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| <section id="fortify"> |
| <h1>Fortify<a class="headerlink" href="#fortify" title="Permalink to this heading"></a></h1> |
| <section id="overview"> |
| <h2>Overview<a class="headerlink" href="#overview" title="Permalink to this heading"></a></h2> |
| <p>A common error in C programs is invoking functions that might exceed memory bounds, |
| causing crashes or undefined behavior. Examples include incorrect usage of functions like |
| <code class="docutils literal notranslate"><span class="pre">memcpy</span></code> and <code class="docutils literal notranslate"><span class="pre">memset</span></code>. <cite>FORTIFY_SOURCE</cite> is a mechanism designed to help developers quickly |
| detect and mitigate boundary-related issues caused by improper use of library functions.</p> |
| </section> |
| <section id="support"> |
| <h2>Support<a class="headerlink" href="#support" title="Permalink to this heading"></a></h2> |
| <p><cite>FORTIFY_SOURCE</cite> is implemented as a software check by the compiler and is supported across all architectures. |
| It works by adding additional validation checks to standard library function calls.</p> |
| </section> |
| <section id="usage"> |
| <h2>Usage<a class="headerlink" href="#usage" title="Permalink to this heading"></a></h2> |
| <p>To enable <cite>FORTIFY_SOURCE</cite>, configure the kernel with the following option:</p> |
| <p><code class="docutils literal notranslate"><span class="pre">CONFIG_FORTIFY_SOURCE=level</span></code></p> |
| <p>Where <cite>level</cite> can be set as:</p> |
| <ol class="arabic simple"> |
| <li><p><strong>Compile-time Checks</strong>: |
| Detects issues during compilation by analyzing source code.</p></li> |
| <li><p><strong>Stack Variable Checks</strong>: |
| Extends level 1 by checking stack variables at runtime.</p></li> |
| <li><p><strong>Heap Memory Checks</strong>: |
| Builds on level 2 by adding checks for memory allocated with <code class="docutils literal notranslate"><span class="pre">malloc</span></code>. |
| (Requires GCC version 12 or later.)</p></li> |
| </ol> |
| <section id="fortify-source-overview"> |
| <h3>FORTIFY_SOURCE Overview<a class="headerlink" href="#fortify-source-overview" title="Permalink to this heading"></a></h3> |
| <p><cite>FORTIFY_SOURCE</cite> detects potential security vulnerabilities by statically analyzing source code at compile time. |
| It replaces standard library function calls with safer versions that include additional boundary checks. |
| These safer versions validate the operation’s boundaries and the input’s validity before performing certain operations.</p> |
| </section> |
| </section> |
| <section id="gcc-built-in-functions"> |
| <h2>GCC Built-in Functions<a class="headerlink" href="#gcc-built-in-functions" title="Permalink to this heading"></a></h2> |
| <p>The GCC compiler internally implements two key functions for <cite>FORTIFY_SOURCE</cite>:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">__builtin_object_size</span></code>: Determines the size of a statically allocated object.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">__builtin_dynamic_object_size</span></code>: Determines the size of dynamically allocated objects (e.g., via <code class="docutils literal notranslate"><span class="pre">malloc</span></code>).</p></li> |
| </ul> |
| <p>Starting with GCC 12, these functions support retrieving the size of variables allocated with <code class="docutils literal notranslate"><span class="pre">malloc</span></code>.</p> |
| <p>By passing a variable or buffer as an argument to these functions, the compiler can compute the corresponding size. |
| Using this size, it is possible to check for potential out-of-bounds behavior in runtime operations.</p> |
| </section> |
| <section id="example-memcpy-implementation-in-nuttx"> |
| <h2>Example: memcpy Implementation in NuttX<a class="headerlink" href="#example-memcpy-implementation-in-nuttx" title="Permalink to this heading"></a></h2> |
| <p>The following example demonstrates how <cite>FORTIFY_SOURCE</cite> can be used to enhance security in a <code class="docutils literal notranslate"><span class="pre">memcpy</span></code> |
| implementation in NuttX:</p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">fortify_function</span><span class="p">(</span><span class="n">memcpy</span><span class="p">)</span> |
| <span class="n">FAR</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">memcpy</span><span class="p">(</span><span class="n">FAR</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">dest</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">FAR</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">src</span><span class="p">,</span> |
| <span class="w"> </span><span class="kt">size_t</span><span class="w"> </span><span class="n">n</span><span class="p">)</span> |
| <span class="p">{</span> |
| <span class="w"> </span><span class="n">fortify_assert</span><span class="p">(</span><span class="n">n</span><span class="w"> </span><span class="o"><=</span><span class="w"> </span><span class="n">fortify_size</span><span class="p">(</span><span class="n">dest</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="w"> </span><span class="o">&&</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="o"><=</span><span class="w"> </span><span class="n">fortify_size</span><span class="p">(</span><span class="n">src</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">));</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">__real_memcpy</span><span class="p">(</span><span class="n">dest</span><span class="p">,</span><span class="w"> </span><span class="n">src</span><span class="p">,</span><span class="w"> </span><span class="n">n</span><span class="p">);</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>In this implementation, the <code class="docutils literal notranslate"><span class="pre">fortify_assert</span></code> macro ensures that the size of the source and destination buffers |
| is sufficient to handle the requested memory operation. If the assertion fails, it indicates a potential buffer |
| overflow, helping developers quickly identify and address such vulnerabilities.</p> |
| </section> |
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