Flattened Image Tree (FIT) overview

The Linux kernel boot process involves a number of artifacts that must be placed in memory and booted together. This typically includes:

  • the kernel image

  • a device tree

  • optionally, device tree overlays

  • optionally, a ramdisk rootfs

The Flattened Image Tree (FIT) provides a flexible and extensible format to put all these artifacts in a single file.

The FIT format supports multiple configurations so you can use the same FIT image to boot multiple boards that have a common component (e.g., kernel) but use other artifacts specific to each board (e.g., device tree or overlays). A FIT file also includes hashes to verify the integrity of every artifact. To learn more about the FIT format, see Flattened Image Tree (FIT) Format.

FIT image signing

The signature feature allows you to use a private key to sign the hashes of the FIT artifacts. This way, any image or configuration inside the FIT can later be authenticated using the public key.

The public key must reside in a trusted place for FIT image artifacts to be authenticated. Digi Embedded Yocto places the public key in the U-Boot bootloader device tree because that is authenticated against the keys programmed in the OTP bits.

As explained in Set up secure boot, the bootloader must be signed and authenticated. If authentication fails, the target does not boot at all. If it succeeds, the bootloader runs and has the public keys available to authenticate the artifacts in the FIT image. This guarantees the chain of trust.

The following diagram explains the process of signing and authenticating the FIT image:

FIT image signing
FIT image authentication

To instruct Digi Embedded Yocto to generate the keys and sign the FIT image, enable TrustFence in your project’s conf/local.conf:

# Required to include trustfence support.
INHERIT += "trustfence"

For more information on U-Boot FIT signature verification, see U-Boot FIT Signature Verification.

Build your project image

Build your Digi Embedded Yocto project image recipe, for instance:

$ bitbake core-image-base

This process:

  • Generates the FIT sign keys (if they don’t exist).

  • Generates a FIT image with the kernel, base device tree, device tree overlays, and U-Boot boot script.

  • Signs the FIT image hashes with the private keys.

  • Inserts the FIT public keys inside the U-Boot device tree.

  • Signs the FIP image (containing U-Boot + OP-TEE) with TrustFence keys.

  • Generates a file *.boot.ubifs that contains the FIT image.

Deploy images to the target

Program the bootloader

The first time you program a signed FIT image, you must first program the bootloader partition with the U-Boot that contains the FIT public keys. This step is not necessary on subsequent FIT image updates.

To update partition fip-a (for example from TFTP) with file fip-ccmp13-dvk-optee_Signed.bin:

=> update fip-a tftp fip-ccmp13-dvk-optee_Signed.bin

Reset the target to boot with the new U-Boot. Some environment variables such as the default boot command bootcmd may have changed to deal with FIT images. Digi recommends you reset the U-Boot environment to default after this first update.

=> env default -a
=> saveenv

Program the FIT image

The FIT image itself is contained inside your project’s image file, for instance core-image-base-ccmp13-dvk.boot.ubifs. To program it (for example from TFTP):

=> update linux tftp core-image-base-ccmp13-dvk.boot.ubifs
The FIT image is the only file that’s stored in the linux partition, as it includes all the required files to boot the system.

Boot the FIT image

The default U-Boot is prepared to boot a regular Linux kernel with zImage format. This is determined by variable dboot_kernel_var. Change this variable to instruct U-Boot to boot a FIT image instead:

=> setenv dboot_kernel_var fitimage
=> saveenv

As part of the Digi Embedded Yocto project, the U-Boot recipe includes a U-Boot boot script to boot the ConnectCore MP13 Development Kit. This script determines the variant of the ConnectCore MP13 SoM and the version of the carrier board and selects a number of device tree overlays to apply on top of the base device tree (wireless, Bluetooth, etc.) before booting the kernel. This boot script is embedded and signed in the FIT image and is run by default when you start the device.

Power the device on and let it run, or run the boot command.

Loading file 'fitImage-ccmp13-dvk.bin' to addr 0xc2000000...
## Executing script at c2000000
sha256,rsa2048:fitimg+ sha256+ Booting from system A
Skip re-loading of FIT image

Explanation of output:

  • The FIT image is loaded into RAM memory and the boot script is authenticated against the public key.

  • Authentication succeeds and the boot script prints the message: "Booting from system A".

  • The boot script calls the dboot command to boot the system. While this command typically loads the image into RAM, in this case the FIT image is not reloaded because it was already loaded to run the boot script.

Next, U-Boot successfully authenticates the default configuration:

## Loading kernel from FIT Image at c2000000 ...
   Using 'conf-ccmp157-dvk.dtb' configuration
   Verifying Hash Integrity ... sha256,rsa2048:fitcfg+ OK
   Trying 'kernel-1' kernel subimage
     Description:  Linux kernel
     Created:      2023-11-09  14:38:10 UTC
     Type:         Kernel Image
     Compression:  uncompressed
     Data Start:   0xc20000e8
     Data Size:    6840888 Bytes = 6.5 MiB
     Architecture: ARM
     OS:           Linux
     Load Address: 0xc0800000
     Entry Point:  0xc0800000
     Hash algo:    sha256
     Hash value:   9c146479659fb34f23c1f4cd2975f8c8be175b2efd2db385ab7f753cd276e6ba
     Sign algo:    sha256,rsa2048:fitimg
     Sign value:   a86ebef17395f6e1df4b67c6e421fd36d27ef1b4217f19d14492cb4b2e17d0992e377b36c6873f850c780ce408b88f04d04aca35c1acaf68488cfdcbc6c2c711d04c
     Timestamp:    2023-11-09  14:38:10 UTC
   Verifying Hash Integrity ... sha256,rsa2048:fitimg+ sha256+ OK

Explanation of output:

  • The first item in the default configuration is the Linux kernel.

  • U-Boot also authenticates the kernel image successfully.

  • Similar messages appear for the base device tree and the selected overlays that the boot script populated as a list in U-Boot environment variable overlays.

  • After all the artifacts have been authenticated, the Linux system boots.

Advanced options

Use custom FIT sign keys

The FIT signature key pairs are created automatically by the Digi Embedded Yocto build system. You can also provide your own key pairs:

  1. Customize the location of the keys using variable TRUSTFENCE_SIGN_KEY_PATH:

    # Use a custom path to the signature keys and certificates.
    TRUSTFENCE_SIGN_KEYS_PATH = "/mnt/secure/PKI_tree"

    If not defined, Digi Embedded Yocto creates a folder called trustfence/ inside your project to hold all keys.

  2. If it doesn’t already exist, create a subdirectory fit to hold the key pairs.

    $ mkdir <your-trustfence-sign-keys-path>/fit
  3. Optionally, edit your project config file conf/local.conf and set the following variables to give your keys custom names. (If not provided, the default filenames are fitcfg and fitimg.)

  4. Generate an RSA private key to sign the FIT image nodes and another one to sign the configuration nodes:

    $ openssl genrsa -F4 -out <your-trustfence-sign-keys-path>/fit/<your-fitcfg-name>.key 2048
    $ openssl genrsa -F4 -out <your-trustfence-sign-keys-path>/fit/<your-fitimg-name>.key 2048
  5. Generate public certificates out of the private keys:

    $ openssl req -batch -new -x509 -key <your-trustfence-sign-keys-path>/fit/<your-fitcfg-name>.key -out <your-trustfence-sign-keys-path>/fit/<your-fitcfg-name>.crt
    $ openssl req -batch -new -x509 -key <your-trustfence-sign-keys-path>/fit/<your-fitimg-name>.key -out <your-trustfence-sign-keys-path>/fit/<your-fitimg-name>.crt

Manually run a boot script inside the FIT image

To manually run a boot script artifact inside the FIT:

  1. Load the FIT image into memory. For instance, to read it from the NAND:

    => ubifsmount ubi0:linux_a
    => ubifsload $loadaddr $fitimage
  2. Run the source command, with the RAM address where the FIT image is, followed by a colon and the name of the boot script, noting that Yocto appends prefix bootscr- to the name of the boot script:

    => source $loadaddr:bootscr-boot.txt

Manually boot a configuration inside the FIT image

As explained in the overview, a FIT image contains images (standalone artifacts) and configurations. A configuration is a set of one or more images. The FIT image that Digi Embedded Yocto builds contains a default configuration that comprises the Linux kernel and the base device tree. There are additional configurations, one per device tree overlay.

To manually boot a configuration:

  1. Set the variable overlays with a comma-separated list of the device tree overlays that you want to use:

    => setenv overlays overlay1.dtb,overlay2.dtb,overlay3.dtb
  2. Run the dboot command:

    => dboot linux nand

Disable FIT signing

You may want to disable image signing to separate the generation of the image from the signing process. For instance, you may want to avoid managing security keys on development machines and instead only sign images on a separate, more secure computer.

To generate FIT images without signature, add the following to your project’s conf/local.conf:


Rebuild your project after making this change.