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2019-07-09 06:49:24 +00:00
---
2019-07-09 08:58:49 +00:00
title: AFL-爱之初体验
2019-07-09 06:49:24 +00:00
date: 2019-07-09 14:46:07
tags:
- AFL
- 模糊测试
categories: 二进制
---
这篇文章是对afl的简单使用可大致分为黑盒测试和白盒测试两个部分。白盒测试从对目标程序的插桩编译开始然后使用fuzzer对其模糊测试发现崩溃最后对测试的代码覆盖率进行评估。黑盒测试则演示得较简略。
参考https://paper.seebug.org/841/#_1
**部署afl**
> ```
> wget http://lcamtuf.coredump.cx/afl/releases/afl-latest.tgz
> tar -zxvf afl-latest.tgz
> cd afl-2.52b/
> make
> sudo make install
> ```
**部署qemu**
> ```
> $ CPU_TARGET=x86_64 ./build_qemu_support.sh
> [+] Build process successful!
> [*] Copying binary...
> -rwxr-xr-x 1 han han 10972920 7月 9 10:43 ../afl-qemu-trace
> [+] Successfully created '../afl-qemu-trace'.
> [!] Note: can't test instrumentation when CPU_TARGET set.
> [+] All set, you can now (hopefully) use the -Q mode in afl-fuzz!
> ```
-------------
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# 0x01 白盒测试
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## 目标程序编译
1. 源代码
```
#undef _FORTIFY_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
void vulnerable_function() {
char buf[128];
read(STDIN_FILENO, buf, 256);
}
int main(int argc, char** argv) {
vulnerable_function();
write(STDOUT_FILENO, "Hello, World\n", 13);
}
```
2. gcc编译不插桩
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```
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$ gcc v1.c -o v1
$ ./v1
what
Hello, World
```
生成v1的目的一是为了和afl-gcc的编译做对比二是为黑盒测试做铺垫。
3. 使用afl-gcc进行编译
*-fno-stack-protector 该选项会禁止stack canary保护
-z execstack 允许堆栈可执行*
```
$ ../afl-2.52b/afl-gcc -fno-stack-protector -z execstack v1.c -o v1-afl
afl-cc 2.52b by <lcamtuf@google.com>
afl-as 2.52b by <lcamtuf@google.com>
[+] Instrumented 2 locations (64-bit, non-hardened mode, ratio 100%).
```
## 测试插桩程序
**afl-showmap** 跟踪单个输入的执行路径并打印程序执行的输出、捕获的元组tuplestuple用于获取分支信息从而衡量衡量程序覆盖情况。
```
$ ./afl-showmap -o /dev/null -- ../vuln/v1 <<(echo test)
afl-showmap 2.52b by <lcamtuf@google.com>
[*] Executing '../vuln/v1'...
-- Program output begins --
Hello, World
-- Program output ends --
[-] PROGRAM ABORT : No instrumentation detected
Location : main(), afl-showmap.c:773
```
```
$ ./afl-showmap -o /dev/null -- ../vuln/v1-afl <<(echo test)
afl-showmap 2.52b by <lcamtuf@google.com>
[*] Executing '../vuln/v1-afl'...
-- Program output begins --
Hello, World
-- Program output ends --
[+] Captured 1 tuples in '/dev/null'.
```
可见afl-gcc相对于gcc的不同在于采用了插桩计算覆盖率在这个实例程序中捕捉到了一个元组
## 执行FUZZER
1. 修改core
在执行afl-fuzz前如果系统配置为将核心转储文件core通知发送到外部程序。
```
$ ./afl-fuzz -i ../vuln/testcase/ -o ../vuln/out/ ../vuln/v1-afl
afl-fuzz 2.52b by <lcamtuf@google.com>
[+] You have 2 CPU cores and 2 runnable tasks (utilization: 100%).
[*] Checking CPU core loadout...
[+] Found a free CPU core, binding to #0.
[*] Checking core_pattern...
[-] Hmm, your system is configured to send core dump notifications to an
external utility. This will cause issues: there will be an extended delay
between stumbling upon a crash and having this information relayed to the
fuzzer via the standard waitpid() API.
To avoid having crashes misinterpreted as timeouts, please log in as root
and temporarily modify /proc/sys/kernel/core_pattern, like so:
echo core >/proc/sys/kernel/core_pattern
[-] PROGRAM ABORT : Pipe at the beginning of 'core_pattern'
Location : check_crash_handling(), afl-fuzz.c:7275
```
将导致将崩溃信息发送到Fuzzer之间的延迟增大进而可能将崩溃被误报为超时所以我们得临时修改core_pattern文件如下所示
```
echo core >/proc/sys/kernel/core_pattern
```
2. 通用fuzz语法
afl-fuzz对于直接从stdin接受输入的目标二进制文件通常的语法是
```
$ ./afl-fuzz -i testcase_dir -o findings_dir / path / to / program [... params ...]
```
对于从文件中获取输入的程序,使用“@@”标记目标命令行中应放置输入文件名的位置。模糊器将替换为您:
```
$ ./afl-fuzz -i testcase_dir -o findings_dir / path / to / program @@
```
此时afl会给我们返回一些信息这里提示我们有些测试用例无效
```
afl-fuzz 2.52b by <lcamtuf@google.com>
[+] You have 2 CPU cores and 2 runnable tasks (utilization: 100%).
[*] Checking CPU core loadout...
[+] Found a free CPU core, binding to #0.
[*] Checking core_pattern...
[*] Setting up output directories...
[+] Output directory exists but deemed OK to reuse.
[*] Deleting old session data...
[+] Output dir cleanup successful.
[*] Scanning '../vuln/testcase/'...
[+] No auto-generated dictionary tokens to reuse.
[*] Creating hard links for all input files...
[*] Validating target binary...
[*] Attempting dry run with 'id:000000,orig:1'...
[*] Spinning up the fork server...
[+] All right - fork server is up.
len = 3, map size = 1, exec speed = 295 us
[*] Attempting dry run with 'id:000001,orig:2'...
len = 23, map size = 1, exec speed = 125 us
[!] WARNING: No new instrumentation output, test case may be useless.
[+] All test cases processed.
[!] WARNING: Some test cases look useless. Consider using a smaller set.
[+] Here are some useful stats:
Test case count : 1 favored, 0 variable, 2 total
Bitmap range : 1 to 1 bits (average: 1.00 bits)
Exec timing : 125 to 295 us (average: 210 us)
[*] No -t option specified, so I'll use exec timeout of 20 ms.
[+] All set and ready to roll!
```
3. 状态窗口
我们可以看到afl很快就给我们制造了崩溃
```
american fuzzy lop 2.52b (v1-afl)
┌─ process timing ─────────────────────────────────────┬─ overall results ─────┐
│ run time : 0 days, 0 hrs, 4 min, 19 sec │ cycles done : 2477 │
│ last new path : 0 days, 0 hrs, 2 min, 27 sec │ total paths : 3 │
│ last uniq crash : 0 days, 0 hrs, 4 min, 19 sec │ uniq crashes : 1 │
│ last uniq hang : 0 days, 0 hrs, 2 min, 12 sec │ uniq hangs : 1 │
├─ cycle progress ────────────────────┬─ map coverage ─┴───────────────────────┤
│ now processing : 2 (66.67%) │ map density : 0.00% / 0.00% │
│ paths timed out : 0 (0.00%) │ count coverage : 1.00 bits/tuple │
├─ stage progress ────────────────────┼─ findings in depth ────────────────────┤
│ now trying : havoc │ favored paths : 1 (33.33%) │
│ stage execs : 1433/1536 (93.29%) │ new edges on : 2 (66.67%) │
│ total execs : 2.32M │ total crashes : 93.1k (1 unique) │
│ exec speed : 0.00/sec (zzzz...) │ total tmouts : 8 (1 unique) │
├─ fuzzing strategy yields ───────────┴───────────────┬─ path geometry ────────┤
│ bit flips : 0/1152, 0/1149, 0/1143 │ levels : 2 │
│ byte flips : 0/144, 0/14, 0/10 │ pending : 0 │
│ arithmetics : 0/888, 0/25, 0/0 │ pend fav : 0 │
│ known ints : 0/98, 0/390, 0/440 │ own finds : 1 │
│ dictionary : 0/0, 0/0, 0/0 │ imported : n/a │
│ havoc : 2/1.50M, 0/819k │ stability : 100.00% │
│ trim : 11.88%/64, 80.00% ├────────────────────────┘
└─────────────────────────────────────────────────────┘ [cpu000:102%] │
│ stage execs : 1432/1536 (93.23%) │ new edges on : 2 (66.67%) │
+++ Testing aborted by user +++ │ total crashes : 93.1k (1 unique) │
[+] We're done here. Have a nice day! │ total tmouts : 8 (1 unique) │
├─ fuzzing strategy yields ───────────┴───────────────┬─ path geometry ────────┤
```
由上面AFL状态窗口
① Process timing:Fuzzer运行时长、以及距离最近发现的路径、崩溃和挂起超时经过了多长时间。
已经运行4m19s距离上一个最新路径已经过去2min27s距离上一个独特崩溃已经过去4min19s可见找到崩溃的速度非常快距离上一次挂起已经过去2m12s。
② Overall resultsFuzzer当前状态的概述。
③ Cycle progress我们输入队列的距离。队列一共有3个用例现在是第二个66.67%
④ Map coverage目标二进制文件中的插桩代码所观察到覆盖范围的细节。
⑤ Stage progressFuzzer现在正在执行的文件变异策略、执行次数和执行速度。
⑥ Findings in depth有关我们找到的执行路径异常和挂起数量的信息。
⑦ Fuzzing strategy yields关于突变策略产生的最新行为和结果的详细信息。
⑧ Path geometry有关Fuzzer找到的执行路径的信息。
⑨ CPU loadCPU利用率
## afl何时结束
(1) 状态窗口中”cycles done”字段颜色变为绿色该字段的颜色可以作为何时停止测试的参考随着周期数不断增大其颜色也会由洋红色逐步变为黄色、蓝色、绿色。当其变为绿色时继续Fuzzing下去也很难有新的发现了这时便可以通过Ctrl-C停止afl-fuzz。
(2) 距上一次发现新路径(或者崩溃)已经过去很长时间
(3) 目标程序的代码几乎被测试用例完全覆盖
## 处理输出结果
> 确定造成这些crashes的bug是否可以利用怎么利用
afl在fuzzing的过程中同时也产生了这些文件
```
$ tree ../vuln/out/
../vuln/out/
├── crashes
│   ├── id:000000,sig:11,src:000000,op:havoc,rep:64
│   └── README.txt
├── fuzz_bitmap
├── fuzzer_stats
├── hangs
├── plot_data
└── queue
├── id:000000,orig:1
└── id:000001,orig:2
3 directories, 7 files
```
在输出目录中创建了三个子目录并实时更新:
* queue 测试每个独特执行路径的案例,以及用户提供的所有起始文件。
* crashes 导致被测程序接收致命信号的独特测试用例例如SIGSEGVSIGILLSIGABRT。条目按接收信号分组。
* hangs 导致测试程序超时的独特测试用例。将某些内容归类为挂起之前的默认时间限制是1秒内的较大值和-t参数的值。可以通过设置AFL_HANG_TMOUT来微调该值但这很少是必需的。
* 崩溃和挂起被视为“唯一” :如果相关的执行路径涉及在先前记录的故障中未见的任何状态转换。如果可以通过多种方式达到单个错误,那么在此过程中会有一些计数通货膨胀,但这应该会迅速逐渐减少。
* fuzzer_statsafl-fuzz的运行状态。
* plot_data用于afl-plot绘图。
## 崩溃类型和可利用性
1. triage_crashes
AFL源码的experimental目录中有一个名为triage_crashes.sh的脚本可以帮助我们触发收集到的crashes。例如下面的例子中11代表了SIGSEGV信号有可能是因为缓冲区溢出导致进程引用了无效的内存06代表了SIGABRT信号可能是执行了abort\assert函数或double free导致这些结果可以作为简单的参考。
```
$ experimental/crash_triage/triage_crashes.sh ../vuln/out/ ../vuln/v1-afl 2>&1 | grep SIGNAL
+++ ID 000000, SIGNAL 11 +++
```
2. crashwalk
如果你想得到更细致的crashes分类结果以及导致crashes的具体原因那么crashwalk就是不错的选择之一。这个工具基于gdb的exploitable插件安装也相对简单在ubuntu上只需要如下几步即可
```
$ apt-get install gdb golang
$ mkdir tools
$ cd tools
$ git clone https://github.com/jfoote/exploitable.git
$ mkdir go
$ export GOPATH=~/tools/go
$ export CW_EXPLOITABLE=~/tools/exploitable/exploitable/exploitable.py
$ go get -u github.com/bnagy/crashwalk/cmd/...
```
- [ ] 这部分我好像还没完成
3. afl-collect
```
$ afl-collect -d crashes.db -e gdb_script -j 8 -r ../vuln/out/ ../vuln/testcase -- ../vuln/v1-afl
*** GDB+EXPLOITABLE SCRIPT OUTPUT ***
[00001] out:id:000000,sig:11,src:000000,op:havoc,rep:64.................: EXPLOITABLE [ReturnAv (1/22)]
*** ***************************** ***
```
-------------
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# 0x02 代码覆盖率及其相关概念
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> 代码覆盖率是模糊测试中一个极其重要的概念使用代码覆盖率可以评估和改进测试过程执行到的代码越多找到bug的可能性就越大毕竟在覆盖的代码中并不能100%发现bug在未覆盖的代码中却是100%找不到任何bug的。
> 代码覆盖率是一种度量代码的覆盖程度的方式也就是指源代码中的某行代码是否已执行对二进制程序还可将此概念理解为汇编代码中的某条指令是否已执行。其计量方式很多但无论是GCC的GCOV还是LLVM的SanitizerCoverage都提供函数function、基本块basic-block、边界edge三种级别的覆盖率检测。
## 计算代码覆盖率
**GCOV**:插桩生成覆盖率 **LCOV**:图形展示覆盖率 **afl-cov**调用前两个工具计算afl测试用例的覆盖率
1. gcc插桩
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**-fprofile-arcs -ftest-coverage**
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```
$ gcc -fprofile-arcs -ftest-coverage ./v1.c -o v1-cov
```
2. afl-cov计算之前fuzzer的过程结束后
```
$ ../afl-2.52b/afl-cov/afl-cov -d ./out/ --enable-branch-coverage -c . -e "cat AFL_FILE | ./v1-cov AFL_FILE"
Non-zero exit status '1' for CMD: /usr/bin/readelf -a cat
*** Imported 2 new test cases from: ./out//queue
[+] AFL test case: id:000000,orig:1 (0 / 2), cycle: 0
lines......: 100.0% (6 of 6 lines)
functions..: 100.0% (2 of 2 functions)
branches...: no data found
Coverage diff (init) id:000000,orig:1
diff (init) -> id:000000,orig:1
New src file: /home/han/ck/vuln/v1.c
New 'function' coverage: main()
New 'function' coverage: vulnerable_function()
New 'line' coverage: 11
New 'line' coverage: 12
New 'line' coverage: 13
New 'line' coverage: 6
New 'line' coverage: 8
New 'line' coverage: 9
++++++ BEGIN - first exec output for CMD: cat ./out//queue/id:000000,orig:1 | ./v1-cov ./out//queue/id:000000,orig:1
Hello, World
++++++ END
[+] AFL test case: id:000001,orig:2 (1 / 2), cycle: 0
lines......: 100.0% (6 of 6 lines)
functions..: 100.0% (2 of 2 functions)
branches...: no data found
[+] Processed 2 / 2 test cases.
[+] Final zero coverage report: ./out//cov/zero-cov
[+] Final positive coverage report: ./out//cov/pos-cov
lines......: 100.0% (6 of 6 lines)
functions..: 100.0% (2 of 2 functions)
branches...: no data found
[+] Final lcov web report: ./out//cov/web/index.html
```
3. LCOV展示
![](https://res.cloudinary.com/dozyfkbg3/image/upload/v1562570048/afl/1.png)
------------------
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# 0x03 黑盒测试使用qemu
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```
$ ./afl-fuzz -i ../vuln/testcase/ -o ../vuln/outQemu -Q ../vuln/v1
american fuzzy lop 2.52b (v1)
┌─ process timing ─────────────────────────────────────┬─ overall results ─────┐
│ run time : 0 days, 0 hrs, 0 min, 41 sec │ cycles done : 232 │
│ last new path : none yet (odd, check syntax!) │ total paths : 2 │
│ last uniq crash : 0 days, 0 hrs, 0 min, 41 sec │ uniq crashes : 1 │
│ last uniq hang : none seen yet │ uniq hangs : 0 │
├─ cycle progress ────────────────────┬─ map coverage ─┴───────────────────────┤
│ now processing : 0* (0.00%) │ map density : 0.04% / 0.04% │
│ paths timed out : 0 (0.00%) │ count coverage : 1.00 bits/tuple │
├─ stage progress ────────────────────┼─ findings in depth ────────────────────┤
│ now trying : havoc │ favored paths : 1 (50.00%) │
│ stage execs : 255/256 (99.61%) │ new edges on : 1 (50.00%) │
│ total execs : 121k │ total crashes : 33 (1 unique) │
│ exec speed : 2860/sec │ total tmouts : 0 (0 unique) │
├─ fuzzing strategy yields ───────────┴───────────────┬─ path geometry ────────┤
│ bit flips : 0/56, 0/54, 0/50 │ levels : 1 │
│ byte flips : 0/7, 0/5, 0/1 │ pending : 0 │
│ arithmetics : 0/392, 0/25, 0/0 │ pend fav : 0 │
│ known ints : 0/36, 0/138, 0/44 │ own finds : 0 │
│ dictionary : 0/0, 0/0, 0/0 │ imported : n/a │
│ havoc : 1/120k, 0/0 │ stability : 100.00% │
│ trim : 82.61%/5, 0.00% ├────────────────────────┘
^C────────────────────────────────────────────────────┘ [cpu000:102%]
```
- [ ] 待完成对黑盒测试原理的分析