Modifying memory of a running process in Linux

Everything said in this post is based on my half-understanding of how memory works, especially in Linux. There are certainly going to be parts in this post that are misunderstood by me. Don’t rely on this post for your homework and take everything in this post with a grain of salt. However, I hope to continue to refine the content in this post as I understand more about how a computer works. Consider this as a disclaimer. That said, let’s begin!

I’ve been trying out 6502 and x86 Assembly to learn some basic reverse engineering and also to learn think in these low-level programming languages, hopefully to understand more of how everything works under-the-hood. So I thought what better than to pun some other process’s memory. I figured I’ll try to modify some stuff assigned to memory by one process from some second process. I know Windows has tools like Cheat Engine while Linux has scanmem. I still wanted to create my own tiny little program which would be enough to demonstrate how these big tools worked at their heart.

Most modern Operating Systems have ASLR (Address Space Layout Randomization) turned on by default which can help prevent many buffer overflow and code injection attacks. These attacks if successful could possibly allow the user to gain access to sensitive information not intended for access to the end user. Although, I’ve tried Cheat Engine and to my knowledge popular debuggers still allow for a way to access and modify the disassembled code of a binary when loaded into memory. They also have a way to watch for code whenever a specified memory location is accessed by the concerned process, so I’m not completely sure whether these debuggers somehow workaround ASLR or if this completely unrelated. Irrespective of this, to make things not more complicated, we’ll already know the address of the memory location we wish to modify, as you’ll see later.

From now on, let’s refer to a primary innocent process as Process A and the secondary process which attempts to hack into the memory of Process A as Process B. In Linux, we have a system call ptrace which acts as a middle man between Process B and the memory of Process A. One cannot directly read or write to the memory of another independent process without ptrace for some reason I don’t understand. My current guess is that it’s there so that a process doesn’t unintentionally messes up with the memory of some other process. So, if you see someone with ptrace it’s probably because they ought to either save the world from extinction or be the very cause itself. Similar in Windows you have ReadProcessMemory and WriteProcessMemory functions available in Windows API acting as the middle man.

Let’s say we have this simple C code:

// process_a.c
#include <stdio.h>
#include <unistd.h>

int main() {
    int n = 10;

    printf("%d\n", n);
    printf("%p\n", &n);

    sleep(25);
    printf("%d\n", n);
    printf("%p\n", &n);
    return 0;
}

All it does is:

  • assign a variable a value
  • display the variable’s value and its location in memory
  • sleep the main thread for a while
  • display the variable’s value and its location in memory again

Let’s run it:

$ gcc process_a.c -o process_a && ./process_a
10
0x7fffcfd52924
10
0x7fffcfd52924

This is our Process A. Our goal is to have a Process B change the value of the assigned variable to something like 20 while the main thread is sleeping. If we succeed, it should show us the value 20 after waking up from sleep while the memory location should remain the same.

I’ve only tried this on Linux so that’s what I’ll be talking about. As I mentioned earlier, you need to fire the ptrace system call and you can do that natively via C. So, what we’ll do is write another program which will act as Process B. To overwrite the memory of another process in Linux, you need to know three things - the PID of the Process A, the memory location to be modified, and obviously the data you need to overwrite with. It’s easy to know the PID of a process in Linux, and we can know the memory location of the variable since our Process A prints the variable’s memory location itself. The data can be anything, I’ll use 20 for the purpose of this post.

Here’s how we’ll use ptrace:

// process_b.c
#include <sys/ptrace.h>
#include <stdio.h>

int main() {
    int pid = 5831;
    int *address = (int *)0x7fffcfd52924;
    int data = 20;

    ptrace(PTRACE_ATTACH, pid, NULL, NULL);
    perror("attach");
    ptrace(PTRACE_POKEDATA, pid, address, data);
    perror("pokedata");
    ptrace(PTRACE_DETACH, pid, NULL, NULL);
    perror("detach");

    return 0;
}

The code is pretty self-explanatory - you attach to the process, write data to a memory address and detach.

Another thing to take care is that the PID and the memory of any process A are allotted at execution. As far as I know, one cannot predict the PID and where in memory a process will go live, before execution of the concerned program. There are probably ways to automate this after the concerned process is executed but let’s avoid complicating the code. So here’s our plan, we’ll execute the Process A and then we’re going to grab its PID manually and update our Process B’s source accordingly. Finally, we’re going to compile and execute our Process B. If we are fast enough and make it through all of this while Process A is in the middle of its sleep, we would have successfully overwritten Process A’s memory and it should be visible when it wakes up from sleep to display the value!

Let’s do it step wise:

In a terminal, run:

$ ./process_a &; fg

You should get something like this:

[1] 32555
[1]  + 32555 running    ./process_a
10
0x7fffcfd52924

The main thread will now go in sleep mode for another 25 seconds. By executing the program in background mode, we also get its PID, which in my case is 32555. We could then just call it in the foreground with fg. Also, the memory address is 0x7fffcfd52924. Now, we have everything we need to call ptrace.

In a new terminal, edit process_b.c’s variables with what you got from above. For me, I’ll replace the variables with:

    int pid = 32555;
    int *address = (int *)0x7fffcfd52924;
    int data = 20;

Now compile and execute the code with:

$ gcc process_b.c -o process_b && sudo ./process_b

Only root can modify another process’s memory, so you’ll need sudo to run the executable generated.

If everything goes fine and you were able to make it this far while Process A was asleep, you should get output similar to this:

attach: Success
pokedata: Success
detach: Success

That’s it! Now when Process A completes its sleep: it should look something like this:

[1] 32555
[1]  + 32555 running    ./process_a
10
0x7fffcfd52924
20
0x7fffcfd52924
*** stack smashing detected ***: <unknown> terminated
[1]  + 32555 abort      ./process_a

Yay, The value just got to 20. Awesome!!

Now if you see the last few lines, you’ll noticed this weird error I dunno much about:

*** stack smashing detected ***: <unknown> terminated
[1]  + 32555 abort      ./process_a

Turns out I got rid of it by simply assigning a pointer to our variable in Process A. What I mean is replace:

    int n = 10;

with

    int n = 10;
    int *ptr = &n;

For some reason, this works and I no longer smash the stack while hacking into Process A’s memory.

Well, this is the heart of how a memory editor works underneath. One could now imagine how this can scale into bigger beasts like Cheat Engine and scanmem as I talked in the first paragraph.

Written on June 26, 2019