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Drow ELF patcher
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<i class="fa fa-calendar padding-small"></i><time datetime="2020-05-12T10:02:00+02:00"> Tue 12 May 2020</time>
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<p>In the <a href="https://github.com/zznop/drow">drow</a> readme there is a short explanation on how the tool works but here I will try to provide a bit of background information regarding ELF segments and sections and how they are aligned in memory. </p>
<h2 id="elf-segments-and-sections">ELF segments and sections</h2>
<p>The ELF file structures are defined in <a href="http://man7.org/linux/man-pages/man5/elf.5.html">elf.h</a> header. </p>
<p>To access the ELF header we can <code>mmap</code> executable in and assign it to the <code>Elf64_Ehdr</code> structure.</p>
<p>Something like:</p>
<div class="highlight"><pre><span></span><code><span class="kt">uint8_t</span><span class="w"> </span><span class="o">*</span><span class="n">data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="p">)</span><span class="n">mmap</span><span class="p">(</span><span class="nb">NULL</span><span class="p">,</span><span class="w"> </span><span class="n">size</span><span class="p">,</span><span class="w"> </span><span class="n">PROT_READ</span><span class="o">|</span><span class="n">PROT_WRITE</span><span class="p">,</span><span class="w"> </span><span class="n">MAP_PRIVATE</span><span class="p">,</span><span class="w"> </span><span class="n">fd</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span>
<span class="n">Elf64_Ehdr</span><span class="w"> </span><span class="o">*</span><span class="n">ehdr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n">Elf64_Ehdr</span><span class="w"> </span><span class="o">*</span><span class="p">)</span><span class="n">data</span><span class="p">;</span>
</code></pre></div>
<p>An executable <code>program header table</code> is an array of structures, each describing a segment or other information the system needs to prepare the program for execution. The program is mapped from disk to virtual memory according to defined segments. Each entry is <code>Elf64_Phdr</code> structure. It is important to note that there are different types (<code>p_type</code>) of segments and that only <code>PT_LOAD</code> segments get loaded in memory.
Program headers are meaningful only for executable and shared object files.</p>
<p>We can view all segments of ELF using:</p>
<div class="highlight"><pre><span></span><code>readelf -l <target>
</code></pre></div>
<p>A file's <code>section header table</code> lets one locate all the file's sections. The section header table is an array of <code>Elf64_Shdr</code> structures. Sections contain important data for linking and relocation. Each segment contains 0 or more sections.</p>
<p>We can view all sections of ELF using:</p>
<div class="highlight"><pre><span></span><code>readelf -S <target>
</code></pre></div>
<p>Both tables can be accessed trough ELF header:</p>
<div class="highlight"><pre><span></span><code><span class="n">Elf64_Phdr</span><span class="w"> </span><span class="o">*</span><span class="n">phdr</span><span class="p">;</span>
<span class="n">Elf64_Shdr</span><span class="w"> </span><span class="o">*</span><span class="n">shtable</span><span class="p">;</span>
<span class="n">phdr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n">Elf64_Phdr</span><span class="w"> </span><span class="o">*</span><span class="p">)((</span><span class="kt">uintptr_t</span><span class="p">)</span><span class="n">data</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">ehdr</span><span class="o">-></span><span class="n">e_phoff</span><span class="p">);</span>
<span class="n">shtable</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n">Elf64_Shdr</span><span class="w"> </span><span class="o">*</span><span class="p">)((</span><span class="kt">uintptr_t</span><span class="p">)</span><span class="n">data</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">ehdr</span><span class="o">-></span><span class="n">e_shoff</span><span class="p">);</span>
</code></pre></div>
<p>A good illustration of the relation between sections (on the left) and segments (on the right) is given in the image bellow <a href="https://intezer.com/blog/research/executable-linkable-format-101-part1-sections-segments/">source</a>.</p>
<p><img alt="Sections and Segments" class="img-fluid centerimage" src="https://f3real.github.io/images/2020_12_5_SegmentsAndSections.png"></p>
<p>Sections are grouped by attributes into segments in order to make the loading process of the executable more efficient, instead of loading each individual section into memory. </p>
<p>For segments following must hold:</p>
<div class="highlight"><pre><span></span><code>({Virt,Phys}Addr - Offset) % PageSize == 0
</code></pre></div>
<p>To align value to for example 8 bytes we need to use an alignment mask of 7 (8 - 1). The <code>drow</code> is using this procedure to align the new section size to page size.</p>
<div class="highlight"><pre><span></span><code>value & ~(alignment - 1)
</code></pre></div>
<p>To successfully modify the size of one section, we need to increase the offset of all the following sections by the same amount. The same holds true for segment entries in <code>program header table</code> as well as offsets of tables themselves in the ELF header if they are after the modified section.</p>
<p>And that's about it. There are few more tricks like writing stager for payload for those interested but I won't go into details how they work here. In general it's a pretty straightforward working tool that provides a nice way to get more familiar with ELF structure.</p>
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