How do I change EIP in a different process? - c

Question

I'm trying to hack another program by changing the EIP of it. There are two programs running, one is the target, that tells where the function that is the "core-function"(e.g. a function that receive a password string as a parameter and returns true or false) is in memory.
Then now that I know where the core-function is I wanna modify the EIP with the other program so the target program can call my function and simply get a true out of it and print out a beautiful "access granted".
My code is now like this:
Target Program:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int checkPwd(char *pwd)
{
printf("\nstill in the function\n");
if(strcmp(pwd, "patrick") == 0) return true;
else return false;
}
int main()
{
char pwd[16];
printf("%d", checkPwd);
scanf("%s", &pwd);
system("pause");
if(checkPwd(pwd)) printf("Granted!\n");
else printf("Not granted\n");
system("pause");
}
Attacker Program:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <memory.h>
int returnTrue()
{
return true;
}
int main()
{
int hex;
scanf("%d", &hex);
memcpy((void*)hex, (void*)returnTrue, sizeof(char)*8);
system("pause");
}
I wanna add that I tried to put the hex code directly(without the scanf part) in the attacker program and did not work, it crashed.
So I think I'm missing some part of the theory in here. I'd be glad to know what is it.
Thanks in advance.

Answer 1

This won't work—the processes occupy different memory spaces!
Modern operating systems are designed to protect user programs from exactly this kind of attack. One process doesn't have access to the memory of another—and indeed, the addresses of data are only valid inside that process.
When a program is running, it has its own view of memory, and only can "see" memory that the kernel has instructed the memory management unit (MMU) to map for it.
Some references:
Mapping of Virtual Address to Physical Address
Printing same physical address in a c program
Why are these two addresses not the same?

Answer 2

It is possible to inject a function into another process but it is a little more involved than you think. The first thing is you need the proper length of the function you can do this by creating two functions.
static int realFunction() { ... }
static void realFunctionEnd() {}
Now when you copy the function over you do the length of:
realFunctionEnd - realFunction
This will give you the size. Now you cannot just call the other functions because as stated they are not guranteed to be at the same address in the other process, but you can assume that , I will assume windows, that kernal32.dll is at the same address so you can actually pass that to the realFunction when you create a remote thread.
Now, as to your real issue. What you need to do is to either inject a dll or copy a function over into the other process and then hook the function that you need to change. You can do this by copying another function over and making that code executable and then overwriting the first five bytes of the target function with a jump to your injected code, or you can do a proper detour type hook. In either case it should work. Or, you can find the offset into the function and patch it yourself by writing the proper op codes in place of the real code, such as a return of true.
Some kind of injection or patching is required to complete this, you have the basic idea, but there is a little more to it than you think at the moment. I have working code for windows to copy a function into another process, but I believe it is a good learning experience.