143 lines
5.6 KiB
Markdown
143 lines
5.6 KiB
Markdown
# Building and using MCUboot with Zephyr
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MCUboot began its life as the bootloader for Mynewt. It has since
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acquired the ability to be used as a bootloader for Zephyr as well.
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There are some pretty significant differences in how apps are built
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for Zephyr, and these are documented here.
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Please see the [design document](design.md) for documentation on the design
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and operation of the bootloader itself. This functionality should be the same
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on all supported RTOSs.
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The first step required for Zephyr is making sure your board has flash
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partitions defined in its device tree. These partitions are:
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- `boot_partition`: for MCUboot itself
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- `slot0_partition`: the primary image slot
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- `slot1_partition`: the secondary image slot
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- `scratch_partition`: the scratch slot
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Currently, the two image slots must be contiguous. If you are running
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MCUboot as your stage 1 bootloader, `boot_partition` must be configured
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so your SoC runs it out of reset.
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The flash partitions are typically defined in the Zephyr boards folder, in a
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file named `boards/<arch>/<board>/<board>.dts`. An example `.dts` file with
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flash partitions defined is the frdm_k64f's in
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`boards/arm/frdm_k64f/frdm_k64f.dts`. Make sure the labels in your board's
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`.dts` file match the ones used there.
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## Building the bootloader itself
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The bootloader is an ordinary Zephyr application, at least from
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Zephyr's point of view. There is a bit of configuration that needs to
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be made before building it. Most of this can be done as documented in
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the `CMakeLists.txt` file in boot/zephyr. There are comments there for
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guidance. It is important to select a signature algorithm, and decide
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if slot0 should be validated on every boot.
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To build MCUboot, create a build directory in boot/zephyr, and build
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it as usual:
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```
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cd boot/zephyr
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mkdir build && cd build
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cmake -GNinja -DBOARD=<board> ..
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ninja
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```
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In addition to the partitions defined in DTS, some additional
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information about the flash layout is currently required to build
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MCUboot itself. All the needed configuration is collected in
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`boot/zephyr/include/target.h`. Depending on the board, this information
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may come from board-specific headers, Device Tree, or be configured by
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MCUboot on a per-SoC family basis.
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After building the bootloader, the binaries should reside in
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`build/zephyr/zephyr.{bin,hex,elf}`, where `build` is the build
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directory you chose when running `cmake`. Use the Zephyr build
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system `flash` target to flash these binaries, usually by running
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`make flash` (or `ninja flash`, etc.) from the build directory.
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## Building Applications for the bootloader
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In addition to flash partitions in DTS, some additional configuration
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is required to build applications for MCUboot.
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This is handled internally by the Zephyr configuration system and is wrapped
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in the `CONFIG_BOOTLOADER_MCUBOOT` Kconfig variable, which must be enabled in
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the application's `prj.conf` file.
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The directory `samples/zephyr/hello-world` in the MCUboot tree contains
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a simple application with everything you need. You can try it on your
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board and then just make a copy of it to get started on your own
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application; see samples/zephyr/README.md for a tutorial.
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The Zephyr `CONFIG_BOOTLOADER_MCUBOOT` configuration option
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[documentation](http://docs.zephyrproject.org/reference/kconfig/CONFIG_BOOTLOADER_MCUBOOT.html)
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provides additional details regarding the changes it makes to the image
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placement and generation in order for an application to be bootable by
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MCUboot.
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With this, build the application as your normally would.
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### Signing the application
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In order to upgrade to an image (or even boot it, if
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`MCUBOOT_VALIDATE_SLOT0` is enabled), the images must be signed.
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To make development easier, MCUboot is distributed with some example
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keys. It is important to stress that these should never be used for
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production, since the private key is publicly available in this
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repository. See below on how to make your own signatures.
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There is a `sign.sh` script that gives some examples of how to make
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these signatures.
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### Flashing the application
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The application itself can flashed with regular flash tools, but will
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need to be loaded at the offset of SLOT-0 for this particular target.
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These images can also be marked for upgrade, and loaded into SLOT-1,
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at which point the bootloader should perform an upgrade. It is up to
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the image to mark slot-0 as "image ok" before the next reboot,
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otherwise the bootloader will revert the application.
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## Managing signing keys
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The signing keys used by MCUboot are represented in standard formats,
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and can be generated and processed using conventional tools. However,
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the Mynewt project has developed some tools to make this easier, and
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the `imgtool` directory contains a small program to use these tools,
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as well as some additional tools for generating and extracting public
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keys. If you will be using your own keys, it is recommended to build
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this tool following the directions within the directory.
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### Generating a new keypair
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Generating a keypair with imgtool is a matter of running the keygen
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subcommand:
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```
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$ imgtool keygen -k mykey.pem -t rsa-2048
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```
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The argument to `-t` should be the desired key type. See the
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imgtool README.rst for more details on the possible key types.
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### Extracting the public key
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The generated keypair above contains both the public and the private
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key. It is necessary to extract the public key and insert it into the
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bootloader. The keys live in `boot/zephyr/keys.c`, and can be
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extracted using imgtool:
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```
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$ imgtool getpub -k mykey.pem
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```
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This will output the public key as a C array that can be dropped
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directly into the `keys.c` file.
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Once this is done, this new keypair file (`mykey.pem` in this
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example) can be used to sign images.
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