SBCTL(8)   SBCTL(8)

sbctl - Secure Boot Manager

sbctl <command>

sbctl is a tool that allows one to create keys for secure boot, securely enroll them and keep track of files to sign.

status

Shows the current secure boot status of the system. It checks if you are currently booted in UEFI with Secure Boot, and whether or not Setup Mode has been enabled.

create-keys

Creates a set of signing keys used to sign EFI binaries. Currently it will create the following keys:
•Platform Key
•Key Exchange key
•Signature Database Key

enroll-keys

Enrolls the creates key into the EFI variables. This puts the computer out of SetupMode and enables Secure Boot.
Note that some devices have hardware firmware that is signed and
validated when Secure Boot is enabled. Failing to validate this firmware
could brick devices. It's recommended to enroll your own keys with
Microsoft certificates.

-m, --microsoft

Enroll UEFI vendor certificates from Microsoft into the signature database. See Option ROM*.

-t, --tpm-eventlog

Enroll checksums from the TPM Eventlog into the signature database. See Option ROM*.
This feature is experimental.

--yes-this-might-brick-my-machine, --yolo

Ignore the Option ROM error and continue enrolling keys into the UEFI firmware. See Option ROM*.

-i, --ignore-immutable

Ignore checking /sys/firmware/efi/efivars/ for immutable files and unset the immutable attribute before enrolling certificates.

sign <'FILE'>...

Signs a EFI binary with the created key. The file will be checked for valid signatures to avoid duplicates.

-o PATH, --output PATH

Output filename. Default replaces the file.

-s, --save

Save file to the database.

sign-all

Signs all enrolled EFI binaries.

-g, --generate

Generate all bundles before signing.

list-files

Lists all enrolled EFI binaries.

remove-file <FILE>

Removes the file from the signing database.

verify

Looks for EFI binaries with the mime type application/x-dosexec in the ESP partition, and looks at the file database. Checks if they have been signed with the Signature Database Key.

reset

Resets the Platform Key. This sets the machine out of Secure Boot mode and allows to rotate keys.

help <'FILE'>...

Displays a help message.

bundle [FLAGS] <'NAME'>

Creates a bundle that should produce EFI binaries. See BUNDLES below for more details.

-a PATH, --amducode PATH

AMD microcode location.

-c PATH, --cmdline PATH

Cmdline location. (default "/etc/kernel/cmdline")

-e PATH, --efi-stub PATH

EFI Stub location. (default "/usr/lib/systemd/boot/efi/linuxx64.efi.stub")

-p PATH, --esp PATH

ESP location. (default "/efi")

-h, --help

Help for bundle.

-f PATH, --initramfs PATH

Initramfs location. (default "/efi/initramfs-linux.img")

-i PATH, --intelucode PATH

Intel microcode location.

-k PATH, --kernel-img PATH

Kernel image location. (default "/efi/vmlinuz-linux")

-o PATH, --os-release PATH

OS Release file location. (default "/usr/lib/os-release")

-s, --save

Save bundle to the database.

-l PATH, --splash-img PATH

Boot splash image location.

generate-bundles

This command generates all bundles.

-s, --sign

Sign all the generated bundles.

remove-bundle <'NAME'>

Removes a bundle from the list. This does not delete the bundle itself.

list-bundles

List all registered bundles to generate.

-j, --json

This enables supported commands to output their values in json instead of human readable text. This is practical for parsing data with tools like jq.

Normally, only the kernel is signed with your secure boot keys. This means the kernel command line and initramfs can be changed without possibility of verification.

Bundles are EFI executables which pack all three (initramfs, kernel and cmdline) into a single file which is easy to sign. Avoiding any unsigned files during boot makes the whole process more tamper-proof.

When a bundle is generated, its configuration is stored into the bundle database (see FILES). Subsequent executions of sbctl generate-bundles will rebuild these bundles, so you don’t need to re-specify all parameters after each system update.

Tip: systemd-boot will automatically show entries for any bundles found in esp/EFI/Linux/*.efi.

See https://github.com/Foxboron/sbctl/wiki/FAQ#option-rom

Note: To use Secure Boot it’s important to reboot the machine, navigate through the BIOS menu into the Secure Boot options and enable "User Mode". Please also ensure you have reset the current platform keys. If this step is not completed the following step might not work depending on the firmware.

Next is creating the keys for secure boot. create-keys creates the key hierarchy needed for secure boot into "/usr/share/secureboot".

$ sbctl create-keys
Created Owner UUID a9fbbdb7-a05f-48d5-b63a-08c5df45ee70
Creating secure boot keys...✔
Secure boot keys created!

Next up is enrolling the keys into the efi firmware. sbctl supports doing this on a live system instead of having to boot or run a key management tool from the UEFI shell.

Note: This can fail because of firmware issues and unique options in the machine BIOS menu.

$ sbctl enroll-keys
Enrolling keys to EFI variables...✔
Enrolled keys to the EFI variables!

After we have successfully enrolled the keys we need to sign our current boot chain. Traditionally on UEFI systems one can have an EFI System Partition (ESP) on /efi, /boot or /boot/efi. One can usually find the correct one by looking at mount points or finding the 'EFI" directory on the ESP.

The most important file to sign is the kernel. This location differs between distributions but can usually be found on the ESP or /boot. We use --save to store the file path so we don’t need to manually sign it later.

Note that sbctl can only keep track of file paths. On versioned kernels this might prove tricky.

$ sbctl sign --save /efi/vmlinuz-linux
✔ Signed /efi/vmlinuz-linux

Next is to sign the bootloader. This can usually be found on the standard path below, but might differ between installations.

$ sbctl sign --save /efi/EFI/BOOT/BOOTX64.EFI
✔ Signed /efi/EFI/BOOT/BOOTX64.EFI

sbctl is able to find and verify the ESP, along with any saved files to verify we have signed the files we need.

$ sbctl verify
Verifying file database and EFI images in /efi...
✔ /efi/EFI/BOOT/BOOTX64.EFI is signed
✔ /efi/vmlinuz-linux is signed

Once we have confirmed everything works, we can reboot. Remember to re enable Secure Boot in the BIOS menu. Once we have logged back inn we can very the state of the system.

$ sbctl status
Installed:  ✓ sbctl is installed
Owner GUID: a7b893cc-949d-408c-b5cc-6e7d0370fdb6
Setup Mode: ✓ Disabled
Secure Boot:    ✓ Enabled

When we do system updated we can run sign-all to resign all the saved files from earlier.

$ sbctl sign-all
File have already been signed /boot/vmlinuz-linux
✓ Signed /efi/EFI/BOOT/BOOTX64.EFI

sbctl supports creating unified kernel images. These UEFI executables bundles the initramfs, kernel and cmdline into one executable which can be signed for secure boot. This allows you to authenticate larger parts of the bootchain instead of only signing the kernel.

$ sbctl bundle -i /boot/intel-ucode.img

-l /usr/share/systemd/bootctl/splash-arch.bmp
-k /boot/vmlinuz-linux
-f /boot/initramfs-linux-lts.img
-c /etc/kernel/cmdline
/efi/EFI/Linux/linux-linux.efi

Note that dracut(8) and mkinitcpio(8) supports unified kernel features, and they should be preferred over the sbctl implementation. It is mostly provided in the cases where this feature is not supported by the initramfs generator of the distribution.

All commands that take path arguments convert them into absolute paths when saving them to the database.

On success, 0 is returned, a non-zero failure code otherwise.

SYSTEMD_ESP_PATH, ESP_PATH

Defines the EFI system partition (ESP) location. This overrides the behaviour from sbctl where we query for the correct partition with lsblk. No checks are performed on this path and can be usefull for testing purposes.

SBCTL_UNICODE

If this value is "0" sbctl will replace the unicode symbols to equivalent ascii ones. The default value is assumed to be 1.

/usr/share/secureboot

Default storage directory.

/usr/share/secureboot/GUID

Owner identification. This is a randomly generated UUID.

/usr/share/secureboot/files.db

Contains a list of EFI binaries to be signed by the generated key.

/usr/share/secureboot/bundles.db

Contains a list of EFI bundles to be generated.

/usr/share/secureboot/keys/db/db.{pem,key}

Contains the Signature Database key used for signing EFI binaries.

/usr/share/secureboot/keys/KEK/KEK.{pem,key}

Contains the Key Exchange Key.

/usr/share/secureboot/keys/PK/PK.{pem,key}

Contains the Platform Key.

bootctl(1) jq(1)

•Morten Linderud <morten@linderud.pw>
12/18/2021