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The NandBug Software

Overview

This software is part of the NandBug Project. Hardware files are available here.

NandBug is a small FPGA based board that I used in my Running Custom Code on a Google Home Mini project.

The goal of NandBug is to make the NAND Flash of a Google Home Mini In-System Programmable.

You can refer to the article for more background information and details.

Dumping the Flash

NandBugDumper.py is used to dump the NAND Flash content.

./NandBugDumper.py -h
usage: NandBugDumper.py [-h] filename

Dump the nand flash content

positional arguments:
  filename    output filename

optional arguments:
  -h, --help  show this help message and exit

This script will:

  • Generate a Dump bitstream and upload it to the FPGA.
  • Receive the NAND Flash data and write it to the output filename.

Programming the Flash

NandBugPatcher.py is used to alter the NAND Flash content.

./NandBugPatcher.py -h
usage: NandBugPatcher.py [-h] [--last-dump LAST_DUMP] filename

Patch the nand flash content

positional arguments:
  filename              input filename

optional arguments:
  -h, --help            show this help message and exit
  --last-dump LAST_DUMP
                        use this dump instead of reading the flash content

This script will:

  • Generate a Dump bitstream and upload it to the FPGA.
  • Receive the NAND Flash data and compare it to the content of filename.
  • Generate a list of blocks to erase and pages to program. This step can optionally be skipped if a LAST_DUMP file is provided.
  • Generate a Erase Blocks bitstream & upload it to the FPGA.
  • Send a list of blocks to erase to the FPGA.
  • Generate a Program Pages bitstream & upload it to the FPGA.
  • Send the pages addresses and data to the FPGA.

Passthrough

The NandBugPassthrough.py script will simply generate a Passthrough bitstream and upload it to the FPGA.

Effectively, this makes the NAND Flash directly connected to the Google Home Mini.

Technical Details

  • nMigen is used to generate bitstreams uploaded in the FPGA of NandBug.
  • pylibftdi is used for configuring and communicating with NandBug.
  • bchlib is used to perform error correction.
  • For now, the code is very specific to the NAND Flash and SoC used by the Google Home Mini (memory size and layout, ECC scheme, ...) and shouldn't be used with anything else without a couple of modifications.
  • Please note it's my first time using nMigen in a real project, so the code is likely suboptimal. A known issue is the absence of reliable testbenches for the HDL modules.

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