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38 <code>2019-02-08</code>
39 <h1>Python for Reverse Engineering #1: ELF Binaries</h1>
40 <h2>Building your own disassembly tooling for — that’s right — fun and profit</h2>
41 <p>While solving complex reversing challenges, we often use established tools like radare2 or IDA for disassembling and debugging. But there are times when you need to dig in a little deeper and understand how things work under the hood.</p>
42
43<p>Rolling your own disassembly scripts can be immensely helpful when it comes to automating certain processes, and eventually build your own homebrew reversing toolchain of sorts. At least, that’s what I’m attempting anyway.</p>
44
45<h3 id="setup">Setup</h3>
46
47<p>As the title suggests, you’re going to need a Python 3 interpreter before
48anything else. Once you’ve confirmed beyond reasonable doubt that you do,
49in fact, have a Python 3 interpreter installed on your system, run</p>
50
51<div class="codehilite"><pre><span></span><code><span class="gp">$</span> pip install capstone pyelftools
52</code></pre></div>
53
54<p>where <code>capstone</code> is the disassembly engine we’ll be scripting with and <code>pyelftools</code> to help parse ELF files.</p>
55
56<p>With that out of the way, let’s start with an example of a basic reversing
57challenge.</p>
58
59<div class="codehilite"><pre><span></span><code><span class="cm">/* chall.c */</span>
60
61<span class="cp">#include</span> <span class="cpf"><stdio.h></span><span class="cp"></span>
62<span class="cp">#include</span> <span class="cpf"><stdlib.h></span><span class="cp"></span>
63<span class="cp">#include</span> <span class="cpf"><string.h></span><span class="cp"></span>
64
65<span class="kt">int</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
66 <span class="kt">char</span> <span class="o">*</span><span class="n">pw</span> <span class="o">=</span> <span class="n">malloc</span><span class="p">(</span><span class="mi">9</span><span class="p">);</span>
67 <span class="n">pw</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="sc">'a'</span><span class="p">;</span>
68 <span class="k">for</span><span class="p">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span> <span class="n">i</span> <span class="o"><=</span> <span class="mi">8</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">){</span>
69 <span class="n">pw</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="n">pw</span><span class="p">[</span><span class="n">i</span> <span class="o">-</span> <span class="mi">1</span><span class="p">]</span> <span class="o">+</span> <span class="mi">1</span><span class="p">;</span>
70 <span class="p">}</span>
71 <span class="n">pw</span><span class="p">[</span><span class="mi">9</span><span class="p">]</span> <span class="o">=</span> <span class="sc">'\0'</span><span class="p">;</span>
72 <span class="kt">char</span> <span class="o">*</span><span class="n">in</span> <span class="o">=</span> <span class="n">malloc</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span>
73 <span class="n">printf</span><span class="p">(</span><span class="s">"password: "</span><span class="p">);</span>
74 <span class="n">fgets</span><span class="p">(</span><span class="n">in</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="n">stdin</span><span class="p">);</span> <span class="c1">// 'abcdefghi'</span>
75 <span class="k">if</span><span class="p">(</span><span class="n">strcmp</span><span class="p">(</span><span class="n">in</span><span class="p">,</span> <span class="n">pw</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span> <span class="p">{</span>
76 <span class="n">printf</span><span class="p">(</span><span class="s">"haha yes!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
77 <span class="p">}</span>
78 <span class="k">else</span> <span class="p">{</span>
79 <span class="n">printf</span><span class="p">(</span><span class="s">"nah dude</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
80 <span class="p">}</span>
81<span class="p">}</span>
82</code></pre></div>
83
84<p>Compile it with GCC/Clang:</p>
85
86<div class="codehilite"><pre><span></span><code><span class="gp">$</span> gcc chall.c -o chall.elf
87</code></pre></div>
88
89<h3 id="scripting">Scripting</h3>
90
91<p>For starters, let’s look at the different sections present in the binary.</p>
92
93<div class="codehilite"><pre><span></span><code><span class="c1"># sections.py</span>
94
95<span class="kn">from</span> <span class="nn">elftools.elf.elffile</span> <span class="kn">import</span> <span class="n">ELFFile</span>
96
97<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'./chall.elf'</span><span class="p">,</span> <span class="s1">'rb'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
98 <span class="n">e</span> <span class="o">=</span> <span class="n">ELFFile</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
99 <span class="k">for</span> <span class="n">section</span> <span class="ow">in</span> <span class="n">e</span><span class="o">.</span><span class="n">iter_sections</span><span class="p">():</span>
100 <span class="k">print</span><span class="p">(</span><span class="nb">hex</span><span class="p">(</span><span class="n">section</span><span class="p">[</span><span class="s1">'sh_addr'</span><span class="p">]),</span> <span class="n">section</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
101</code></pre></div>
102
103<p>This script iterates through all the sections and also shows us where it’s loaded. This will be pretty useful later. Running it gives us</p>
104
105<div class="codehilite"><pre><span></span><code><span class="go">› python sections.py</span>
106<span class="go">0x238 .interp</span>
107<span class="go">0x254 .note.ABI-tag</span>
108<span class="go">0x274 .note.gnu.build-id</span>
109<span class="go">0x298 .gnu.hash</span>
110<span class="go">0x2c0 .dynsym</span>
111<span class="go">0x3e0 .dynstr</span>
112<span class="go">0x484 .gnu.version</span>
113<span class="go">0x4a0 .gnu.version_r</span>
114<span class="go">0x4c0 .rela.dyn</span>
115<span class="go">0x598 .rela.plt</span>
116<span class="go">0x610 .init</span>
117<span class="go">0x630 .plt</span>
118<span class="go">0x690 .plt.got</span>
119<span class="go">0x6a0 .text</span>
120<span class="go">0x8f4 .fini</span>
121<span class="go">0x900 .rodata</span>
122<span class="go">0x924 .eh_frame_hdr</span>
123<span class="go">0x960 .eh_frame</span>
124<span class="go">0x200d98 .init_array</span>
125<span class="go">0x200da0 .fini_array</span>
126<span class="go">0x200da8 .dynamic</span>
127<span class="go">0x200f98 .got</span>
128<span class="go">0x201000 .data</span>
129<span class="go">0x201010 .bss</span>
130<span class="go">0x0 .comment</span>
131<span class="go">0x0 .symtab</span>
132<span class="go">0x0 .strtab</span>
133<span class="go">0x0 .shstrtab</span>
134</code></pre></div>
135
136<p>Most of these aren’t relevant to us, but a few sections here are to be noted. The <code>.text</code> section contains the instructions (opcodes) that we’re after. The <code>.data</code> section should have strings and constants initialized at compile time. Finally, the <code>.plt</code> which is the Procedure Linkage Table and the <code>.got</code>, the Global Offset Table. If you’re unsure about what these mean, read up on the ELF format and its internals.</p>
137
138<p>Since we know that the <code>.text</code> section has the opcodes, let’s disassemble the binary starting at that address.</p>
139
140<div class="codehilite"><pre><span></span><code><span class="c1"># disas1.py</span>
141
142<span class="kn">from</span> <span class="nn">elftools.elf.elffile</span> <span class="kn">import</span> <span class="n">ELFFile</span>
143<span class="kn">from</span> <span class="nn">capstone</span> <span class="kn">import</span> <span class="o">*</span>
144
145<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'./bin.elf'</span><span class="p">,</span> <span class="s1">'rb'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
146 <span class="n">elf</span> <span class="o">=</span> <span class="n">ELFFile</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
147 <span class="n">code</span> <span class="o">=</span> <span class="n">elf</span><span class="o">.</span><span class="n">get_section_by_name</span><span class="p">(</span><span class="s1">'.text'</span><span class="p">)</span>
148 <span class="n">ops</span> <span class="o">=</span> <span class="n">code</span><span class="o">.</span><span class="n">data</span><span class="p">()</span>
149 <span class="n">addr</span> <span class="o">=</span> <span class="n">code</span><span class="p">[</span><span class="s1">'sh_addr'</span><span class="p">]</span>
150 <span class="n">md</span> <span class="o">=</span> <span class="n">Cs</span><span class="p">(</span><span class="n">CS_ARCH_X86</span><span class="p">,</span> <span class="n">CS_MODE_64</span><span class="p">)</span>
151 <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">md</span><span class="o">.</span><span class="n">disasm</span><span class="p">(</span><span class="n">ops</span><span class="p">,</span> <span class="n">addr</span><span class="p">):</span>
152 <span class="k">print</span><span class="p">(</span><span class="n">f</span><span class="s1">'0x{i.address:x}:</span><span class="se">\t</span><span class="s1">{i.mnemonic}</span><span class="se">\t</span><span class="s1">{i.op_str}'</span><span class="p">)</span>
153</code></pre></div>
154
155<p>The code is fairly straightforward (I think). We should be seeing this, on running</p>
156
157<div class="codehilite"><pre><span></span><code><span class="go">› python disas1.py | less </span>
158<span class="go">0x6a0: xor ebp, ebp</span>
159<span class="go">0x6a2: mov r9, rdx</span>
160<span class="go">0x6a5: pop rsi</span>
161<span class="go">0x6a6: mov rdx, rsp</span>
162<span class="go">0x6a9: and rsp, 0xfffffffffffffff0</span>
163<span class="go">0x6ad: push rax</span>
164<span class="go">0x6ae: push rsp</span>
165<span class="go">0x6af: lea r8, [rip + 0x23a]</span>
166<span class="go">0x6b6: lea rcx, [rip + 0x1c3]</span>
167<span class="go">0x6bd: lea rdi, [rip + 0xe6]</span>
168<span class="go">**0x6c4: call qword ptr [rip + 0x200916]**</span>
169<span class="go">0x6ca: hlt</span>
170<span class="go">... snip ...</span>
171</code></pre></div>
172
173<p>The line in bold is fairly interesting to us. The address at <code>[rip + 0x200916]</code> is equivalent to <code>[0x6ca + 0x200916]</code>, which in turn evaluates to <code>0x200fe0</code>. The first <code>call</code> being made to a function at <code>0x200fe0</code>? What could this function be?</p>
174
175<p>For this, we will have to look at <strong>relocations</strong>. Quoting <a href="http://refspecs.linuxbase.org/elf/gabi4+/ch4.reloc.html">linuxbase.org</a></p>
176
177<blockquote>
178 <p>Relocation is the process of connecting symbolic references with symbolic definitions. For example, when a program calls a function, the associated call instruction must transfer control to the proper destination address at execution. Relocatable files must have “relocation entries’’ which are necessary because they contain information that describes how to modify their section contents, thus allowing executable and shared object files to hold the right information for a process’s program image.</p>
179</blockquote>
180
181<p>To try and find these relocation entries, we write a third script.</p>
182
183<div class="codehilite"><pre><span></span><code><span class="c1"># relocations.py</span>
184
185<span class="kn">import</span> <span class="nn">sys</span>
186<span class="kn">from</span> <span class="nn">elftools.elf.elffile</span> <span class="kn">import</span> <span class="n">ELFFile</span>
187<span class="kn">from</span> <span class="nn">elftools.elf.relocation</span> <span class="kn">import</span> <span class="n">RelocationSection</span>
188
189<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'./chall.elf'</span><span class="p">,</span> <span class="s1">'rb'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
190 <span class="n">e</span> <span class="o">=</span> <span class="n">ELFFile</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
191 <span class="k">for</span> <span class="n">section</span> <span class="ow">in</span> <span class="n">e</span><span class="o">.</span><span class="n">iter_sections</span><span class="p">():</span>
192 <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">section</span><span class="p">,</span> <span class="n">RelocationSection</span><span class="p">):</span>
193 <span class="k">print</span><span class="p">(</span><span class="n">f</span><span class="s1">'{section.name}:'</span><span class="p">)</span>
194 <span class="n">symbol_table</span> <span class="o">=</span> <span class="n">e</span><span class="o">.</span><span class="n">get_section</span><span class="p">(</span><span class="n">section</span><span class="p">[</span><span class="s1">'sh_link'</span><span class="p">])</span>
195 <span class="k">for</span> <span class="n">relocation</span> <span class="ow">in</span> <span class="n">section</span><span class="o">.</span><span class="n">iter_relocations</span><span class="p">():</span>
196 <span class="n">symbol</span> <span class="o">=</span> <span class="n">symbol_table</span><span class="o">.</span><span class="n">get_symbol</span><span class="p">(</span><span class="n">relocation</span><span class="p">[</span><span class="s1">'r_info_sym'</span><span class="p">])</span>
197 <span class="n">addr</span> <span class="o">=</span> <span class="nb">hex</span><span class="p">(</span><span class="n">relocation</span><span class="p">[</span><span class="s1">'r_offset'</span><span class="p">])</span>
198 <span class="k">print</span><span class="p">(</span><span class="n">f</span><span class="s1">'{symbol.name} {addr}'</span><span class="p">)</span>
199</code></pre></div>
200
201<p>Let’s run through this code real quick. We first loop through the sections, and check if it’s of the type <code>RelocationSection</code>. We then iterate through the relocations from the symbol table for each section. Finally, running this gives us</p>
202
203<div class="codehilite"><pre><span></span><code><span class="go">› python relocations.py</span>
204<span class="go">.rela.dyn:</span>
205<span class="go"> 0x200d98</span>
206<span class="go"> 0x200da0</span>
207<span class="go"> 0x201008</span>
208<span class="go">_ITM_deregisterTMCloneTable 0x200fd8</span>
209<span class="go">**__libc_start_main 0x200fe0**</span>
210<span class="go">__gmon_start__ 0x200fe8</span>
211<span class="go">_ITM_registerTMCloneTable 0x200ff0</span>
212<span class="go">__cxa_finalize 0x200ff8</span>
213<span class="go">stdin 0x201010</span>
214<span class="go">.rela.plt:</span>
215<span class="go">puts 0x200fb0</span>
216<span class="go">printf 0x200fb8</span>
217<span class="go">fgets 0x200fc0</span>
218<span class="go">strcmp 0x200fc8</span>
219<span class="go">malloc 0x200fd0</span>
220</code></pre></div>
221
222<p>Remember the function call at <code>0x200fe0</code> from earlier? Yep, so that was a call to the well known <code>__libc_start_main</code>. Again, according to <a href="http://refspecs.linuxbase.org/LSB_3.1.0/LSB-generic/LSB-generic/baselib—libc-start-main-.html">linuxbase.org</a></p>
223
224<blockquote>
225 <p>The <code>__libc_start_main()</code> function shall perform any necessary initialization of the execution environment, call the <em>main</em> function with appropriate arguments, and handle the return from <code>main()</code>. If the <code>main()</code> function returns, the return value shall be passed to the <code>exit()</code> function.</p>
226</blockquote>
227
228<p>And its definition is like so</p>
229
230<div class="codehilite"><pre><span></span><code><span class="kt">int</span> <span class="nf">__libc_start_main</span><span class="p">(</span><span class="kt">int</span> <span class="o">*</span><span class="p">(</span><span class="n">main</span><span class="p">)</span> <span class="p">(</span><span class="kt">int</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span> <span class="o">*</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span> <span class="o">*</span><span class="p">),</span>
231<span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span> <span class="o">*</span> <span class="n">ubp_av</span><span class="p">,</span>
232<span class="kt">void</span> <span class="p">(</span><span class="o">*</span><span class="n">init</span><span class="p">)</span> <span class="p">(</span><span class="kt">void</span><span class="p">),</span>
233<span class="kt">void</span> <span class="p">(</span><span class="o">*</span><span class="n">fini</span><span class="p">)</span> <span class="p">(</span><span class="kt">void</span><span class="p">),</span>
234<span class="kt">void</span> <span class="p">(</span><span class="o">*</span><span class="n">rtld_fini</span><span class="p">)</span> <span class="p">(</span><span class="kt">void</span><span class="p">),</span>
235<span class="kt">void</span> <span class="p">(</span><span class="o">*</span> <span class="n">stack_end</span><span class="p">));</span>
236</code></pre></div>
237
238<p>Looking back at our disassembly</p>
239
240<pre><code>0x6a0: xor ebp, ebp
2410x6a2: mov r9, rdx
2420x6a5: pop rsi
2430x6a6: mov rdx, rsp
2440x6a9: and rsp, 0xfffffffffffffff0
2450x6ad: push rax
2460x6ae: push rsp
2470x6af: lea r8, [rip + 0x23a]
2480x6b6: lea rcx, [rip + 0x1c3]
249**0x6bd: lea rdi, [rip + 0xe6]**
2500x6c4: call qword ptr [rip + 0x200916]
2510x6ca: hlt
252... snip ...
253</code></pre>
254
255<p>but this time, at the <code>lea</code> or Load Effective Address instruction, which loads some address <code>[rip + 0xe6]</code> into the <code>rdi</code> register. <code>[rip + 0xe6]</code> evaluates to <code>0x7aa</code> which happens to be the address of our <code>main()</code> function! How do I know that? Because <code>__libc_start_main()</code>, after doing whatever it does, eventually jumps to the function at <code>rdi</code>, which is generally the <code>main()</code> function. It looks something like this</p>
256
257<p><img src="https://cdn-images-1.medium.com/max/800/0*oQA2MwHjhzosF8ZH.png" alt="" /></p>
258
259<p>To see the disassembly of <code>main</code>, seek to <code>0x7aa</code> in the output of the script we’d written earlier (<code>disas1.py</code>).</p>
260
261<p>From what we discovered earlier, each <code>call</code> instruction points to some function which we can see from the relocation entries. So following each <code>call</code> into their relocations gives us this</p>
262
263<pre><code>printf 0x650
264fgets 0x660
265strcmp 0x670
266malloc 0x680
267</code></pre>
268
269<p>Putting all this together, things start falling into place. Let me highlight the key sections of the disassembly here. It’s pretty self-explanatory.</p>
270
271<pre><code>0x7b2: mov edi, 0xa ; 10
2720x7b7: call 0x680 ; malloc
273</code></pre>
274
275<p>The loop to populate the <code>*pw</code> string</p>
276
277<pre><code>0x7d0: mov eax, dword ptr [rbp - 0x14]
2780x7d3: cdqe
2790x7d5: lea rdx, [rax - 1]
2800x7d9: mov rax, qword ptr [rbp - 0x10]
2810x7dd: add rax, rdx
2820x7e0: movzx eax, byte ptr [rax]
2830x7e3: lea ecx, [rax + 1]
2840x7e6: mov eax, dword ptr [rbp - 0x14]
2850x7e9: movsxd rdx, eax
2860x7ec: mov rax, qword ptr [rbp - 0x10]
2870x7f0: add rax, rdx
2880x7f3: mov edx, ecx
2890x7f5: mov byte ptr [rax], dl
2900x7f7: add dword ptr [rbp - 0x14], 1
2910x7fb: cmp dword ptr [rbp - 0x14], 8
2920x7ff: jle 0x7d0
293</code></pre>
294
295<p>And this looks like our <code>strcmp()</code></p>
296
297<pre><code>0x843: mov rdx, qword ptr [rbp - 0x10] ; *in
2980x847: mov rax, qword ptr [rbp - 8] ; *pw
2990x84b: mov rsi, rdx
3000x84e: mov rdi, rax
3010x851: call 0x670 ; strcmp
3020x856: test eax, eax ; is = 0?
3030x858: jne 0x868 ; no? jump to 0x868
3040x85a: lea rdi, [rip + 0xae] ; "haha yes!"
3050x861: call 0x640 ; puts
3060x866: jmp 0x874
3070x868: lea rdi, [rip + 0xaa] ; "nah dude"
3080x86f: call 0x640 ; puts
309</code></pre>
310
311<p>I’m not sure why it uses <code>puts</code> here? I might be missing something; perhaps <code>printf</code> calls <code>puts</code>. I could be wrong. I also confirmed with radare2 that those locations are actually the strings “haha yes!” and “nah dude”.</p>
312
313<p><strong>Update</strong>: It’s because of compiler optimization. A <code>printf()</code> (in this case) is seen as a bit overkill, and hence gets simplified to a <code>puts()</code>.</p>
314
315<h3 id="conclusion">Conclusion</h3>
316
317<p>Wew, that took quite some time. But we’re done. If you’re a beginner, you might find this extremely confusing, or probably didn’t even understand what was going on. And that’s okay. Building an intuition for reading and grokking disassembly comes with practice. I’m no good at it either.</p>
318
319<p>All the code used in this post is here: <a href="https://github.com/icyphox/asdf/tree/master/reversing-elf">https://github.com/icyphox/asdf/tree/master/reversing-elf</a></p>
320
321<p>Ciao for now, and I’ll see ya in #2 of this series — PE binaries. Whenever that is.</p>
322
323 </div>
324 <hr />
325 <p class="muted">Questions or comments? Open an issue at <a href="https://github.com/icyphox/site">this repo</a>, or send a plain-text email to <a href="mailto:x@icyphox.sh">x@icyphox.sh</a>.</p>
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