Fixing Buffer Overflow: Segment Fault

TL;DR

A buffer overflow causes a “segment fault” when your program tries to write data beyond the allocated memory for a buffer. This guide shows you how to identify and fix these issues, typically by using safer functions or implementing bounds checking.

Understanding Buffer Overflows

Imagine a container (the buffer) designed to hold 10 items. A buffer overflow happens when you try to stuff 15 items into it. The extra items spill over and overwrite other important data in your program’s memory, leading to crashes or even security vulnerabilities.

Identifying the Problem

1. Look for Error Messages: A “segment fault” is a common symptom. Debuggers will often point you to the line of code causing the issue.
2. Review Code: Focus on areas where you’re copying data into buffers, especially using functions like strcpy, strcat, gets, or sprintf. These are notorious for buffer overflow vulnerabilities.
3. Use a Debugger (GDB): A debugger allows you to step through your code line by line and inspect the values of variables. This helps pinpoint exactly where the overflow is happening.

   gdb ./your_program

   Then, set a breakpoint near the suspected buffer:

   break function_name

   Run the program with input that triggers the crash:

   run < input_file

4. Valgrind: A memory debugging tool. It can detect buffer overflows and other memory errors.

   valgrind --leak-check=full ./your_program < input_file

Fixing Buffer Overflows

1. Use Safer Functions: Replace unsafe functions with their safer counterparts.
   • Instead of strcpy(dest, src), use strncpy(dest, src, sizeof(dest) - 1); dest[sizeof(dest)-1] = '';. The strncpy function limits the number of characters copied to prevent overflows. Remember to null-terminate the string manually!
   • Instead of strcat(dest, src), use strncat(dest, src, sizeof(dest) - strlen(dest) - 1);. Again, ensure proper null termination.
   • Never use gets(). It’s inherently unsafe. Use fgets(buffer, sizeof(buffer), stdin) instead.
   • Instead of sprintf(buffer, format_string, ...), use snprintf(buffer, sizeof(buffer), format_string, ...). This limits the output length.
2. Bounds Checking: Explicitly check the size of the input data before copying it into a buffer.

   #include <string.h>

   int main() {

    char dest[20];

    char src[] = "This is a long string";

    if (strlen(src) < sizeof(dest)) {

      strcpy(dest, src);

    } else {

      printf("Input string too long!");

    }

    return 0;

   }

3. Stack Canaries: Compilers can insert “canary” values onto the stack to detect buffer overflows. Enable this feature during compilation (usually enabled by default with modern compilers, but check your compiler flags).
4. Address Space Layout Randomization (ASLR): ASLR randomizes the memory addresses of key program components, making it harder for attackers to exploit buffer overflows. Ensure ASLR is enabled in your operating system.

   # Linux: Check /proc/sys/kernel/randomize_va_space

Example Fix

Let’s say you have this vulnerable code:

char buffer[10];

strcpy(buffer, "This is a very long string!"); // Potential overflow!

A fix using strncpy would be:

char buffer[10];

strncpy(buffer, "This is a very long string!", sizeof(buffer) - 1);

buffer[sizeof(buffer) - 1] = ''; // Ensure null termination.