Understanding Stack Corruption in Assembly

Stack corruption occurs when a program modifies the stack in an unintended manner, leading to incorrect function return addresses, overwritten variables, and application crashes.

Common symptoms include:

  • Segmentation faults (SIGSEGV) or access violations
  • Unpredictable function return values
  • Crashes when executing ret instructions
  • Corrupt local variables after a function call

Key Causes of Stack Corruption

Several factors contribute to stack corruption in Assembly:

  • Incorrect function prologue/epilogue: Failure to maintain stack balance in function calls.
  • Mismatched calling conventions: Differences in register usage and stack cleanup between caller and callee.
  • Misaligned stack frames: Stack alignment requirements not met for specific architectures.
  • Unintended stack pointer modifications: Incorrect usage of push and pop instructions.
  • Buffer overflows: Writing beyond allocated stack space, overwriting return addresses.

Diagnosing Stack Corruption in Assembly

To identify and resolve stack corruption, systematic debugging is required.

1. Checking Stack Alignment

Ensure proper stack alignment before function calls:

and rsp, -16  ; Align stack to 16 bytes on x86-64

2. Inspecting Function Prologue and Epilogue

Compare function prologues and epilogues:

push rbp mov rbp, rsp ; Function setup mov rsp, rbp pop rbp ret

3. Verifying Calling Convention Compliance

Ensure consistent function parameter passing:

stdcall: caller cleans up stack cdecl: callee cleans up stack

4. Detecting Stack Pointer Mismatches

Monitor rsp before and after function calls:

gdb -ex "break *main" -ex "run" -ex "info registers"

5. Checking for Buffer Overflows

Detect buffer overflows by padding stack space:

sub rsp, 32  ; Reserve stack space

Fixing Stack Corruption Issues

1. Using Correct Function Prologues

Ensure functions properly save and restore stack state:

push rbp mov rbp, rsp sub rsp, 32  ; Allocate stack space

2. Matching Calling Conventions

Ensure caller and callee follow the same convention:

extern _stdcall_function ; Ensure matching declaration

3. Aligning Stack for SIMD Instructions

Ensure 16-byte alignment before using movaps:

and rsp, -16

4. Validating Stack Pointer Before Function Return

Check stack pointer consistency at function exit:

cmp rsp, rbp jne stack_corruption_detected

5. Avoiding Unnecessary push/pop Instructions

Balance stack operations:

push rax pop rax  ; Ensure balanced stack usage

Conclusion

Stack corruption in Assembly due to incorrect calling conventions can lead to serious application crashes and unpredictable behavior. By ensuring proper stack alignment, matching calling conventions, and correctly implementing function prologues and epilogues, developers can prevent these issues and improve program stability.

Frequently Asked Questions

1. Why is my Assembly function crashing on return?

Likely causes include incorrect stack alignment, mismatched calling conventions, or an overwritten return address.

2. How do I check if my stack is aligned correctly?

Use and rsp, -16 to enforce 16-byte alignment before function calls.

3. What is the difference between cdecl and stdcall?

In cdecl, the caller cleans up the stack, whereas in stdcall, the callee is responsible.

4. How can I detect stack corruption at runtime?

Use a debugger like GDB to inspect register values and track stack pointer mismatches.

5. Should I always use a function prologue and epilogue?

Yes, unless the function is extremely simple and does not modify the stack.