Library Buffer Overflow Protection

TL;DR

This guide shows how to add basic buffer overflow protections to a custom library using compiler flags and runtime checks. We’ll focus on stack canaries, address space layout randomization (ASLR), and bounds checking.

1. Understand the Risks

Buffer overflows happen when a program writes data beyond the allocated memory for a buffer. This can overwrite important data or even execute malicious code. Custom libraries are vulnerable if they don’t carefully manage input sizes.

2. Compiler Flags: Stack Canaries & ASLR

Most compilers offer built-in protections. These flags don’t *prevent* overflows, but make exploitation much harder.

• Stack Canaries (-fstack-protector or -fstack-protector-all): Adds a random value (the canary) before the return address on the stack. If an overflow overwrites the canary, the program detects it and terminates. -fstack-protector-all provides more comprehensive protection but may have a slight performance impact.
• Address Space Layout Randomization (-pie -fPIE): Randomizes the base addresses of libraries and executables in memory. This makes it harder for attackers to predict where code is located.

Example compilation command (GCC/Clang):

gcc -shared -fPIC -pie -fstack-protector-all mylibrary.c -o libmylibrary.so

3. Bounds Checking

The most effective defense is to prevent overflows in the first place by checking input sizes before copying data.

• Explicit Length Checks: Always verify that the input size doesn’t exceed the buffer capacity.
• Safe String Functions: Avoid functions like strcpy, strcat, and sprintf which don’t perform bounds checking. Use safer alternatives like strncpy, strncat, and snprintf.

Example (C):

#include <string.h>

void safe_copy(char *dest, const char *src, size_t dest_size) {
  if (strlen(src) >= dest_size) {
    // Handle error - don't copy!
    fprintf(stderr, "Input too long for destination buffer.n");
    return;
  }
  strncpy(dest, src, dest_size - 1); // Leave space for null terminator
  dest[dest_size - 1] = '';
}

4. Runtime Checks (Optional)

For critical functions, you can add runtime checks to verify buffer boundaries.

• Heap Overflow Detection: Tools like AddressSanitizer (ASan) and Valgrind can detect heap overflows during program execution.

Example compilation command with ASan:

gcc -shared -fPIC -pie -fstack-protector-all -fsanitize=address mylibrary.c -o libmylibrary.so

5. Testing

• Fuzzing: Use fuzzing tools to automatically generate a large number of random inputs and test your library for vulnerabilities.
• Manual Testing: Create specific test cases designed to trigger buffer overflows (e.g., very long strings, unexpected input formats).