Chained-CFG-obfuscation-pass

LLVM obfuscation out-of-tree pass, flattening at the basic block's level and turning each basic block into a dispacher and each instruction into a new basic block.

Pass description

This CFG flattening pass is applied at the scope of the basic block which means that even programs with few basic blocks will be obfuscated. The larger the size of the basic blocks, the better the obfuscation will be. This is why this pass can be pretty effective combined with some MBA operation : they can potentially increase the size of each basic block and therefore the number of case in the basic block's dispacher. To find the right way the dispacher is going to use a local variable for each function with a random value and calculate the next case's value.

• On the left we can see the main function of a non obfuscated binary.
• On the right the same function but obfuscated with chained CFG flattening.

The raw size of the file is not much larger (in this case 20 % larger) but the graph is drastically different with much more basic bloc which symbolize a single program's instruction, the other part of the basic block is a xor operation on the local variable used by the dispacher to dinamically find his path in this giant mess.

Build this pass

• You first need to get LLVM working by either using your 5 Tb of RAM to build it from the sources or simply install it with apt install clang build-essential llvm-12-dev.
• We'll use cmake to build a makefile for the pass so you can modify the Pass/CMakelists.txt with your own LLVM installation/includes directories.
• You can then emit the LLVM IR from the source code you want with clang -O0 -S -emit-llvm input.c -o output.ll.
• build the pass with cmake <your CMakeLists.txt file location> and make.
• run the pass with path/to/llvm/bin/opt -load-pass-plugin /build/folder/libPassTheFishe.so -passes=Pass-Fishe -O0 output.ll -o output_mod.ll.
• You can now recompile your LLVM IR.
• Note that if you want to use the legacy pass manager you will need to change the pass implementation and registration in the Pass/Pass.h file check this link to see how the legacy PM works.

What to do next?

• This CFG flattening is an intra-basic block obfuscation thus merging this pass with CFG flattening on other levels would be really fun/hard to deobfuscate:
  • At the function level(inter-basic block)
  • At the module level (inter-function)
  • At the OS level (inter-process) lmao
• As previously said, combining this pass with MBA operation could also increase reverser's pain.