If UEFI SecureBoot is enabled and the ".cmdline" section is present in the executed image,
       any attempts to override the kernel command line by passing one as invocation parameters
       to the EFI binary are ignored. Thus, in order to allow overriding the kernel command line,
       either disable UEFI SecureBoot, or don't include a kernel command line PE section in the
       kernel image file. If a command line is accepted via EFI invocation parameters to the EFI
       binary it is measured into TPM PCR 12 (if a TPM is present).

       If a DeviceTree is embedded in the ".dtb" section, it replaces an existing DeviceTree in
       the corresponding EFI configuration table. systemd-stub will ask the firmware via the
       "EFI_DT_FIXUP_PROTOCOL" for hardware specific fixups to the DeviceTree.

       The contents of seven of these eight PE sections are measured into TPM PCR 11, that is
       otherwise not used. Thus, it can be pre-calculated without too much effort. The ".pcrsig"
       section is not included in this PCR measurement, since it's supposed to contain signatures
       for the expected results for these measurements, i.e. of the outputs of the measurement
       operation, and thus cannot also be input to it.

       When ".pcrsig" and/or ".pcrpkey" are present in a unified kernel image their contents are
       passed to the booted kernel in an synthetic initrd cpio archive that places them in the
       /.extra/tpm2-pcr-signature.json and /.extra/tpm2-pcr-public-key.pem files. Typically, a
       tmpfiles.d(5) line then ensures they are copied into /run/systemd/tpm2-pcr-signature.json
       and /run/systemd/tpm2-pcr-public-key.pem where they remain accessible even after the
       system transitions out of the initrd environment into the host file system. Tools such
       systemd-cryptsetup@.service(8), systemd-cryptenroll(1) and systemd-creds(1) will
       automatically use files present under these paths to unlock protected resources (encrypted
       storage or credentials) or bind encryption to booted kernels.

COMPANION FILES

       The systemd-stub UEFI boot stub automatically collects two types of auxiliary companion
       files optionally placed in drop-in directories on the same partition as the EFI binary,
       dynamically generates cpio initrd archives from them, and passes them to the kernel.
       Specifically:

       •   For a kernel binary called foo.efi, it will look for files with the .cred suffix in a
           directory named foo.efi.extra.d/ next to it. A cpio archive is generated from all
           files found that way, placing them in the /.extra/credentials/ directory of the initrd
           file hierarchy. The main initrd may then access them in this directory. This is
           supposed to be used to store auxiliary, encrypted, authenticated credentials for use
           with LoadCredentialEncrypted= in the UEFI System Partition. See systemd.exec(5) and
           systemd-creds(1) for details on encrypted credentials. The generated cpio archive is
           measured into TPM PCR 12 (if a TPM is present).

       •   Similarly, files foo.efi.extra.d/*.raw are packed up in a cpio archive and placed in
           the /.extra/sysext/ directory in the initrd file hierarchy. This is supposed to be
           used to pass additional system extension images to the initrd. See systemd-sysext(8)
           for details on system extension images. The generated cpio archive containing these
           system extension images is measured into TPM PCR 13 (if a TPM is present).

       •   Files /loader/credentials/*.cred are packed up in a cpio archive and placed in the
           /.extra/global_credentials/ directory of the initrd file hierarchy. This is supposed
           to be used to pass additional credentials to the initrd, regardless of the kernel
           being booted. The generated cpio archive is measured into TPM PCR 12 (if a TPM is
           present)

       These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable
       initrd images in a reasonably safe way: all data they contain is measured into TPM PCRs.
       On access they should be further validated: in case of the credentials case by
       encrypting/authenticating them via TPM, as exposed by systemd-creds encrypt -T (see
       systemd-creds(1) for details); in case of the system extension images by using signed
       Verity images.

TPM PCR NOTES

       Note that when a unified kernel using systemd-stub is invoked the firmware will measure it
       as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself,
       the core kernel, the embedded initrd and kernel command line (see above for a full list).

       Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This
       means every type of initrd will be measured two or three times: the initrd embedded in the
       kernel image will be measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from
       credentials will be measured to both PCR 9 and PCR 12; the initrd synthesized from system
       extensions will be measured to both PCR 4 and PCR 9. Let's summarize the OS resources and
       the PCRs they are measured to:

       Table 1. OS Resource PCR Summary
       ┌─────────────────────────────────┬─────────────────┐
       │OS ResourceMeasurement PCR │
       ├─────────────────────────────────┼─────────────────┤
       │systemd-stub code (the entry     │ 4               │
       │point of the unified PE binary)  │                 │
       ├─────────────────────────────────┼─────────────────┤
       │Core kernel code (embedded in    │ 4 + 11          │
       │unified PE binary)               │                 │
       ├─────────────────────────────────┼─────────────────┤
       │OS release information (embedded │ 4 + 11          │
       │in the unified PE binary)        │                 │
       ├─────────────────────────────────┼─────────────────┤
       │Main initrd (embedded in unified │ 4 + 9 + 11      │
       │PE binary)                       │                 │
       ├─────────────────────────────────┼─────────────────┤
       │Default kernel command line      │ 4 + 11          │
       │(embedded in unified PE binary)  │                 │
       ├─────────────────────────────────┼─────────────────┤
       │Overridden kernel command line   │ 12              │
       ├─────────────────────────────────┼─────────────────┤
       │Boot splash (embedded in the     │ 4 + 11          │
       │unified PE binary)               │                 │
       ├─────────────────────────────────┼─────────────────┤
       │TPM2 PCR signature JSON          │ 4 + 9           │
       │(embedded in unified PE binary,  │                 │
       │synthesized into initrd)         │                 │
       ├─────────────────────────────────┼─────────────────┤
       │TPM2 PCR PEM public key          │ 4 + 9 + 11      │
       │(embedded in unified PE binary,  │                 │
       │synthesized into initrd)         │                 │
       ├─────────────────────────────────┼─────────────────┤
       │Credentials (synthesized initrd  │ 9 + 12          │
       │from companion files)            │                 │
       ├─────────────────────────────────┼─────────────────┤
       │System Extensions (synthesized   │ 9 + 13          │
       │initrd from companion files)     │                 │
       └─────────────────────────────────┴─────────────────┘

EFI VARIABLES

       The following EFI variables are defined, set and read by systemd-stub, under the vendor
       UUID "4a67b082-0a4c-41cf-b6c7-440b29bb8c4f", for communication between the boot stub and
       the OS:

       LoaderDevicePartUUID
           Contains the partition UUID of the EFI System Partition the EFI image was run from.
           systemd-gpt-auto-generator(8) uses this information to automatically find the disk
           booted from, in order to discover various other partitions on the same disk
           automatically.

       LoaderFirmwareInfo, LoaderFirmwareType
           Brief firmware information. Use bootctl(1) to view this data.

       LoaderImageIdentifier
           The path of EFI executable, relative to the EFI System Partition's root directory. Use
           bootctl(1) to view this data.

       StubInfo
           Brief stub information. Use bootctl(1) to view this data.

       StubPcrKernelImage
           The PCR register index the kernel image, initrd image, boot splash, devicetree
           database, and the embedded command line are measured into, formatted as decimal ASCII
           string (e.g.  "11"). This variable is set if a measurement was successfully completed,
           and remains unset otherwise.

       StubPcrKernelParameters
           The PCR register index the kernel command line and credentials are measured into,
           formatted as decimal ASCII string (e.g.  "12"). This variable is set if a measurement
           was successfully completed, and remains unset otherwise.

       StubPcrInitRDSysExts
           The PCR register index the systemd extensions for the initrd, which are picked up from
           the file system the kernel image is located on. Formatted as decimal ASCII string
           (e.g.  "13"). This variable is set if a measurement was successfully completed, and
           remains unset otherwise.

       Note that some of the variables above may also be set by the boot loader. The stub will
       only set them if they aren't set already. Some of these variables are defined by the Boot
       Loader Interface[1].

INITRD RESOURCES

       The following resources are passed as initrd cpio archives to the booted kernel, and thus
       make up the initial file system hierarchy in the initrd execution environment:

       /
           The main initrd from the ".initrd" PE section of the unified kernel image.

       /.extra/credentials/*.cred
           Credential files (suffix ".cred") that are placed next to the unified kernel image (as
           described above) are copied into the /.extra/credentials/ directory in the initrd
           execution environment.

       /.extra/global_credentials/*.cred
           Similar, credential files in the /loader/credentials/ directory in the file system the
           unified kernel image is placed in are copied into the /.extra/global_credentials/
           directory in the initrd execution environment.

       /.extra/sysext/*.raw
           System extension image files (suffix ".raw") that are placed next to the unified
           kernel image (as described above) are copied into the /.extra/sysext/ directory in the
           initrd execution environment.

       /.extra/tpm2-pcr-signature.json
           The TPM2 PCR signature JSON object included in the ".pcrsig" PE section of the unified
           kernel image is copied into the /.extra/tpm2-pcr-signature.json file in the initrd
           execution environment.

       /.extra/tpm2-pcr-pkey.pem
           The PEM public key included in the ".pcrpkey" PE section of the unified kernel image
           is copied into the /.extra/tpm2-pcr-public-key.pem file in the initrd execution
           environment.

       Note that all these files are located in the "tmpfs" file system the kernel sets up for
       the initrd file hierarchy and are thus lost when the system transitions from the initrd
       execution environment into the host file system. If these resources shall be kept around
       over this transition they need to be copied to a place that survives the transition first,
       for example via a suitable tmpfiles.d(5) line. By default, this is done for the TPM2 PCR
       signature and public key files.

ASSEMBLING KERNEL IMAGES

       In order to assemble an UEFI PE kernel image from various components as described above,
       use an objcopy(1) command line like this:

           objcopy \
               --add-section .osrel=os-release --change-section-vma .osrel=0x20000 \
               --add-section .cmdline=cmdline.txt --change-section-vma .cmdline=0x30000 \
               --add-section .dtb=devicetree.dtb --change-section-vma .dtb=0x40000 \
               --add-section .splash=splash.bmp --change-section-vma .splash=0x100000 \
               --add-section .linux=vmlinux --change-section-vma .linux=0x2000000 \
               --add-section .initrd=initrd.cpio --change-section-vma .initrd=0x3000000 \
               /usr/lib/systemd/boot/efi/linuxx64.efi.stub \
               foo-unsigned.efi

       Note that these PE section offsets are example values and a properly assembled image must
       not contain any overlapping sections (this includes already existing sections inside the
       stub before assembly) or boot may fail.

       This generates one PE executable file foo-unsigned.efi from the six individual files for
       OS release information, kernel command line, boot splash image, kernel image, main initrd
       and UEFI boot stub.

       To then sign the resulting image for UEFI SecureBoot use an sbsign(1) command like the
       following:

           sbsign \
               --key mykey.pem \
               --cert mykey.crt \
               --output foo.efi \
               foo-unsigned.efi

       This expects a pair of X.509 private key and certificate as parameters and then signs the
       UEFI PE executable we generated above for UEFI SecureBoot and generates a signed UEFI PE
       executable as result.

       See systemd-measure(1) for an example involving the ".pcrsig" and ".pcrpkey" sections.

SEE ALSO

       systemd-boot(7), systemd.exec(5), systemd-creds(1), systemd-sysext(8), Boot Loader
       Specification[2], Boot Loader Interface[1], objcopy(1), sbsign(1), systemd-measure(1)

NOTES

        1. Boot Loader Interface
           https://systemd.io/BOOT_LOADER_INTERFACE

        2. Boot Loader Specification
           https://systemd.io/BOOT_LOADER_SPECIFICATION