DeathSleep

First of all

Everything stated here comes from my understanding of the different topics covered, either from reading or experience during development. Im aware that Im not an expert and the last thing I want to do is spread misinformation, so if you think that something is not correct, I would love you to make me aware of it, you can contact me on twitter, or opening issues in this repo. Thank you very much for your understanding. :)

Basics

The main objective of this technique is clear, terminating the current thread and restoring it before resuming execution, but what exactly does this mean, and which new constraints it puts in place?

To be able to restore the execution, we need to save two things before terminating the thread, first, the CPU state, and secondly the stack, and effectively set them up again after the new thread is launched.

I talked about new constraints that will appear in this techniche, and there are two big ones: First we need to store outside the stack anything that is needed from the moment the thread terminates until the stack is restored, and as you will see, it creates some new challenges.

Second, we always need at least another thread running in our process, since we are terminating our thread, if there are no other threads the process will end. I don't think this a big problem, since most agents are injected in other processes, we can assume that this process will keep at least one thread running.

DeathSleep components:

We can view on this POC 4 core functions:

• Main program: This is where you would write your agent code, and it's the portion of the code that will make use of DeathSleep
• Awake function: this is the entry point of all our threads, and it's in charge of saving the starting point of the stack that we will be restoring. Also, it's in charge of restoring the stack and CPU context when it's needed, or just launching our main program.
• DeathSleep: this is the main function of this technique, and is in charge of backing up the thread context and stack, and also setting everything up for the magic to happen.
• Rebirth: A simple function only in charge of launching our new threads.

Saving the stack.

When we are about to save the stack, a question is raised: how much of the stack needs to be saved?

Lets review first what is in the stack after we called the DeathSleep function (this is the function that saves the context, stack and prepares everything for the obfuscation and restoration)

As we can see, every function has three parts:

• Shadow space: this is a 32 bytes space, allocated by the caller, but used by the callee. As far as I know and I could see, its main function is to hold, if needed, the arguments passed to the callee function in registers, but it can be used for anything the callee function decides.
• Return address: this is the address of the next instruction to be executed in the caller function, pushed by the CALL instruction, so the RET instruction in the callee will just take this address and “jump” to it when it ends.
• Function stack space: This is the space reserved by the callee to store the value of registers that need to be restored and the value of its local variables.

The minimum portion of the stack that we obviously need to save is everything inside our main program, that means its shadow space, its return address and everything until the DeathSleep function. Anything before that is not really required (saving the stack used by the entry function has its advantages, but we will discuss that later), as that is the stack used by windows routines for launching our new thread. Apart from this, I decided to also store the shadow space of the DeathSleep function (not really needed, but it makes the calculation of the Rsp at the moment of waking up easier).

So at the end, we are saving this:

How to find our stack addresses:

Every function on a standart compilation should be composed of 3 parts, the prologue, the function code, and the epilogue.

The Rsp (stack pointer), should only be modified on the function prologue and epilogue. The prologue increases the stack pointer (remember increasing the stack means reducing addresses, since they go in opposite directions), to save registers, to hold all its local variables and then to hold the shadow space, and the epilogue does exactly the opposite.

This means that the stack pointer inside the function code should always point to the end of the shadow space (purple on the above image), and the sum of the stack size of the function and the shadow space can be found by calculating how much the prologue increases the stack pointer. This value can be easily calculated using the information held in the unwind tables, an explanation about their usage is something we will not cover here, but as a summary, those tables are used to allow any other thread or process to correctly move through the stack to see its content, to handle exceptions or to analyze it.

Capturing and preparing the context to restore.

Capturing the context is probably one of the easiest things to do, as we can simply do RtlCaptureContext() at the first line of DeathSleep, before any modification is done to the non-volatile registers. We still need to do two modifications to the context where we will restore the execution.