Source Code: https://github.com/HexHive/BOPC
Summary: Vulnerable software with an active defense system (e.g. Control-Flow Integrity, Shadow Stack, Address Space Randomization etc.) is hard to exploit. Control Flow Integrity (CFI) restrict execution within valid control flows, although because of the weak control flow graph (CFG), the coarse-grained CFI system allows overapproximating control transfers. This keeps open the exploit door for an attacker to lead the execution to a certain status (e.g. pwn the shell, information leak etc.). In this research work, the author proposes a high-level language to write a tuning complete exploit, a new approach to generate exploit using basic blocks as gadgets and validate their control flow following valid path in CFG. This approach can beat shadow stack which ROP gadgets cannot do.
Design: When a vulnerable system is built with CFI and shadow stack, it is neither possible to target any arbitrary location in the program nor possible to return anywhere. The major contribution here is that instead of using ROP gadgets, using basic blocks as gadgets, an attacker can let the program follow the valid control transfers when still he can achieve his goal. But before that, an attacker has to express his desire what he is hoping for to exploit. So, they propose a language for writing exploit payloads (SPL) which is architecture independent and tuning complete. An attacker can use abstract registers as volatile vars, a subset of C library calls (e.g. execve etc.). SPL can contain multiple statements and for each of them, the block oriented programming (BOP) will find out a set of basic blocks that satisfy that statement. For example, if a statement expects a register to hold value 7, it will find out basic blocks in the vulnerable program that ends up with any register containing value 7. These set of basic blocks are identified as candidate blocks. Next, the system will mix the set of basic blocks each represents one statement without one basic block clobbering another basic blocks purpose. They are called functional blocks and now it is time to identify their dispatcher blocks. Dispatcher blocks are those blocks which are not clobbered blocks and connect one functional blocks to another. Searching for dispatcher blocks are NP-hard problems, even approximation algorithm will not work (because of large code space in real-world programs). So, they propose a backtracking system with a minimum threshold for search depth. This may overlook lots of possible dispatcher path, but it is important for scalability. With functional blocks with their dispatcher blocks, they now have delta graph which is a context-sensitive shortest path to lead a vulnerable entry to exploit zone. Although the exploit is valid in presence of CFI and shadow stack, it is important to validate them by solving their constraints in different control transfers. A concolic execution system is used to prove a subgraph of the CFG that can use for the exploit is possible to execute. Now, if an attacker has found a vulnerability in a system, he can use this exploit generation system to figure out what exploit could be useful.
Advantage: Their exploit generation technique is a forward approach that’s why it is easy to follow. The approach of block-oriented gadgets is a real expectation (ROP gadgets are an insane idea for the secured system).
Disadvantage: Too many thresholds limit the power. The SPL idea is not necessary, it is still a C code. The evaluation shows the system mostly fails to generate exploits for a major breakthrough like pwn the shell.
Discussion: The concept of using basic blocks and follow the control flow graph as the base to help find out a subgraph which is also valid is a good one. But, due to heuristics, the expectation limits a lot. A forward approach also limits what else a vulnerable system can be able to deliver.