=================== ``mcuboot`` MCUboot =================== Description ----------- The NuttX port of MCUboot secure boot library expects that the platform provides a Flash storage with the following partitions: - ``CONFIG_MCUBOOT_PRIMARY_SLOT_PATH``: MTD partition for the application firmware image PRIMARY slot; - ``CONFIG_MCUBOOT_SECONDARY_SLOT_PATH``: MTD partition for the application firmware image SECONDARY slot; - ``CONFIG_MCUBOOT_SCRATCH_PATH``: MTD partition for the Scratch area; Also, these are optional features that may be enabled: - ``CONFIG_MCUBOOT_WATCHDOG``: If ``CONFIG_WATCHDOG`` is enabled, MCUboot shall reset the watchdog timer indicated by ``CONFIG_MCUBOOT_WATCHDOG_DEVPATH`` to the current timeout value, preventing any imminent watchdog timeouts. The porting layer of MCUboot library consists of the following interfaces: - ````, for enabling MCUboot to manage the application firmware image slots in the device storage. - ````, for configuration of MCUboot's features. - ````, for providing logging capabilities. - ````, for providing MCUboot access to the OS memory management interfaces. - ````, for configuration of the system's flash area organization. The NuttX port of MCUboot is implemented at application-level and requires minimal knowledge about characteristics of the underlying storage device. This is achieved by means of the ``BCH`` and ``FTL`` subsystems, which enable MCUboot to manage MTD partitions via character device drivers using standard POSIX filesystem operations (e.g. ``open()`` / ``close()`` / ``read()`` / ``write()``). Creating MCUboot-compatible application firmware images ------------------------------------------------------- One common use case for MCUboot is to integrate it to a firmware update agent, which is an important component of a secure firmware update subsystem. Through MCUboot APIs an application is able to install a newly received application firmware image and, once this application firmware image is assured to be valid, the application may confirm it as a stable image. In case that application firmware image is deemed bogus, MCUboot provides an API for invalidating that update, which will induce a rollback procedure to the most recent stable application firmware image. The ``CONFIG_EXAMPLES_MCUBOOT_UPDATE_AGENT`` example demonstrates this workflow by downloading an application firmware image from a webserver, installing it and triggering the firmware update process for the next boot after a system reset. There is also the ``CONFIG_EXAMPLES_MCUBOOT_SLOT_CONFIRM``, which is a fairly simple example that just calls an MCUboot API for confirming the executing application firmware image as stable. For more information about all MCUboot examples, see ``examples/mcuboot`` directory. Using MCUboot on NuttX as a secure boot solution ------------------------------------------------ NuttX port for MCUboot also enables the creation of a secure bootloader application requiring minimal platform-specific implementation. The logical implementation for the secure boot is performed at application-level by the MCUboot library. Once MCUboot validates the application firmware image, it delegates the loading and execution of the application firmware image to a platform-specific routine, which is accessed via ``boardctl(BOARDIOC_BOOT_IMAGE)`` call. Each platform must then provide an implementation for the ``board_boot_image()`` for executing the required actions in order to boot a new application firmware image (e.g. deinitialize peripherals, load the Program Counter register with the application firmware image entry point address). The MCUboot bootloader application may be enabled by selecting the ``CONFIG_MCUBOOT_BOOTLOADER`` option. Assumptions ----------- IOCTL MTD commands ~~~~~~~~~~~~~~~~~~ The implementation of ```` expects that the MTD driver for a given image partition handles the following ``ioctl`` commands: - ``MTDIOC_GEOMETRY``, for retrieving information about the geometry of the MTD, required for the configuration of the size of each flash area. - ``MTDIOC_ERASESTATE``, for retrieving the byte value of an erased cell of the MTD, required for the implementation of ``flash_area_erased_val()`` interface. Write access alignment ~~~~~~~~~~~~~~~~~~~~~~ Through ``flash_area_align()`` interface MCUboot expects that the implementation provides the shortest data length that may be written via ``flash_area_write()`` interface. The NuttX implementation passes through the ``BCH`` and ``FTL`` layers, which appropriately handle the write alignment restrictions of the underlying MTD. So The NuttX implementation of ``flash_area_align()`` is able to return a fixed value of 1 byte, even if the MTD does not support byte operations. Limitations ----------- ```` functions are not multitasking-safe ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ MCUboot's documentation imposes no restrictions regarding the usage of its public interfaces, which doesn't mean they are thread-safe. But, regarding NuttX implementation of the ````, it is safe to state that they are **not** multitasking-safe. NuttX implementation manages the MTD partitions via character device drivers. As file-descriptors cannot be shared between different tasks, if one task calls ``flash_area_open`` and another task calls ``flash_area_`` passing the same ``struct flash_area`` instance, it will result in failure.