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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

Design

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

The directory layout is as follows:

Folder Description  
/doc Documentation
/pki Crypto keys for testing
/ Bootloader source
/src/board Board support
/src/arch Architecture support
/src/plat Platform support
/src/drivers Drivers
/tools Tools

Supported architectures:

Architecture Supported
armv7a Yes
armv8a Yes
armv7m Yes

Supported platforms:

Platform Supported USB EMMC HW Crypto Secure Boot Fusebox
NXP imx6ul Yes Yes Yes Yes Yes Yes
NXP imx8m Yes Yes Yes Yes No Yes
NXP imx8x Yes Yes Yes Yes Yes Yes

Supported boards:

Board Supported More info
Jiffy Fully supported https://github.com/jonasblixt/jiffy
Bebop Fully supported https://github.com/jonasblixt/bebop
Technexion PICO-IMX8M Partial support https://www.technexion.com/products/system-on-modules/pico/pico-compute-modules/detail/PICO-IMX8M
NXP IMX8QXP MEK Fully supported https://www.nxp.com/products/processors-and-microcontrollers/arm-based-processors-and-mcus/i.mx-applications-processors/i.mx-8-processors/i.mx-8-multisensory-enablement-kit:i.MX8-MEK

Hardware accelerated signature verification

Platform RSA4096 EC secp256r1 EC secp384r1 EC secp521
NXP imx6ul Yes Yes No No
NXP imx8m Yes Yes No No
nxp imx8x yes yes yes yes
nxp imx RT no no no no

Hardware accelerated hash algorithms

Platform MD5 SHA256 SHA384 SHA512
NXP imx6ul Yes Yes No No
NXP imx8m Yes Yes No No
NXP imx8x Yes Yes Yes Yes
NXP imx RT No No No No

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

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'

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
  • Call board specific functions

The tool is written in Python and some parts in C to allow bindings for other languages. The tool is built on top of a library to make it possible to integrate with other tooling and environments.

The tool is distributed through PyPi. Binary wheels are available for Windows, Linux and macos (x86_64 and arm64).

Install using pypi:

$ pip install punchboot

Image format

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

Authentication token

Punchboot enforces authentication when the SLC (Security Life Cycle) is locked. To interact with Punchboot the session must be authenticated by using a password or a signed token.

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:

Authenticating the session:

$ punchboot auth token ./0B177094-6B62-3572-902E-C1DE339ECB01.token <name of a key>
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 of course 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:

Parameter Value Unit
Power On Reset 28 ms
Bootloader init 7 ms
Blockdev read 13 ms
SHA256 Hash 4 ms
RSA 4096 Signaure 5 ms
Total 57 ms

Using libtomcrypt for SHA and RSA:

Parameter Value Unit
Power On Reset 28 ms
Bootloader init 7 ms
Blockdev read 13 ms
SHA256 Hash 431 ms
RSA 4096 Signaure 567 ms
Total 1046 ms

Measurements taken on IMX8QXP, loading a 14296kByte binary.

Using hardware accelerators for SHA and RSA signatures:

Parameter Value Unit
Power On Reset 175 ms
Bootloader init 6.358 ms
Blockdev read / hash 107 ms
RSA 4096 Signature 0.676 ms
Total 288 ms

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