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wolfBoot/docs/firmware_update.md

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Firmware update

This section documents the complete firmware update procedure, enabling secure boot for an existing embedded application.

Updating Microcontroller FLASH

The steps to complete a firmware update with wolfBoot are:

  • Compile the firmware with the correct entry point
  • Sign the firmware
  • Transfer the image using a secure connection, and store it to the secondary firmware slot
  • Trigger the image swap
  • Reboot to let the bootloader begin the image swap

At any given time, an application or OS running on a wolfBoot system can receive an updated version of itself, and store the updated image in the second partition in the FLASH memory.

Update and Rollback

Applications or OS threads can be linked to the libwolfboot library, which exports the API to trigger the update at the next reboot, and some helper functions to access the flash partition for erase/write through the target specific HAL.

Update procedure description

Using the API provided to the application, wolfBoot offers the possibility to initiate, confirm or rollback an update.

After storing the new firmware image in the UPDATE partition, the application should initiate the update by calling wolfBoot_update_trigger(). By doing so, the UPDATE partition is marked for update. Upon the next reboot, wolfBoot will:

  • Validate the new firmware image stored in the UPDATE partition
  • Verify the signature attached against a known public key stored in the bootloader image
  • Swap the content of the BOOT and the UPDATE partitions
  • Mark the new firmware in the BOOT partition as in state STATE_TESTING
  • Boot into the newly received firmware

If the system is interrupted during the swap operation and reboots, wolfBoot will pick up where it left off and continue the update procedure.

Successful boot

Upon a successful boot, the application should inform the bootloader by calling wolfBoot_success(), after verifying that the system is up and running again. This operation confirms the update to a new firmware.

Failing to set the BOOT partition to STATE_SUCCESS before the next reboot triggers a roll-back operation. Roll-back is initiated by the bootloader by triggering a new update, this time starting from the backup copy of the original (pre-update) firmware, which is now stored in the UPDATE partition due to the swap occurring earlier.

Building a new firmware image

Firmware images are position-dependent, and can only boot from the origin of the BOOT partition in FLASH. This design constraint implies that the chosen firmware is always stored in the BOOT partition, and wolfBoot is responsible for pre-validating an update image and copy it to the correct address.

All the firmware images must therefore have their entry point set to the address corresponding to the beginning of the BOOT partition, plus an offset of 256 Bytes to account for the image header.

Once the firmware is compiled and linked, it must be signed using the sign tool. The tool produces a signed image that can be transferred to the target using a secure connection, using the same key corresponding to the public key currently used for verification.

The tool also adds all the required Tags to the image header, containing the signatures and the SHA256 hash of the firmware.

Self-update

wolfBoot can update itself if RAM_CODE is set. This procedure operates almost the same as firmware update with a few key differences. The header of the update is marked as a bootloader update (use --wolfboot-update for the sign tools).

The new signed wolfBoot image is loaded into the UPDATE parition and triggered the same as a firmware update. Instead of performing a swap, after the image is validated and signature verified, the bootloader is erased and the new image is written to flash. This operation is not safe from interruption. Interruption will prevent the device from rebooting.

wolfBoot can be used to deploy new bootloader versions as well as update keys.

Incremental updates

wolfBoot supports incremental updates, based on a specific older version. The sign tool can create a small "patch" that only contains the binary difference between the version currently running on the target and the update package. This reduces the size of the image to be transferred to the target, while keeping the same level of security through public key verification, and integrity due to the repeated check (on the patch and the resulting image).

The format of the patch is based on the mechanism suggested by Bentley/McIlroy, which is particularly effective to generate small binary patches. This is useful to minimize time and resources needed to transfer, authenticate and install updates.

Incremental update: example

Requirement: wolfBoot is compiled with DELTA_UPDATES=1

Version "1" is signed as usual, as a standalone image:

tools/keytools/sign.py --ecc256 --sha256 test-app/image.bin ecc256.der 1

When updating from version 1 to version 2, you can invoke the sign tool as:

tools/keytools/sign.py --delta test-app/image_v1_signed.bin --ecc256 --sha256 test-app/image.bin ecc256.der 2

Besides the usual output file image_v2_signed.bin, the sign tool creates an additional image_v2_signed_diff.bin which should be noticeably smaller in size as long as the two binary files contain overlapping areas.

The image_v2_signed_diff.bin file can be now transferred to the update partition on the target like a full update image.

At next reboot, wolfBoot recognizes the incremental update, checks the integrity, the authenticity and the versions of the patch. If all checks succeed, the new version is installed by applying the patch on the current firmware image.