ASLR: Strengthening Memory Protection

TL;DR

Address Space Layout Randomisation (ASLR) is a vital cyber security technique that makes it harder for attackers to exploit vulnerabilities. This guide explains how ASLR works, checks its status on your system, and shows you how to improve its effectiveness by understanding different memory regions.

What is ASLR?

ASLR randomises the positions of key data areas in a program’s memory space – things like the base of the executable, libraries, heap, and stack. This means that an attacker can’t reliably predict where to find specific code or data, making exploits much more difficult.

Checking ASLR Status (Linux)

1. Check if ASLR is enabled globally: Use the following command in your terminal.

   sysctl -a | grep kernel.randomize_va_space

   • A value of ‘2’ means full ASLR is active, which is what you want.
   • ‘1’ indicates a weaker form of ASLR.
   • ‘0’ means ASLR is disabled.
2. Check ASLR status for a specific executable: Use the getconf command.

   getconf LONG_BIT /bin/ls

   This shows you if the program is compiled with PIE (Position Independent Executable), which is essential for effective ASLR. A value of ’64’ means it’s likely PIE-enabled.

Checking ASLR Status (Windows)

1. Check if ASLR is enabled: Open PowerShell as an administrator and run.

   Get-Process -Id $(tasklist /fi "imagename eq notepad.exe" /fo csv | ConvertFrom-Csv).PID | Select-Object -ExpandProperty AslrEnabled

   (Replace ‘notepad.exe’ with the process you want to check.) A value of True means ASLR is enabled for that process.

Understanding Memory Regions

ASLR works best when multiple memory regions are randomised. Here’s a breakdown:

• Executable Stack: Allows code to be executed from the stack. Disable this! It significantly increases attack surface.
• Heap: Dynamically allocated memory. Randomising the heap base is crucial.
• Stack: Used for function calls and local variables. Randomisation here makes stack-based exploits harder.
• Libraries (Shared Objects): Code shared between programs. Randomising library load addresses is vital.
• Base Address of Executable: The starting point of the main program code. This *must* be randomised.

Improving ASLR Effectiveness

1. Compile with PIE (Position Independent Executable): When compiling your own programs, use the `-fPIE` flag.

   gcc -fPIE -o myprogram myprogram.c

   Also link with `-pie`:

   ld -pie myprogram.o -o myprogram

2. Disable Executable Stack (Linux): Use the following command to disable it for a process.

   echo 0 | sudo tee /proc/sys/kernel/exec-stack

   To make this permanent, edit `/etc/sysctl.conf` and add `kernel.exec-stack = 0`. Then run `sudo sysctl -p`.

3. Enable Data Execution Prevention (DEP) / No-Execute (NX): This prevents code execution from data regions like the heap and stack. Windows usually enables this by default, but check your system settings. On Linux, ensure the NX bit is set for memory pages.
4. Keep Software Updated: Updates often include ASLR improvements and fix vulnerabilities that attackers could exploit even with ASLR in place.

Further Considerations

• Entropy: The more random the addresses generated by ASLR, the better. Modern systems generally have good entropy sources.
• Information Leaks: Vulnerabilities that leak memory addresses can undermine ASLR. Address any such leaks promptly.