Fix ASLR: Text Section Not Randomizing

G5 Cyber Security

2 weeks ago

TL;DR

Address Space Layout Randomisation (ASLR) isn’t working properly for the text section of your program. This guide shows you how to check if it’s enabled, identify why it might not be randomising, and steps to fix it.

Checking ASLR Status

Verify System-Wide ASLR is Enabled: Most modern operating systems have ASLR enabled by default. You can check this on Linux with:
```
sysctl -a | grep kernel.randomize_va_space
```
A value of ‘2’ means full ASLR is active. Values 0 and 1 indicate partial or no ASLR.
Check Executable Support: The executable itself needs to be compiled with support for ASLR. Use the following command on Linux:
```
readelf -h /path/to/your/executable | grep 'Type:'
```
Look for ‘EXEC’ in the output. If it’s not an executable, ASLR won’t apply.
Confirm PIE (Position Independent Executable): For full text section randomisation, your program *must* be compiled as a Position Independent Executable (PIE). Check with:
```
readelf -h /path/to/your/executable | grep 'Flags:'
```
If you don’t see the flag `EXEC_P`, it’s likely not PIE.

Why ASLR Might Not Be Working

Not Compiled as PIE: This is the most common reason. If your program isn’t compiled with `-fPIE` and linked with `-pie`, the text section won’t randomise.
Static Linking: Static linking can sometimes interfere with ASLR, especially if libraries aren’t built with ASLR in mind. Prefer dynamic linking where possible.
System Restrictions: Some security policies or sandboxing environments might disable ASLR for specific programs.
Debugging: Debuggers often temporarily disable ASLR to make debugging easier. Ensure you’re testing outside of a debugger.

Fixing the Problem

Recompile with PIE: This is almost always the solution. Use these compiler and linker flags:
- Compile: -fPIE (Position Independent Executable)
- Link: -pie (Create a PIE executable)
Example compilation command:
```
gcc -fPIE -pie your_program.c -o your_program
```
Dynamic Linking: Ensure you are linking dynamically against libraries.
Check Compiler Version: Older compilers might have limited ASLR support. Update to a recent version if possible.
Verify Address Randomisation at Runtime (Linux): After recompiling, check the addresses of functions in memory using tools like `gdb` or by printing function pointers:
```
#include <stdio.h>
int main() {
  printf("Address of main: %pn", (void *)main);
  return 0;
}
```
Run the program multiple times and observe if the address changes each time. If it doesn’t, ASLR isn’t working.
Security Policy Review: Investigate any system-level security policies that might be interfering with ASLR. This is less common but possible in restricted environments.

Further Troubleshooting

If the problem persists after these steps, consider providing more details about your operating system, compiler version, and linking process for further assistance.