---
template: text.html
title: Return Oriented Programming on ARM (32-bit)
subtitle: Making stack-based exploitation great again!
date: 2019-06-06
url: rop-on-arm
---

Before we start _anything_, you’re expected to know the basics of ARM
assembly to follow along. I highly recommend
[Azeria’s](https://twitter.com/fox0x01) series on [ARM Assembly
Basics](https://azeria-labs.com/writing-arm-assembly-part-1/). Once you’re
comfortable with it, proceed with the next bit -- environment setup.

## Setup

Since we’re working with the ARM architecture, there are two options to go
forth with: 

1. Emulate -- head over to [qemu.org/download](https://www.qemu.org/download/) and install QEMU. 
And then download and extract the ARMv6 Debian Stretch image from one of the links [here](https://blahcat.github.io/qemu/).
The scripts found inside should be self-explanatory.
2. Use actual ARM hardware, like an RPi.

For debugging and disassembling, we’ll be using plain old `gdb`, but you
may use `radare2`, IDA or anything else, really. All of which can be
trivially installed.

And for the sake of simplicity, disable ASLR:

```shell
$ echo 0 > /proc/sys/kernel/randomize_va_space
```

Finally, the binary we’ll be using in this exercise is [Billy Ellis’](https://twitter.com/bellis1000)
[roplevel2](/static/files/roplevel2.c). 

Compile it:
```sh
$ gcc roplevel2.c -o rop2
```

With that out of the way, here’s a quick run down of what ROP actually is.

## A primer on ROP

ROP or Return Oriented Programming is a modern exploitation technique that’s
used to bypass protections like the **NX bit** (no-execute bit) and **code sigining**.
In essence, no code in the binary is actually modified and the entire exploit
is crafted out of pre-existing artifacts within the binary, known as **gadgets**.

A gadget is essentially a small sequence of code (instructions), ending with
a `ret`, or a return instruction. In our case, since we’re dealing with ARM
code, there is no `ret` instruction but rather a `pop {pc}` or a `bx lr`.
These gadgets are _chained_ together by jumping (returning) from one onto the other
to form what’s called as a **ropchain**. At the end of a ropchain,
there’s generally a call to `system()`, to acheive code execution.

In practice, the process of executing a ropchain is something like this:

- confirm the existence of a stack-based buffer overflow
- identify the offset at which the instruction pointer gets overwritten
- locate the addresses of the gadgets you wish to use
- craft your input keeping in mind the stack’s layout, and chain the addresses
of your gadgets

[LiveOverflow](https://twitter.com/LiveOverflow) has a [beautiful video](https://www.youtube.com/watch?v=zaQVNM3or7k&list=PLhixgUqwRTjxglIswKp9mpkfPNfHkzyeN&index=46&t=0s) where he explains ROP using “weird machines”. 
Check it out, it might be just what you needed for that “aha!” moment :)

Still don’t get it? Don’t fret, we’ll look at _actual_ exploit code in a bit and hopefully
that should put things into perspective.

## Exploring our binary

Start by running it, and entering any arbitrary string. On entering a fairly
large string, say, “A” × 20, we
see a segmentation fault occur.

![string and segfault](/static/img/string_segfault.png)

Now, open it up in `gdb` and look at the functions inside it.

![gdb functions](/static/img/gdb_functions.png)

There are three functions that are of importance here, `main`, `winner` and 
`gadget`. Disassembling the `main` function:

![gdb main disassembly](/static/img/gdb_main_disas.png)

We see a buffer of 16 bytes being created (`sub	sp, sp, #16`), and some calls
to `puts()`/`printf()` and `scanf()`. Looks like `winner` and `gadget` are 
never actually called.

Disassembling the `gadget` function:

![gdb gadget disassembly](/static/img/gdb_gadget_disas.png)

This is fairly simple, the stack is being initialized by `push`ing `{r11}`,
which is also the frame pointer (`fp`). What’s interesting is the `pop {r0, pc}`
instruction in the middle. This is a **gadget**.

We can use this to control what goes into `r0` and `pc`. Unlike in x86 where
arguments to functions are passed on the stack, in ARM the registers `r0` to `r3`
are used for this. So this gadget effectively allows us to pass arguments to
functions using `r0`, and subsequently jumping to them by passing its address
in `pc`. Neat.

Moving on to the disassembly of the `winner` function:

![gdb winner disassembly](/static/img/gdb_disas_winner.png)

Here, we see a calls to `puts()`, `system()` and finally, `exit()`.
So our end goal here is to, quite obviously, execute code via the `system()`
function.

Now that we have an overview of what’s in the binary, let’s formulate a method
of exploitation by messing around with inputs.

## Messing around with inputs :^)

Back to `gdb`, hit `r` to run and pass in a patterned input, like in the
screenshot.

![gdb info reg post segfault](/static/img/gdb_info_reg_segfault.png)

We hit a segfault because of invalid memory at address `0x46464646`. Notice
the `pc` has been overwritten with our input.
So we smashed the stack alright, but more importantly, it’s at the letter ‘F’.

Since we know the offset at which the `pc` gets overwritten, we can now
control program execution flow. Let’s try jumping to the `winner` function.

Disassemble `winner` again using `disas winner` and note down the offset
of the second instruction -- `add r11, sp, #4`. 
For this, we’ll use Python to print our input string replacing `FFFF` with
the address of `winner`. Note the endianness.

```shell
$ python -c 'print("AAAABBBBCCCCDDDDEEEE\x28\x05\x01\x00")' | ./rop2
```

![jump to winner](/static/img/python_winner_jump.png)

The reason we don’t jump to the first instruction is because we want to control the stack
ourselves. If we allow `push {rll, lr}` (first instruction) to occur, the program will `pop`
those out after `winner` is done executing and we will no longer control 
where it jumps to.

So that didn’t do much, just prints out a string “Nothing much here...”. 
But it _does_ however, contain `system()`. Which somehow needs to be populated with an argument
to do what we want (run a command, execute a shell, etc.).

To do that, we’ll follow a multi-step process: 

1. Jump to the address of `gadget`, again the 2nd instruction. This will `pop` `r0` and `pc`.
2. Push our command to be executed, say “`/bin/sh`” onto the stack. This will go into
`r0`.
3. Then, push the address of `system()`. And this will go into `pc`.

The pseudo-code is something like this:
```
string = AAAABBBBCCCCDDDDEEEE
gadget = # addr of gadget
binsh  = # addr of /bin/sh
system = # addr of system()

print(string + gadget + binsh + system)
```
Clean and mean.


## The exploit

To write the exploit, we’ll use Python and the absolute godsend of a library -- `struct`.
It allows us to pack the bytes of addresses to the endianness of our choice.
It probably does a lot more, but who cares.

Let’s start by fetching the address of `/bin/sh`. In `gdb`, set a breakpoint
at `main`, hit `r` to run, and search the entire address space for the string “`/bin/sh`”:


```
(gdb) find &system, +9999999, "/bin/sh"
```
![gdb finding /bin/sh](/static/img/gdb_find_binsh.png)

One hit at `0xb6f85588`. The addresses of `gadget` and `system()` can be
found from the disassmblies from earlier. Here’s the final exploit code:
```python
import struct

binsh = struct.pack("I", 0xb6f85588)
string = "AAAABBBBCCCCDDDDEEEE"
gadget = struct.pack("I", 0x00010550)
system = struct.pack("I", 0x00010538)

print(string + gadget + binsh + system)

```
Honestly, not too far off from our pseudo-code :)

Let’s see it in action:

![the shell!](/static/img/the_shell.png)

Notice that it doesn’t work the first time, and this is because `/bin/sh` terminates
when the pipe closes, since there’s no input coming in from STDIN.
To get around this, we use `cat(1)` which allows us to relay input through it
to the shell. Nifty trick.

## Conclusion

This was a fairly basic challenge, with everything laid out conveniently. 
Actual ropchaining is a little more involved, with a lot more gadgets to be chained
to acheive code execution.

Hopefully, I’ll get around to writing about heap exploitation on ARM too. That’s all for now.