- The paper demonstrates that enforcing integrity through compiler and runtime mechanisms can significantly limit control-flow hijacking despite persistent memory bugs.
- It employs stack integrity, control-flow integrity, and heap partitioning to restrict attacker exploitation paths and reduce the potential for arbitrary code execution.
- The approach leverages production-proven, low-overhead techniques that integrate seamlessly into existing systems to pragmatically mitigate exploitation risks.
Securing existing C and C++ software without attaining complete memory safety involves deploying practical compiler and runtime mechanisms that eliminate the vast majority of execution paths attackers exploit following memory corruption. The central premise is that while memory corruption bugs themselves may persist, their utility for achieving arbitrary code execution or full control hijacking can be drastically reduced by enforcing various forms of integrity on program execution (2503.21145). This approach focuses on preventing the consequences of memory corruption, specifically Remote Code Execution (RCE), rather than eliminating the initial corruption event itself. It leverages existing, production-proven technologies that impose low overhead and require no specialized hardware beyond what is increasingly common.
Stack Integrity Enforcement
Stack-based memory corruption, such as buffer overflows, allows attackers to overwrite critical stack data, including return addresses, saved registers, local variables, and function arguments. Exploits like stack smashing and Return-Oriented Programming (ROP) rely on manipulating this data to hijack control flow. Compiler-based stack integrity mechanisms mitigate these threats by fundamentally changing how the stack is managed and accessed. Techniques often involve simulating a segmented stack model. Compilers can enforce rules such as restricting stack pointer updates to constants and relocating pointer-accessed local variables from the traditional stack frame to a dedicated, thread-local heap region. LLVM's SafeStack implementation exemplifies this, separating control-related data (return addresses, saved frame pointers) and potentially attacker-corruptible arrays or variables into distinct stack regions (the "safe stack" and the "unsafe stack"). This segregation prevents buffer overflows in local arrays from directly corrupting return addresses or other critical control data residing on the safe stack, thus thwarting common stack-based control-flow hijacking techniques.
Control-Flow Integrity (CFI)
Control-Flow Integrity (CFI) targets exploits that divert execution by corrupting indirect control-flow transfers, primarily function pointers and C++ virtual table (vtable) pointers. Attackers overwrite these pointers, typically stored on the heap or corrupted via use-after-free vulnerabilities, to redirect program execution to malicious code or gadgets (code reuse attacks). CFI implementations instrument the code, inserting checks before indirect calls or jumps. These checks validate that the target address corresponds to a legitimate destination, as determined statically by the compiler. Common validation methods include ensuring the target is the entry point of a function with a compatible type signature (for function pointer calls) or a valid virtual method within the object's class hierarchy (for vtable calls). Major compilers like LLVM, GCC, and Microsoft Visual C++ (implementing Control Flow Guard - CFG) offer CFI variants. Hardware extensions like Intel Control-flow Enforcement Technology (CET) and ARM Branch Target Identification (BTI) / Pointer Authentication Codes (PAC) provide hardware acceleration for CFI checks, reducing performance overhead. Deployments in systems like Windows, Android, and Chrome demonstrate the practicality and effectiveness of CFI in preventing indirect control-flow hijacking. When combined with stack integrity, CFI ensures that program execution largely follows the statically determined call graph, proceeding as a sequence of well-nested function calls.
Heap Data Integrity
Attackers frequently exploit heap vulnerabilities (e.g., heap overflows, use-after-free) combined with heap layout manipulation techniques ("heap feng shui") to achieve precise corruption of target objects or metadata residing on the heap. This targeted corruption often aims at overwriting function pointers, vtable pointers, or other security-critical data structures. Heap data integrity mechanisms counter this by partitioning the heap into multiple, isolated regions. Objects are allocated into specific partitions based on criteria derived from static analysis, such as the allocation site, object type, size class, or namespace. Even coarse-grained partitioning (e.g., by size) offers some benefit, but finer-grained partitioning provides stronger isolation guarantees, making it significantly harder for an overflow or misuse of an object in one partition to affect objects in another. Examples include Chrome's PartitionAlloc, which partitions based on allocator type and size, and Apple's kalloc_type in the XNU kernel, which uses type-based partitioning. The SafeCode system also proposed techniques to confine pointer usage based on static reachability within partitions. By segregating heap objects, these mechanisms disrupt heap grooming techniques and limit the blast radius of heap corruption, preventing attackers from reliably corrupting specific target data needed for exploitation.
Pointer Integrity and Unforgeability
A fundamental requirement for most memory corruption exploits is the ability to create or modify pointers to point to attacker-controlled locations (e.g., shellcode, ROP gadgets, corrupted data). Pointer integrity mechanisms aim to make valid pointers difficult for attackers to forge or arbitrarily modify, treating them somewhat like capabilities. Address Space Layout Randomization (ASLR) serves as a baseline defense by randomizing the locations of code, stack, heap, and libraries, making pointer targets harder to predict. More advanced techniques involve pointer authentication or randomization, where cryptographic secrets (tags or signatures) are incorporated into pointer values, often dependent on the pointer's type or context. These augmented pointers must be authenticated or "derandomized" before dereferencing. Hardware support, most notably ARM Pointer Authentication (PAC), provides efficient runtime enforcement by signing pointers using secret keys (often context-dependent) and verifying the signature upon use. Instructions like PACIASP (sign stack pointer) and AUTIASP (authenticate stack pointer) are used to protect stack integrity, while similar instructions protect function pointers and return addresses, directly enforcing aspects of stack and control-flow integrity. Apple's platforms extensively use ARM PAC to enforce integrity. Even without runtime checks, pointer randomization significantly increases the entropy attackers must overcome. By making pointers unforgeable and context-bound, these mechanisms prevent attackers from easily crafting pointers to redirect control flow or access arbitrary memory locations.
Conclusion
In summary, securing existing C and C++ software against sophisticated exploits like RCE, without undertaking the often prohibitive task of achieving full memory safety, can be practically accomplished by combining compiler and runtime integrity enforcement mechanisms. Stack integrity, Control-Flow Integrity (CFI), heap data integrity through partitioning, and pointer integrity/unforgeability collectively eliminate the most common pathways attackers use to gain control after inducing memory corruption. While memory bugs might still trigger crashes or localized data corruption, these combined defenses severely constrain the attacker's ability to redirect execution flow arbitrarily, significantly raising the bar for successful exploitation and improving the security posture of vast amounts of legacy code (2503.21145). These techniques are largely available in modern toolchains and operating systems, offering a pragmatic path towards mitigating the most severe consequences of memory unsafety in C and C++.
