Firmware Update Service

Author

Sherry Zhang

Organization

Arm Limited

Contact

Sherry Zhang <Sherry.Zhang2@arm.com>

Table of Contents

• Firmware Update Service

  • Introduction of Firmware Update service

  • Components

  • Service API description

  • Shim Layer between FWU and bootloader

    • Shim layer introduction

    • Interfaces of the shim Layer

  • Additional shared data between BL2 and SPE

  • Build configurations related to FWU partition

  • Limitations of current implementation

  • Benefits Analysis on this Partition

    • Implement the FWU functionality in the non-secure side

    • Pros and Cons for implementing FWU APIs in secure side

  • Reference

Introduction of Firmware Update service

The Firmware Update(FWU) service provides the functionality of updating firmware images. It provides a standard interface for updating firmware and it is platform independent. TF-M defines a shim layer to support cooperation between bootloader and FWU service.

This partition supports the following features:

• Query the firmware store information.

• Image preparation: prepare a new firmware image in the component’s firmware store.

• Image installation: install prepared firmware images on all components that have been prepared for installation.

• Image trial: manage a trial of new firmware images atomically on all components that are in TRIAL state.

A typical flow through the component states is shown below 1.

Components

The structure of the TF-M Firmware Update service is listed below:

Component name	Description	Location
Client API interface	This module exports the client API of PSA Firmware Update to the users.	./interface/src/tfm_fwu_api.c
Manifest	The manifest file is a description of the service components.	./secure_fw/partitions/firmware_update/tfm_firmware_update.yaml
NSPE client API interface	This module exports the client API of PSA Firmware Update to the NSPE(i.e. to the applications).	./interface/src/tfm_fwu_api.c
IPC request handlers	This module handles all the secure requests in IPC model. It maitains the image state context and calls the image ID converter to achieve the firmware update functionalities.	./secure_fw/partitions/firmware_update/tfm_fwu_req_mngr.c
Shim layer between FWU and bootloader	This module provides the APIs with the functionality of operating the bootloader to cooperate with the Firmware Update service	./secure_fw/partitions/firmware_update/bootloader/tfm_bootloader_fwu_abstraction.h
Shim layer example based on MCUboot	This module is the implementation of the shim layer between FWU and bootloader based on MCUboot.	./secure_fw/partitions/firmware_update/bootloader/mcuboot/tfm_mcuboot_fwu.c

Service API description

This service follows the PSA Firmware Update API spec of version 1.0 1. Please refer to Firmware Update spec for the detailed description.

Shim Layer between FWU and bootloader

The firmware update operations are achieved by calling the shim layer APIs between bootloader and FWU.

Shim layer introduction

This shim layer provides the APIs with the functionality of operating the bootloader to cooperate with the Firmware Update service. This shim layer is decoupled from bootloader implementation. Users can specify a specific bootloader by setting TFM_FWU_BOOTLOADER_LIB build configuration and adding the specific build scripts into that file. By default, the MCUboot is chosen as the bootloader.

Interfaces of the shim Layer

fwu_bootloader_init(function)

Prototype

psa_status_t fwu_bootloader_init(void);

Description

Bootloader related initialization for the firmware update. It reads some necessary shared data from the memory if needed. It initializes the flash drivers defined in FLASH_DRIVER_LIST. Platform can define FLASH_DRIVER_LIST in flash_layout.h to overload the default driver list.

Parameters

N/A

fwu_bootloader_staging_area_init(function)

Prototype

psa_status_t fwu_bootloader_staging_area_init(psa_fwu_component_t component,
                                              const void *manifest,
                                              size_t manifest_size);

Description

The component is in READY state. Prepare the staging area of the component for image download. For example, initialize the staging area, open the flash area, and so on.

Parameters

• component: The identifier of the target component in bootloader.

• manifest: A pointer to a buffer containing a detached manifest for the update. If the manifest is bundled with the firmware image, manifest must be NULL.

• manifest_size: Size of the manifest buffer in bytes.

fwu_bootloader_load_image(function)

Prototype

psa_status_t fwu_bootloader_load_image(psa_fwu_component_t component,
                                       size_t        image_offset,
                                       const void    *block,
                                       size_t        block_size);

Description

Load the image into the target component.

Parameters

• component: The identifier of the target component in bootloader.

• image_offset: The offset of the image being passed into block, in bytes.

• block: A buffer containing a block of image data. This might be a complete image or a subset.

• block_size: Size of block.

fwu_bootloader_install_image(function)

Prototype

psa_status_t fwu_bootloader_install_image(psa_fwu_component_t *candidates,
                                          uint8_t number);

Description

Check the authenticity and integrity of the image. If a reboot is required to complete the check, then mark this image as a candidate so that the next time bootloader runs it will take this image as a candidate one to bootup. Return the error code PSA_SUCCESS_REBOOT.

Parameters

• candidates: A list of components in CANDIDATE state.

• number: Number of components in CANDIDATE state.

fwu_bootloader_mark_image_accepted(function)

Prototype

psa_status_t fwu_bootloader_mark_image_accepted(const psa_fwu_component_t *trials,
                                                uint8_t number);

Description

Call this API to mark the TRIAL(running) image in component as confirmed to avoid revert when next time bootup. Usually, this API is called after the running images have been verified as valid.

Parameters

• trials: A list of components in TRIAL state.

• number: Number of components in TRIAL state.

fwu_bootloader_reject_staged_image(function)

Prototype

psa_status_t fwu_bootloader_reject_staged_image(psa_fwu_component_t component);

Description

The component is in STAGED state. Call this API to Uninstall the staged image in the component so that this image will not be treated as a candidate next time bootup.

Parameters

• component: The identifier of the target component in bootloader.

fwu_bootloader_reject_trial_image(function)

Prototype

psa_status_t fwu_bootloader_reject_trial_image(psa_fwu_component_t component);

Description

The component is in TRIAL state. Mark the running image in the component as rejected.

Parameters

• component: The identifier of the target component in bootloader.

fwu_bootloader_clean_component(function)

Prototype

psa_status_t fwu_bootloader_clean_component(psa_fwu_component_t component);

Description

The component is in FAILED or UPDATED state. Clean the staging area of the component.

Parameters

• component: The identifier of the target component in bootloader.

fwu_bootloader_get_image_info(function)

Prototype

psa_status_t fwu_bootloader_get_image_info(psa_fwu_component_t component,
                                           bool query_state,
                                           bool query_impl_info,
                                           psa_fwu_component_info_t *info);

Description

Get the image information of the given bootloader_image_id in the staging area or the running area.

Parameters

• component: The identifier of the target component in bootloader.

• query_state: Whether query the ‘state’ field of psa_fwu_component_info_t.

• query_impl_info: Whether Query ‘impl’ field of psa_fwu_component_info_t.

• info: Buffer containing return the component information.

Additional shared data between BL2 and SPE

An additional TLV area “image version” is added into the shared memory between BL2 and TF-M. So that the firmware update partition can get the image version. Even though the image version information is also included in the BOOT RECORD TLV area which is encoded by CBOR, adding a dedicated image version TLV area is preferred to avoid involving the CBOR encoder which can increase the code size. The FWU partition will read the shared data at the partition initialization.

Build configurations related to FWU partition

• TFM_PARTITION_FIRMWARE_UPDATE Controls whether FWU partition is enabled or not.

• TFM_FWU_BOOTLOADER_LIB Bootloader configure file for FWU partition.

• TFM_CONFIG_FWU_MAX_WRITE_SIZE The maximum permitted size for block in psa_fwu_write, in bytes.

• TFM_FWU_BUF_SIZE Size of the FWU internal data transfer buffer (defaults to TFM_CONFIG_FWU_MAX_WRITE_SIZE if not set).

• FWU_STACK_SIZE The stack size of FWU Partition.

• FWU_DEVICE_CONFIG_FILE The device configuration file for FWU partition. The default value is the configuration file generated for MCUboot. The following macros should be defined in the configuration file:

  • FWU_COMPONENT_NUMBER The number of components on the device.

    Note

    In this design, component ID ranges from 0 to FWU_COMPONENT_NUMBER - 1.

  • FWU_SUPPORT_TRIAL_STATE Whether TRIAL component state is supported.

• TEST_NS_FWU FWU nonsecure tests switch.

• TEST_S_FWU FWU secure tests switch.

  Note

  The running image which supports revert mechanism should be confirmed before initiating a firmware update process. For example, if the running image is built with -DMCUBOOT_UPGRADE_STRATEGY=SWAP_USING_MOVE, the image should be confirmed either by adding -DMCUBOOT_CONFIRM_IMAGE=ON build option or by calling psa_fwu_accept() API before initiating a firmware update process. Otherwise, PSA_ERROR_BAD_STATE will be returned by psa_fwu_start().

Limitations of current implementation

Currently, the MCUboot based implementation does not record image update results like failure or success. And FWU partition does not detect failure errors in bootloader installation. If an image installation fails in the bootloader and the old image still runs after reboot, PSA_FWU_READY state will be returned by psa_fwu_query() after reboot.

Currently, image download recovery after a reboot is not supported. If a reboot happens in image preparation, the downloaded image data will be ignored after the reboot.

Benefits Analysis on this Partition

Implement the FWU functionality in the non-secure side

The APIs listed in PSA Firmware Update API spec 1 can also be implemented in the non-secure side.

Pros and Cons for implementing FWU APIs in secure side

Pros

• It protects the image in the passive or staging area from being tampered with by the NSPE. Otherwise, a malicious actor from NSPE can tamper the image stored in the non-secure area to break image update.

• It protects secure image information from disclosure. In some cases, the non-secure side shall not be permitted to get secure image information.

• It protects the active image from being manipulated by NSPE. Some bootloader supports testing the image. After the image is successfully installed and starts to run, the user should set the image as permanent image if the image passes the test. To achieve this, the area of the active image needs to be accessed. In this case, implementing FWU service in SPE can prevent NSPE from manipulating the active image area.

• On some devices, such as the Arm Musca-B1 board, the passive or staging area is restricted as secure access only. In this case, the FWU partition should be implemented in the secure side.

Cons

• It increases the image size of the secure image.

• It increases the execution latency and footprint. Compared to implementing FWU in NSPE directly, calling the Firmware Update APIs which are implemented in the secure side increases the execution latency and footprint.

• It can increase the attack surface of the secure runtime.

Users can decide whether to call the FWU service in TF-M directly or implement the Firmware Update APIs in the non-secure side based on the pros and cons analysis above.

Reference

1(1,2,3)

PSA Firwmare Update API

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