Dr.赵博栋 Prof.张超 清华大学 网络研究院 INSC
本文主要介绍了通过State Fuzz对Linux驱动程序进行模糊测试，该Fuzz方法由赵博栋博士在InForSec会议上分享，并在USENIX Security上发布.StateFuzz :System Call-Based State-Aware Linux Driver Fuzzing.该篇文章主要介绍了核心方法，为展示测试数据与实验展望.

前言：

模糊测试是当前主流的漏洞挖掘方法，近年来发现了大量的未知漏洞，受到工业界和学术界的广泛关注。其中，以代码覆盖率为进化指标的灰盒测试方案得到大量研究，衍生出了大量优化改进方案。但是，代码覆盖率与漏洞之间存在gap，提高代码覆盖率不一定能够有效发现潜在的安全漏洞。提出了状态敏感的模糊测试方法StateFuzz (USENIX’22)，引入了程序状态作为进化指标，实验结果表明了该方法的有效性，在Linux和Android驱动中发现了数十个未知漏洞。本次报告将与大家探讨这一方案。

背景

漏洞：网络空间重要安全威胁

重要事件：乌克兰断电事件 震网病毒事件 WannaCry HeartBleed(websites) Aurora(Google)

漏洞：网络攻击的突破口

制导部分(漏洞) 战斗部分(漏洞利用) 控制部分(恶意代码)

美国军火商Lockheed-Martin提出的"杀伤链"

• Reconnaissance 目标侦查 （漏洞挖掘）
  Research,identification,and selection of targets

• Weaponization 武器定制 （漏洞利用）
  Pairing remote access malware with exploit into a
  deliverable payload(e.g.Adobe PDF and Microsoft Office files)

• Delivery 武器投放（主动/被动）
  Transmission of weapon to target(e.g. via email attachments websites,r USB drivers)

• Exploitation 武器生效（漏洞触发与劫持）

  Once delivered,the weapon’s code is triggered,exploiting vulnerable applications or systems.

• Installation 持久驻留（恶意代码）

  The weapon installs a backdoor on a target’s system allowing persistent access.

• Command & Control 远程控制（僵尸网络）

  Outside server communicates with the weapons providing "hands on keyboard access"inside the

  target’s network.

• Actions on Objective 最终行动（窃密/破坏/跳板）

  The attacker works to achieve the objective of the intrusion,which can include exfiltration or destruction of data,or intrusion of another target.

漏洞与漏洞利用

漏洞挖掘与漏洞利用生成本质上都是输入空间搜素问题
输入样本空间 -> 漏洞Poc样本空间 -> 目标(软件、硬件、网络)
漏洞示例CVE-2009-4270

int outprintf(const char *fmt,...)
{
  int count;char buf[1024];va_list args;
  va_start(args,fmt);
  count = vsprintf(buf,fmt,args);
  outwrite(buf,count);//print out
}
int main(int argc,char* argv[])
{
  const char *arg;
  while((arg = *argv++)!=0){
    switch(arg[0]){
      case '-':{
        switch(arg[1]){
          case 0;
          default:
              outprintf("unknown switch %s\n",arg[1]);
            }
          }
          default:...
        }
        ...

count = vsprintf(buf,fmt,args);没有对内存拷贝长度进行限制，造成了栈溢出问题
Vul trigger conditions:

• Path constraints 路径约束
• Vul constraints 漏洞约束
  Discover vulnerabilities:
• Symbolic execution 符号执行
• Fuzzing(testing) 模糊测试
  基于代码覆盖率的Fuzzing更擅长解决Path constraints，

漏洞挖掘技术概览

漏洞挖掘技术发展历史

第一阶段(1960s-1970s):人工审核(依赖经验、无法扩展) -> 源代码审计、逆向工程、经验规则
第二阶段(1970s-1990s):规则扫描(误报高/可扩展性差) -> 静态分析、符号执行、模型检验
第三阶段(1990s-2013s):动态测试(漏报高、覆盖率低) -> 随机畸形测试例，模拟攻击者攻击输入
第四阶段(2013s-2023s):智能挖掘(智能进化) -> 知识与数据驱动，遗传进化算法

第二代方案:规则扫描 静态分析(SAST)

• 基于经验规则静态扫描

优点：速度快
缺点:误报高、无法输出poc验证脚本
瓶颈:不可判定(rice定理)

第三代方案：动态测试 DAST、IAST

• 基于动态信息的漏洞挖掘

优点:误报低
缺点:覆盖率低，漏报高
工业化产品:OWASP BURPSUITE VERACODE

第三代方案:模糊测试(fuzzing)

• Fuzzing 模糊测试

生成/变异测试例，测试，检查，重复…
Generator/Mutator -> inputs -> monitor(target program) -> Security violation? -> bugs

• 科学问题/挑战:

在无穷的输入空间中，如何高校搜素有限的漏洞样本？

Fuzzing 1:Generation-based

基于模块生成测试用例(e.g. grammar,specification)

优点: valid inputs,more code coverage
缺点: hard to setup,requires input knowledge(human efforts)
工业界应用:peach bstorm

Fuzzing 2:Mutation-based

变异旧测试用例来生成新的测试用例

优点:easy to setup,no prior knowledge required
缺点:invalid inputs,limited code coverage(checksum,magic number etc.)
工业界应用:Google OSS-Fuzz Micorsoft Project OneFuzz

第四代方案:智能模糊测试

目前学术界的探索方向：
广度：支持不同类型的目标软件
模糊测试系统应用到目标软件里面。
深度：提升种子生成、变异、测试效率
主要在种子变异和种子挑选环节进行方法优化。

提供较好的初始种子测试例 -> 种子池挑选种子 -> 种子变异 ->能量分配(变异次数) -> 新测试例 -> 测试执行(覆盖率跟踪/安全监控)
主要思想是优胜劣汰的方法，覆盖率跟踪使用遗传算法实现，得到的测试例覆盖率如果得到提升(进化)，将会被筛选出作为种子放入种子池中。

VUL337 课题组漏洞挖掘研究成果

广度探索:

• 固件/硬件 IOT 芯片 Bios/TEE
• 内核/驱动 Windows MacOS Linux
• 系统软件 浏览器 hypervisor SGX/TEE应用
• 用户态软件 代码库 二进制程序 GUI程序
• 区块链 符号执行 智能合约 DeFi
• 网络设备/协议 网络服务 5G、路由器 网联车
  深度探索:
• 种子生成 > 自动/智能 输入格式识别
• 种子排序挑选 > 自动/智能程序语义理解
• 种子变异
• 测试性能优化 > 并行化、硬件协同
• 进化信号跟踪 > 精确、轻量化
• 进化策略 > 代码覆盖率、状态制导
• 安全违例检测 > 定向挖掘、瓶颈爆破

状态敏感模糊测试USENIX 2022

Code Coverage - Limitation

• Example:maze game

most code can be explored easily
no guidance to trigger the bug
State:values of maze[y][x]

while(true){
  ox=x; oy=y;

  switch(input[i]) {
    case: 'W': y--;break;
    case: 'S': y+=;break;
    case: 'A': x--;break;
    case: 'D': x+=;break;
      }
  if (maze[y][x]=='#'){Bug();}
  //If target is blocked,do not advance.
  if (maze[y][x] != ' '){x = ox; y =oy;}
}

• Another Example:DNN testing

most (Python) code can be explored easily
State:output of neurons(activated or not)

StateFuzz:State-aware Fuzzing

• Intuition:guide fuzzers to explore more program states
  我们通过引导模糊测试，去探索更多的程序状态(Program State)。
  Program State:A combination of Register values and Memory values.
  所有寄存器的值和内存的值的组合。
  问题：如何去跟踪这样庞大的组合？
• Intuition: guide fuzzers to explore more program states
• Need to answer 3 quesions

Q1: what are appropriate program states?如何定义一个确认的程序状态？
Q2: how to recognize and track program states?如何识别与跟踪程序状态？
Q3: how to guide fuzzers to explore program states?如何去引导模糊测试？

Q1:What are program states?

• Values of all memory and registers?

the number of such states is overwhelmingly large
hard to track in practice

• Manual annotation:

human efforts needed

• Protocal status code:

not always available

• Using variables to represent states is very common
  使用变量来标识状态，我们也可以通过变量作为我们的程序状态。
• Ideally,a state is a combination of all program variables(including memory and register values)

state explodsion!

• Practically,states will persist across interaction boundaries,which will be read by an interaction,and
  written another interaction.

have a long life time
can be updated(i.e… state transition)by users
can affect the program’s control flow or memory access
Ex:FTP Server Program
User -> Pass Packet / User Packet -> FTP Server

int ftpUSER(PFTPCONTEXT context,const char *params);
int ftpPASS(PFTPCONTEXT context,const char *params);

Ex:the variable context -> Access is shared by the Pass and List request

int ftpLIST(PFTPCONTEXT context,const char *params){
  if (context->Access == FTP ACCESS_NOT_LOGGED_IN)
    return sendstring(context,error530);
}

int ftpPASS(PFCONTEXT context,const char *params){
  ...
  if (strcasecmp(temptext,"admin")==0){
    context->Access = FTP_ACCESS_FULL;
  }
}

Q2:How to track states?

• Step1:Recognize State Variables(varialbes shared by different user actions)

  step 1.1:recognize user actions 识别状态变量

  • interaces that could be accessed by users

  step 1.2:recognize variables accessed by actions

  • read/write variables

  step 1.3:intersection of actions’ variable

  • read by one action,and write by the other action
• Example:the Maze game

  variables read by action ‘w’:LVMap[‘w’]={y}
  variables written by action ‘s’：SVMap[‘s’]={y}
  State variable set V=V U (LVMap[‘w’] 交 SVMap[‘s’])

while(true){
  ox=x; oy=y;

  switch(input[i]) {
    case: 'W': y--;break;
    case: 'S': y+=;break;
    case: 'A': x--;break;
    case: 'D': x+=;break;
      }
  if (maze[y][x]=='#'){Bug();}
  //If target is blocked,do not advance.
  if (maze[y][x] != ' '){x = ox; y =oy;}
}

• Step2:Calculate and track states(a combination of all state variables)

how?

Recall:How does AFL track code coverage?

• coverage = combinations of code blocks
• But number of combinations is too large.

Instution:state coverage = combinations of state-variables’ values.

Analyze the value ranges of each state-variable

• e.g. (MIN,0],[0,4],(4,10],(10.MAX))
  跟踪变量的值域范围，而不是跟踪某一个值.
  We identify value ranges by solving constrains of condition statements.
  But the value set of each state-variables is too large,which causes edge explosion.

通过判断变量是否影响相同的程序控制流，对变量进行组合.
The combination of two relevant state-variables values.
Both variables affect the same control-flow path or memory accessing.

if (x<0)
  ...
else if(x<=4)
  ...
else 
  ...

Q3:How to explore program sates?

遗传算法，使用代码覆盖率作为反馈Check Feedback.我们将状态变量的值域也作为遗传算法的指标.

• Based on existing genetic algorithm

  which relies only on code coverage feedback currently

  • Our solution: 3-dimension feedback machanism

• A test case is interesting,if it

discovers new code
discovers new value ranges of state variables
discover new extremum values of state variables

StateFuzz:Implementation

1.Kernel Source code -> Program State Recognition(Static Analysis静态分析->State-variable List状态变量集合->Static Symbolic Execution静态符号执行->提取约束条件State-Variable Value Ranges)
2.Instrumentation(State-variable Tracking Instrumentation &Code Coverage Instrumentation->Instrumented Kernel内核插桩)
3.Fuzzing Loop(根据代码插桩情况选择如何保留种子Seed Preservation -> Seed Selection ->Mutation)

具体实现细节

• State Recognition

  DIFUZE(for program action recognition)
  CRIX(for building call graph)
  Clang Static Analyzer(for static symbolic execution)

• Instrumentation

  LLVM Sancov
  SVF

• Fuzzing loop

  Syzkaller