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

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.