Part 11 - Firmware Boot Procedures
For a complete table of contents of all the lessons please click below as it will give you a brief of each lesson in addition to the topics it will cover. https://github.com/mytechnotalent/Reverse-Engineering-Tutorial
Let’s take a moment to talk about what happens when we first power on our Raspberry Pi device.
As soon as the Pi receives power, the graphics processor is the first thing to run as the processor is held in a reset state to which the GPU starts executing code. The ROM reads from the SD card and reads bootcode.bin to which gets loaded into memory in C2 cache and turns on the rest of the RAM to which start.elf then loads.
The start.elf is an OS for the graphics processor and reads config.txt to which you can mod. The kernel.img then gets loaded into 0x8000 in memory which is the Linux kernel.
Once loaded, kernel.img turns on the CPU and starts running at 0x8000 in memory.
If we wanted, we could create our own kernel.img to which we can hard code machine code into a file and replace the original image and then reboot. Keep in mind the ARM word size is 32 bit long which go from bit 0 to 31.
As stated, when kernel.img is loaded the first byte, which is 8-bits, is loaded into address 0x800.
Lets open up a hex editor and write the following:
FE FF FF EA
Save the file as kernel.img and reboot.
“Ok nothing happens, this sucks!”
Actually something did happen, you created your first bare-metal firmware! Time to break out the champagne!
When the Pi boots, the below code when it reached kernel.img loads the following:
FE FF FF EA
@ address 0x8000, 0xfe gets loaded.
@ address 0x8001, 0xff gets loaded.
@ address 0x8002, 0xff gets loaded.
@ address 0x8003, 0xea gets loaded.
“So what the hell is really going on?”
This set of commands simply executes an infinite loop.
Review the datasheet:
https://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
The above code has 3 parts to it:
1)Conditional – Set To Always
2)Op Code – Branch
3)Offset – How Far To Move Within The Current Location
Condition – bits 31-28: 0xe or 1110
Op Code – bits 27-24: 0xa or 1010
Offset – bits 23-0 -2
I know this may be a lot to wrap your mind around however it is critical that you take the time and read the datasheet linked above. Do not cut corners if you truly have the passion to understand the above. READ THE DATASHEET!
I will go through painstaking efforts to break everything down step-by-step however there are exercises like the above that I am asking you to review the datasheet above so you learn how to better understand where to look when you are stuck on a particular routine or set of machine code. This is one of those times I ask you to please read and research the datasheet above!
“I’m bored! Why the hell does this crap matter?”
Glad you asked! The single most dangerous malware on planet earth today is that of the root-kit variety. If you do not have a basic understanding of the above, you will never begin to even understand what a root-kit is as you progress in your understanding.
Anyone can simply replace the kernel.img file with their own hacked version and you can have total control over the entire process from boot.
Next week we will dive into the Von Neumann Architecture.