Introduction

Punchboot is a secure and fast bootloader for embedded systems. It is designed to:

• Boot as fast as possible
• Integrate with the SoC's secure boot functionality
• Authenticate the next piece of software in the boot chain
• Support A/B system partitions for atomic updates
• Support automatic rollbacks
• Minimize software download time in production
• Be useful for day-to-day development

Punchboot is designed for embedded systems and therefore it has a minimalistic apporach. There is no run-time configuration, everything is configured in the board files.

Punchboot could be useful if you care about the following:

• Boot speed
• Secure boot
• Downloading software quickly in production

Building

The easiest way is using docker.

Building the docker image:

$ docker build -f pb.Dockerfile -t pb_docker_env .

Building the jiffy-board target:

$ ./run_docker.sh
$ cp configs/jiffy_defconfig .config
$ make

Run test suite

Run the built in tests:

$ ./run_docker.sh
$ cp configs/test_defconfig .config
$ make
$ make check

The dockerfile in the top directory details the dependencies on ubuntu xenial

Design

Punchboot is written in C and some assembler. Currently armv7a and armv8 is supported.

The directory layout is as follows:

Supported architectures:

Supported platforms:

Supportd boards:

Hardware accelerated signature verification

Hardware accelerated hash algorithms

Secure Boot

Typical and simplified secure boot flow

• ROM loads a set of public keys, calculates the checksum of the keys and compares the result to a fused checksum
• ROM loads punchboot, calculates checksum and verifies signature using key's in step one
• Run punchboot
• Punchboot loads a PBI bundle, calculates the checksum and verifies the signature using built in keys
• Run next step in boot chain

Most SoC:s have a boot rom that includes meachanisms for calculating a checksum of the bootloader and cryptographically verifying a signature using a public key fused to the device.

Normally fuses are a limited resource and therefor a common way is to calculate a sha256 checksum of the public key(s) and then store this checksum in fuses, this way many different public keys can be stored in a flash memory and every time the device boots it will compute a sha256 checksum and compare it to the fused checksum.

Punchboot is designed to be a part of a secure boot chain. This means that the bootloader is cryptographically signed, the ROM code of the SoC must support a mechanism to validate this signature, otherwise there is no root of trust.

When punchboot has been verified it, in turn, will load and verify the next software component in the boot chain. The bootloader only supports signed binaries.

Testing and integration tests

Punchboot uses QEMU for all module and integration tests. The 'test' platform and board target relies on virtio serial ports and block devices. The punchboot cli can be built with a domain socket transport instead of USB for communicating with an QEMU environment.

The test platform code includes gcov code that calls the QEMU semihosting API for storing test coverage data on the host.

Building and running tests:

$ cp configs/test_defconfig .config
$ make
$ make check

A/B paritions and atomic upgrades

To support a robust way of upgrading the system the simplest way is to have two copies of the system software; System A and System B. When system A is active System B can be reprogrammed and activated only when it is verified. This is known as "Atomic Upgrade"

Punchboot uses a special config partition to store the current state. The state includes information about which system is active, boot count tries and error bits.

Automatic rollback

Sometimes upgrades fail. Punchboot supports a mechanism for so called automatic rollbacks.

The left most column describes a simplified way a linux system could initiate an upgrade. In this case System A is active and System B is to be prepared and eventually activated.

The new software is written to System B and verified then system A verified flag, error bits must be reset and boot try counter is programmed to a desired try-count. A cleared OK bit but set counter constitutes and upgrade state and punchboot will try to start this system and decrement the counter unless the counter has reched zero.

If the counter reaches zero the error bit is set and System A is automatically activated again (Rollback event)

At this point the upgrade is staged, and the OK bit of System A can be cleared and finally the system is reset.

Punchboot recognizes that none of the System partitions has the OK bit set but System B has a non-zero counter. System B is started.

When returning back to the upgrade application in linux final checks can be performed, for example checking connectivity and such before finally setting the OK bit of system B and thus permanently activate System B

Device identity

Most modern SoC's provide some kind of unique identity, that is guaranteed to be unique for that particular type of SoC / Vendor etc but can not be guarateed to be globally unique.

Punchboot provides a UUID3 device identity based on a combination of the unique data from the SoC and an allocated, random, namspace UUID per platform.

When booting a linux system this information is relayed to linux through in-line patching of the device-tree. The device identity can be found in '/proc/device-tree/chosen/device-uuid'

Allocated UUID's

Platform namespace UUID's

Command mode

Command mode is entered when the system can't boot or if the bootloader is forced by a configurable, external event to do so.

In the command mode it is possible to update the bootloader, write data to partitions and install default settings. From v0.3 and forward an 'authentication cookie' must be used to interact with the bootloader to prevent malicious activity. The only command that can be executed without authentication is listing the device information (including the device UUID)

The authentication cookie consists of the device UUID encrypted with one of the active key pair's private key.

punchboot tool

The punchboot CLI is used for interacting with the command mode. A summary of the features available:

• Update the bootloader it self
• Manually start system A or B
• Activate boot partitions
• Load image to ram and execute it
• Display basic device info
• Configure fuses and GPT parition tables

The tools and CLI are from version 0.7.0 separated into another repository: https://github.com/jonasblixt/punchboot-tools

Image format

Punchboot uses the bitpacker file format (https://github.com/jonasblixt/bpak)

Authentication token

Punchboot enforces authentication when a device is enforcing secure boot. It is still possible to access the USB recovert mode after authentication. When the device enters command mode it is still possible to issue the ' dev -l ' command to get the device UUID.

The authentication token is generated by hashing the device UUID and signing it with one of the active key pairs.

Example:

$ punchboot dev --show

Bootloader version: v0.6.1-40-ga47f-dirty
Device UUID:        0b177094-6b62-3572-902e-c1de339ecb01
Board name:         pico8ml

Creating the authentication token using the 'createtoken.sh' script located in the tools folder. In this example the private key is stored on a yubikey 5 HSM.

$ ./createtoken.sh 0B177094-6B62-3572-902E-C1DE339ECB01 pkcs11 -sha256 "pkcs11:id=%02;type=private"

engine "pkcs11" set.
Enter PKCS#11 token PIN for PIV Card Holder pin (PIV_II):
Enter PKCS#11 key PIN for SIGN key:
 
$ punchboot auth --token ./0B177094-6B62-3572-902E-C1DE339ECB01.token --key-id 0xa90f9680

Signature format: secp256r1
Hash: sha256
Authenticating using key index 0 and './0B177094-6B62-3572-902E-C1DE339ECB01.token'
Read 103 bytes
Authentication successful

Now the command mode is fully unlocked. The token is ofcourse only valid for the individual unit with that perticular UUID.

Metrics

Measurements taken on IMX6UL, running at 528 MHz loading a 400kByte binary.

Using hardware accelerators for SHA and RSA signatures:

Using libtomcrypt for SHA and RSA:

Measurements taken on IMX8QXP, loading a 14296kByte binary.

Using hardware accelerators for SHA and RSA signatures:

The POR time is off due to some unidentified problem with the SCU firmware. A guess would be that this metric should be in the 20ms -range.

Contributing

1. Fork the repository
2. Implement new feature or bugfix on a branch
3. Implement test case(s) to ensure that future changes do not break legacy
4. Run checks: cp configs/test_defconfig .config && make check
5. Create pull request