Fixing Buffer Overflow Seg Faults

TL;DR

Your buffer overflow exploit is crashing with a segmentation fault (segfault). This guide helps you diagnose and fix common causes, focusing on correct offset calculation, shellcode injection, and ensuring proper execution permissions.

Understanding the Problem

A segfault usually means your program tried to access memory it shouldn’t. In a buffer overflow exploit, this often happens when:

• You overwrite important data on the stack (like the return address).
• Your shellcode is incorrectly placed or doesn’t have execute permissions.
• The offset you calculate to reach the return address is wrong.

Step-by-Step Fixes

1. Verify Offset Calculation
   • Use a debugger (like GDB) to find the exact offset to the return address on the stack. This is crucial!
   • A common technique is sending a pattern of unique characters and then finding where that pattern overwrites the return address in memory. Tools like pattern_create.rb (from Metasploit) can help generate this pattern.
   • Example using GDB:

     run < input_file
     break *function_where_overflow_occurs
     continue
     x/20gx $rsp  # Examine the stack around the return address

2. Check Shellcode Injection
   • Ensure your shellcode is being correctly injected into the buffer. Double-check the length of your shellcode and that it’s not truncated or corrupted during transmission.
   • Consider using a null byte terminator if necessary, depending on how the input is handled by the vulnerable program (e.g., strcpy stops at null bytes).
3. Address Space Layout Randomization (ASLR)
   • If ASLR is enabled, the addresses of libraries and functions change on each execution. This means a hardcoded address for shellcode or library functions won’t work reliably.
   • Disable ASLR temporarily for testing: sudo sysctl -w kernel.randomize_va_space=0 (remember to re-enable it later with kernel.randomize_va_space=2).
   • For a more robust exploit, use techniques like Return-Oriented Programming (ROP) to bypass ASLR. This involves chaining together existing code snippets in the program or libraries.
4. Execution Permissions
   • The stack is usually not executable by default. You need to find a way to make it executable. This often isn’t possible directly without further exploitation techniques.
   • If the program allows writing to memory, you might be able to copy your shellcode to an executable section of memory (e.g., the .text segment).
5. Stack Alignment
   • Some architectures require the stack to be aligned on a specific boundary (e.g., 16 bytes) before executing shellcode. Misalignment can cause segfaults.
   • Pad your input with NOP instructions (

     x90

     ) to ensure proper alignment. The number of NOPs needed depends on the architecture and current stack pointer value.

6. Non-Executable Stack Mitigation
   • Modern systems often have protections like Data Execution Prevention (DEP) or NX bit that prevent code execution from data sections like the stack.
   • ROP is a common technique to bypass these mitigations.
7. Debugging with GDB
   • Set breakpoints before and after the vulnerable function call to examine the stack contents and register values.
   • Use info frame to see details about the current stack frame, including the return address.
   • Step through the code line by line (using next or step) to identify exactly where the segfault occurs.

Example Scenario

Let’s say you have a program with a buffer overflow in a function called vulnerable_function. You calculate an offset of 40 bytes to reach the return address.

python -c 'print "A" * 40 + "x90" * 100' | ./your_program

If this still segfaults, go back to step 1 and carefully re-examine your offset calculation using GDB. Pay close attention to the stack layout.