Dumping SPI Flash Chip Firmware

Within hardware hacking, there are plenty of techniques that can be used to extract information from a device, interface with it or increase its attack surface - it all really depends on what comes out of your investigations into the device.

One such vector is gaining access to the device's firmware, which commonly includes a small linux filesystem of some description. This can give a big insight into the inner workings of the embedded system, and depending on how far the manufacturer has deviated from best practices, can expose hardcoded secrets or obvious vulnerabilities.

Obtaining the firmware can be achieved through dumping non-volatile flash chips, the most common of which are SPI (Serial Peripheral Interface) flash chips. We can interface with these chips using software, and pretend to be a processor that can send commands to the chip to read the firmware.

Understanding SPI Flash Memory

SPI (Serial Peripheral Interface) is a synchronous serial communication interface that uses a controller-peripheral architecture. The communication interface is done over 8 different pins on the chip, each of which have a specific purpose:

• PICO or MISO or DI - Peripheral In. This is the channel through which we will input data to the chip.
• POCI or MOSI or DO - Peripheral Out. This is the channel through which we receive output data from the chip.
• CS - This is the chip select signal. Typically when this is low voltage, the chip is enabled and ready to receive data.
• CLK - This is the clock signal. This is used to synchronize the data transfer between the controller and the peripheral.
• VCC - This is the power supply pin. This is usually 3.3V, but some chips may use 3V or 1.8V.
• GND - This is the ground pin.
• /WP - This is the write protect pin. This pin is not really used when reading firmware. Depending on the manufacturer, we typically disable this pin by "pulling it high" (in other words, applying 3.3V to it).
• /HOLD or /RESET or both - This pin is used to put the chip into a reset state. Again, we won't be using this pin when reading firmware. Again, depending on the chip it is disabled by pulling it high.

Identifying Flash Chips

Different SPI flash chip packages exist, I have rarely encountered packages outside of the following formats:

• SOIC-8
I have found this format mostly in cheap devices. They have very obvious protruding legs, and are (in my opinion) easier to interface with and de-solder.



• WSON-8
This is common for "name brand", but still cheaper devices. They have a low profile, look more rectangular, and don't have protruding legs. They come in a variety of packages, but the most common ones are 5x6mm and 6x8mm.

Chip Markings

It is also typical for chip manufacturers to print reference markings on the top of their chips. These markings are used to identify the chip, and are usually a combination of the manufacturer, the type of chip and the capacity. So if you think you have found a candidate chip, you can simply google the markings and find its datasheet.

From the above, we know the manufacturer is Winbond, and the chip is a W25Q256JVEQ. If we google this string "W25Q256JVEQ", we will get results including the datasheet for the chip.

Interpreting Datasheets

The datasheet will inform us of the chip's pinout, as well as other specifications including voltage requirements, speeds, chip packages and more.

The below diagram is taken from the datasheet we found for the above chip:

The physical chip is marked with a dot in one of its corners. This dot is used to identify pin 1 on the chip, and helps us line up the pins in the diagram with the pins on the chip.

Chip Off vs Chip On

There are two main ways to actually dump the firmware from a chip; chip off and chip on. These are pretty self explanatory - chip off is when you remove the chip from the board by de-soldering it, and chip on is when you leave the chip on the board and use a test clip or probes to connect to its pins.

• Chip Off
This is the best method, as it completely isolates the chip from the board and makes life easier when trying to connect to it.


• Chip On
This method is hit and miss - you have a much better chance of success if you are trying to dump an SOIC-8 package, but it is possible to achieve with a WSON-8 package if you have the right equipment.

The chip off method is generally preferred because when you introduce the 3.3V power supply to the chip, you can also inadvertenly power up other device components. This can cause clashing behaviours when you are trying to send commands to the chip to read the firmware.

Dumping the Firmware

Here is a part list of equipment you might typically need:

• Bus Pirate - A well known tool for interfacing with chips of many kinds. This is what we will be using to communicate with the chip and extract the firmware.
• Test Clips - If you are performing a chip on, you will need a test clip to connect to the chip. These clips are different depending on the package of the chip you are using.
• Socket Adapter / Chip Cradle - If you are performing a chip off, you will need a socket adapter to connect to the chip. The adapter is specific to the package of your chip. It is essentially a breakout board containing a socket that the chip fits into, which allows you to connect to the chip's pins. You can do it without, but paying the £10 is 100% worth it to avoid the headache.
• Jumper Wires - For connecting the test clip or socket adapter to your programmer.
• Hot Air Reworking Station - If you are performing a chip off, you need a hot air reworking station to desolder the chip from the board. It is possible to use a soldering iron, but this is more difficult.
• Other Soldering Stuff - Again for chip off, you will need a pair of tweezers, solder flux, solder wick and isopropyl alcohol to clean the board and chip. This is often overlooked, but is important to get clean connections to the chip's pins.

Breakout

The first thing we need to do is get reliable connections to the chip's pins. If you are performing a chip on, this is as simple as connecting the test clip to the chip's pins, ensuring that proper contact is made to each pin. Test clips commonly have a red cable, marking it as pin 1.

If you are performing a chip off, this is the point where you will need to de-solder the chip from the board.

1. Apply a healthy amount of solder flux to the chip's pins / connections on the board.

2. Start heating the area of the board with the chip on using a hot air gun. I tend to start around 300C for this.

3. Once the area around the chip is heated, increase your temperature to around 400C (thicker PCB boards may require heating up to 420C). Point your hot air gun at the chip, slowly moving around to apply even heat.

4. You will eventually see the solder around the chip's pins melt and go shiny. This is the point at which you take your tweezers and carefully lift the chip off the board.

5. Once the chip is removed, it is important to clean up. Use solder wick to remove any excess solder (from the chip and the board), and use isopropyl alcohol to remove flux residue.

6. Pop the chip into your socket adapter / chip cradle. There are again usually markings on the adapter which tell you where pin 1 should go.

Connecting to the Chip

Now that we have access to the chip's pins, we will connect the chip to our Bus Pirate. Recent iterations of the Bus Pirate have a cool LED screen, which tells you which connection correlates to which pin. We have to set up the Bus Pirate first to get this information.

The Bus Pirate 5 works over a serial interface, so, using a cable that supports power and data, connect it to your computer. We can then identify the serial interface port either by listing devices in /dev/ if you are on linux, or looking at device manager COM ports if you are on windows. Once identified, connect to the serial interface with a baud rate of 115200 using a terminal emulator like MobaXterm, or a tool like screen or tio.

Now we can configure the bus pirate to be in SPI mode. The default settings are pretty universal for SPI flash chips, but remember, the datasheet will give you clarity on the best settings for your chip. You can accept default settings by pressing enter.

Once configured, we can see that the Bus Pirate's LED screen has changed to show what each pin is expected to be connected to.

As previously mentioned, we need to pull the HOLD/RESET and WP pins to high. We can do this by configuring two of our spare pins to "HIGH" mode.

Now let's connect the chip to the Bus Pirate. Most of the connections are pretty easy to figure out - some things to consider:

• SCLK and CLK are interchangeable.
• MISO, PICO and DO are interchangeable. As are MOSI, POCI and DI. MOSI goes to MOSI, MISO goes to MISO.
• The power supply on the bus pirate is always "VOUT", the first pin. On the chip side, this is VCC.
• Always connect GND to GND. Sometimes GND is labelled as VSS.

Here's a really polished wiring diagram I made at 2AM in MS Paint - please try to contain yourself when viewing:

Once all your connections are made, we can supply some power to the chip. The below commands are specific to the Bus Pirate 5. Again, default settings are usually fine, but you can always consult the datasheet if you are not sure.

To test everything is working, we use the probe command. This will read the first 16 bytes of the chip which stores the manufacturer ID (or JEDEC ID). The bus pirate will then look up the ID in its database and return information about the chip.

If this doesn't work, you can troubleshoot with the following:

• Check your connections.
• Re-clean your chip.
• Check the datasheet for the correct settings.
• If it still doesn't work, but a valid JEDEC is returned (ie not 0xEF), you may have to use software with a different database, such as flashrom.

Otherwise, if everything is working, we can dump the firmware to a file!

Once the dump is complete, you can grab it from the Bus Pirate's internal storage; it shows up like a USB drive on your system. Tools like binwalk and unblob can be used against the bin file to carve out interesting file contents, however this is beyond the scope of this article.

Conclusion

Whether you're exploring the security of your own devices, conducting authorised penetration testing or developing more secure hardware, understanding how to access and analyse flash memory contents provides valuable insights into the inner workings of embedded systems.

Dumping SPI flash chips is a fundamental skill in hardware security assessment and reverse engineering. While the basic process is straightforward, mastering the techniques for analysis and overcoming common challenges takes practice and experience. Practise on devices you find through ebay, cheap manufacturers, e-waste or old unused devices.

Remember that with great power comes great responsibility - always use these techniques ethically and legally, respecting intellectual property rights and applicable laws.