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 38      <p> 8 Feb, 2019 </p>
 39      <h1 id="python-for-reverse-engineering-1-elf-binaries">Python for Reverse Engineering 1: ELF Binaries</h1>
 40
 41<h2 id="building-your-own-disassembly-tooling-for-thats-right-fun-and-profit">Building your own disassembly tooling for — that’s right — fun and profit</h2>
 42
 43<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>
 44
 45<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>
 46
 47<h3 id="setup">Setup</h3>
 48
 49<p>As the title suggests, you’re going to need a Python 3 interpreter before
 50anything else. Once you’ve confirmed beyond reasonable doubt that you do,
 51in fact, have a Python 3 interpreter installed on your system, run</p>
 52
 53<div class="codehilite"><pre><span></span><code><span class="gp">$</span> pip install capstone pyelftools
 54</code></pre></div>
 55
 56<p>where <code>capstone</code> is the disassembly engine we’ll be scripting with and <code>pyelftools</code> to help parse ELF files.</p>
 57
 58<p>With that out of the way, let’s start with an example of a basic reversing
 59challenge.</p>
 60
 61<div class="codehilite"><pre><span></span><code><span class="cm">/* chall.c */</span>
 62
 63<span class="cp">#include</span> <span class="cpf">&lt;stdio.h&gt;</span><span class="cp"></span>
 64<span class="cp">#include</span> <span class="cpf">&lt;stdlib.h&gt;</span><span class="cp"></span>
 65<span class="cp">#include</span> <span class="cpf">&lt;string.h&gt;</span><span class="cp"></span>
 66
 67<span class="kt">int</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
 68   <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>
 69   <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">&#39;a&#39;</span><span class="p">;</span>
 70   <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">&lt;=</span> <span class="mi">8</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">){</span>
 71       <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>
 72   <span class="p">}</span>
 73   <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">&#39;\0&#39;</span><span class="p">;</span>
 74   <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>
 75   <span class="n">printf</span><span class="p">(</span><span class="s">&quot;password: &quot;</span><span class="p">);</span>
 76   <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">// &#39;abcdefghi&#39;</span>
 77   <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>
 78       <span class="n">printf</span><span class="p">(</span><span class="s">&quot;haha yes!</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">);</span>
 79   <span class="p">}</span>
 80   <span class="k">else</span> <span class="p">{</span>
 81       <span class="n">printf</span><span class="p">(</span><span class="s">&quot;nah dude</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">);</span>
 82   <span class="p">}</span>
 83<span class="p">}</span>
 84</code></pre></div>
 85
 86<p>Compile it with GCC/Clang:</p>
 87
 88<div class="codehilite"><pre><span></span><code><span class="gp">$</span> gcc chall.c -o chall.elf
 89</code></pre></div>
 90
 91<h3 id="scripting">Scripting</h3>
 92
 93<p>For starters, let’s look at the different sections present in the binary.</p>
 94
 95<div class="codehilite"><pre><span></span><code><span class="c1"># sections.py</span>
 96
 97<span class="kn">from</span> <span class="nn">elftools.elf.elffile</span> <span class="kn">import</span> <span class="n">ELFFile</span>
 98
 99<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;./chall.elf&#39;</span><span class="p">,</span> <span class="s1">&#39;rb&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
100    <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>
101    <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>
102        <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">&#39;sh_addr&#39;</span><span class="p">]),</span> <span class="n">section</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
103</code></pre></div>
104
105<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>
106
107<div class="codehilite"><pre><span></span><code><span class="go">› python sections.py</span>
108<span class="go">0x238 .interp</span>
109<span class="go">0x254 .note.ABI-tag</span>
110<span class="go">0x274 .note.gnu.build-id</span>
111<span class="go">0x298 .gnu.hash</span>
112<span class="go">0x2c0 .dynsym</span>
113<span class="go">0x3e0 .dynstr</span>
114<span class="go">0x484 .gnu.version</span>
115<span class="go">0x4a0 .gnu.version_r</span>
116<span class="go">0x4c0 .rela.dyn</span>
117<span class="go">0x598 .rela.plt</span>
118<span class="go">0x610 .init</span>
119<span class="go">0x630 .plt</span>
120<span class="go">0x690 .plt.got</span>
121<span class="go">0x6a0 .text</span>
122<span class="go">0x8f4 .fini</span>
123<span class="go">0x900 .rodata</span>
124<span class="go">0x924 .eh_frame_hdr</span>
125<span class="go">0x960 .eh_frame</span>
126<span class="go">0x200d98 .init_array</span>
127<span class="go">0x200da0 .fini_array</span>
128<span class="go">0x200da8 .dynamic</span>
129<span class="go">0x200f98 .got</span>
130<span class="go">0x201000 .data</span>
131<span class="go">0x201010 .bss</span>
132<span class="go">0x0 .comment</span>
133<span class="go">0x0 .symtab</span>
134<span class="go">0x0 .strtab</span>
135<span class="go">0x0 .shstrtab</span>
136</code></pre></div>
137
138<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>
139
140<p>Since we know that the <code>.text</code> section has the opcodes, let’s disassemble the binary starting at that address.</p>
141
142<div class="codehilite"><pre><span></span><code><span class="c1"># disas1.py</span>
143
144<span class="kn">from</span> <span class="nn">elftools.elf.elffile</span> <span class="kn">import</span> <span class="n">ELFFile</span>
145<span class="kn">from</span> <span class="nn">capstone</span> <span class="kn">import</span> <span class="o">*</span>
146
147<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;./bin.elf&#39;</span><span class="p">,</span> <span class="s1">&#39;rb&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
148    <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>
149    <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">&#39;.text&#39;</span><span class="p">)</span>
150    <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>
151    <span class="n">addr</span> <span class="o">=</span> <span class="n">code</span><span class="p">[</span><span class="s1">&#39;sh_addr&#39;</span><span class="p">]</span>
152    <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>
153    <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>        
154        <span class="k">print</span><span class="p">(</span><span class="n">f</span><span class="s1">&#39;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}&#39;</span><span class="p">)</span>
155</code></pre></div>
156
157<p>The code is fairly straightforward (I think). We should be seeing this, on running</p>
158
159<div class="codehilite"><pre><span></span><code><span class="go">› python disas1.py | less      </span>
160<span class="go">0x6a0: xor ebp, ebp</span>
161<span class="go">0x6a2: mov r9, rdx</span>
162<span class="go">0x6a5: pop rsi</span>
163<span class="go">0x6a6: mov rdx, rsp</span>
164<span class="go">0x6a9: and rsp, 0xfffffffffffffff0</span>
165<span class="go">0x6ad: push rax</span>
166<span class="go">0x6ae: push rsp</span>
167<span class="go">0x6af: lea r8, [rip + 0x23a]</span>
168<span class="go">0x6b6: lea rcx, [rip + 0x1c3]</span>
169<span class="go">0x6bd: lea rdi, [rip + 0xe6]</span>
170<span class="go">**0x6c4: call qword ptr [rip + 0x200916]**</span>
171<span class="go">0x6ca: hlt</span>
172<span class="go">... snip ...</span>
173</code></pre></div>
174
175<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>
176
177<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>
178
179<blockquote>
180  <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>
181</blockquote>
182
183<p>To try and find these relocation entries, we write a third script.</p>
184
185<div class="codehilite"><pre><span></span><code><span class="c1"># relocations.py</span>
186
187<span class="kn">import</span> <span class="nn">sys</span>
188<span class="kn">from</span> <span class="nn">elftools.elf.elffile</span> <span class="kn">import</span> <span class="n">ELFFile</span>
189<span class="kn">from</span> <span class="nn">elftools.elf.relocation</span> <span class="kn">import</span> <span class="n">RelocationSection</span>
190
191<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;./chall.elf&#39;</span><span class="p">,</span> <span class="s1">&#39;rb&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
192    <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>
193    <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>
194        <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>
195            <span class="k">print</span><span class="p">(</span><span class="n">f</span><span class="s1">&#39;{section.name}:&#39;</span><span class="p">)</span>
196            <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">&#39;sh_link&#39;</span><span class="p">])</span>
197            <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>
198                <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">&#39;r_info_sym&#39;</span><span class="p">])</span>
199                <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">&#39;r_offset&#39;</span><span class="p">])</span>
200                <span class="k">print</span><span class="p">(</span><span class="n">f</span><span class="s1">&#39;{symbol.name} {addr}&#39;</span><span class="p">)</span>
201</code></pre></div>
202
203<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>
204
205<div class="codehilite"><pre><span></span><code><span class="go">› python relocations.py</span>
206<span class="go">.rela.dyn:</span>
207<span class="go"> 0x200d98</span>
208<span class="go"> 0x200da0</span>
209<span class="go"> 0x201008</span>
210<span class="go">_ITM_deregisterTMCloneTable 0x200fd8</span>
211<span class="go">**__libc_start_main 0x200fe0**</span>
212<span class="go">__gmon_start__ 0x200fe8</span>
213<span class="go">_ITM_registerTMCloneTable 0x200ff0</span>
214<span class="go">__cxa_finalize 0x200ff8</span>
215<span class="go">stdin 0x201010</span>
216<span class="go">.rela.plt:</span>
217<span class="go">puts 0x200fb0</span>
218<span class="go">printf 0x200fb8</span>
219<span class="go">fgets 0x200fc0</span>
220<span class="go">strcmp 0x200fc8</span>
221<span class="go">malloc 0x200fd0</span>
222</code></pre></div>
223
224<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&#8212;libc-start-main-.html">linuxbase.org</a></p>
225
226<blockquote>
227  <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>
228</blockquote>
229
230<p>And its definition is like so</p>
231
232<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> 
233<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> 
234<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> 
235<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> 
236<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> 
237<span class="kt">void</span> <span class="p">(</span><span class="o">*</span> <span class="n">stack_end</span><span class="p">));</span>
238</code></pre></div>
239
240<p>Looking back at our disassembly</p>
241
242<pre><code>0x6a0: xor ebp, ebp
2430x6a2: mov r9, rdx
2440x6a5: pop rsi
2450x6a6: mov rdx, rsp
2460x6a9: and rsp, 0xfffffffffffffff0
2470x6ad: push rax
2480x6ae: push rsp
2490x6af: lea r8, [rip + 0x23a]
2500x6b6: lea rcx, [rip + 0x1c3]
251**0x6bd: lea rdi, [rip + 0xe6]**
2520x6c4: call qword ptr [rip + 0x200916]
2530x6ca: hlt
254... snip ...
255</code></pre>
256
257<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>
258
259<p><img src="https://cdn-images-1.medium.com/max/800/0*oQA2MwHjhzosF8ZH.png" alt="" /></p>
260
261<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>
262
263<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>
264
265<pre><code>printf 0x650
266fgets  0x660
267strcmp 0x670
268malloc 0x680
269</code></pre>
270
271<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>
272
273<pre><code>0x7b2: mov edi, 0xa  ; 10
2740x7b7: call 0x680    ; malloc
275</code></pre>
276
277<p>The loop to populate the <code>*pw</code> string</p>
278
279<pre><code>0x7d0:  mov     eax, dword ptr [rbp - 0x14]
2800x7d3:  cdqe    
2810x7d5:  lea     rdx, [rax - 1]
2820x7d9:  mov     rax, qword ptr [rbp - 0x10]
2830x7dd:  add     rax, rdx
2840x7e0:  movzx   eax, byte ptr [rax]
2850x7e3:  lea     ecx, [rax + 1]
2860x7e6:  mov     eax, dword ptr [rbp - 0x14]
2870x7e9:  movsxd  rdx, eax
2880x7ec:  mov     rax, qword ptr [rbp - 0x10]
2890x7f0:  add     rax, rdx
2900x7f3:  mov     edx, ecx
2910x7f5:  mov     byte ptr [rax], dl
2920x7f7:  add     dword ptr [rbp - 0x14], 1
2930x7fb:  cmp     dword ptr [rbp - 0x14], 8
2940x7ff:  jle     0x7d0
295</code></pre>
296
297<p>And this looks like our <code>strcmp()</code></p>
298
299<pre><code>0x843:  mov     rdx, qword ptr [rbp - 0x10] ; *in
3000x847:  mov     rax, qword ptr [rbp - 8]    ; *pw
3010x84b:  mov     rsi, rdx             
3020x84e:  mov     rdi, rax
3030x851:  call    0x670                       ; strcmp  
3040x856:  test    eax, eax                    ; is = 0? 
3050x858:  jne     0x868                       ; no? jump to 0x868
3060x85a:  lea     rdi, [rip + 0xae]           ; "haha yes!" 
3070x861:  call    0x640                       ; puts
3080x866:  jmp     0x874
3090x868:  lea     rdi, [rip + 0xaa]           ; "nah dude"
3100x86f:  call    0x640                       ; puts  
311</code></pre>
312
313<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>
314
315<p><strong>Update</strong>: It&#8217;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>
316
317<h3 id="conclusion">Conclusion</h3>
318
319<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>
320
321<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>
322
323<p>Ciao for now, and I’ll see ya in #2 of this series — PE binaries. Whenever that is.</p>
324 
325    </div>
326    <hr />
327    <p class="muted">Questions or comments? Open an issue at <a href="https://github.com/icyphox/site">this repo</a>, or send an email to <a href="mailto:icyph0x@pm.me">icyph0x@pm.me</a>.</p>
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