391 lines
18 KiB
Markdown
391 lines
18 KiB
Markdown
---
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title: AFL-爱之初体验
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date: 2019-07-09 14:46:07
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tags:
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- AFL
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- 模糊测试
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categories: 二进制
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---
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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。
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参考：https://paper.seebug.org/841/#_1
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**部署afl**
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> ```
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> wget http://lcamtuf.coredump.cx/afl/releases/afl-latest.tgz
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> tar -zxvf afl-latest.tgz
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> cd afl-2.52b/
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> make
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> sudo make install
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> ```
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**部署qemu**
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> ```
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> $ CPU_TARGET=x86_64 ./build_qemu_support.sh
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> [+] Build process successful!
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> [*] Copying binary...
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> -rwxr-xr-x 1 han han 10972920 7月 9 10:43 ../afl-qemu-trace
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> [+] Successfully created '../afl-qemu-trace'.
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> [!] Note: can't test instrumentation when CPU_TARGET set.
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> [+] All set, you can now (hopefully) use the -Q mode in afl-fuzz!
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> ```
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-------------
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# 0x01 白盒测试
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## 目标程序编译
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1. 源代码
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```
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#undef _FORTIFY_SOURCE
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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void vulnerable_function() {
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char buf[128];
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read(STDIN_FILENO, buf, 256);
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}
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int main(int argc, char** argv) {
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vulnerable_function();
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write(STDOUT_FILENO, "Hello, World\n", 13);
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}
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```
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2. gcc编译（不插桩）
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```
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$ gcc v1.c -o v1
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$ ./v1
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what
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Hello, World
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```
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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。
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3. 使用afl-gcc进行编译
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*-fno-stack-protector 该选项会禁止stack canary保护
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-z execstack 允许堆栈可执行*
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```
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$ ../afl-2.52b/afl-gcc -fno-stack-protector -z execstack v1.c -o v1-afl
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afl-cc 2.52b by <lcamtuf@google.com>
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afl-as 2.52b by <lcamtuf@google.com>
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[+] Instrumented 2 locations (64-bit, non-hardened mode, ratio 100%).
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```
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## 测试插桩程序
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**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（tuples），tuple用于获取分支信息，从而衡量衡量程序覆盖情况。
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```
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$ ./afl-showmap -o /dev/null -- ../vuln/v1 <<(echo test)
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afl-showmap 2.52b by <lcamtuf@google.com>
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[*] Executing '../vuln/v1'...
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-- Program output begins --
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Hello, World
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-- Program output ends --
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[-] PROGRAM ABORT : No instrumentation detected
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Location : main(), afl-showmap.c:773
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```
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```
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$ ./afl-showmap -o /dev/null -- ../vuln/v1-afl <<(echo test)
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afl-showmap 2.52b by <lcamtuf@google.com>
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[*] Executing '../vuln/v1-afl'...
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-- Program output begins --
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Hello, World
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-- Program output ends --
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[+] Captured 1 tuples in '/dev/null'.
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```
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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
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## 执行FUZZER
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1. 修改core
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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。
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```
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$ ./afl-fuzz -i ../vuln/testcase/ -o ../vuln/out/ ../vuln/v1-afl
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afl-fuzz 2.52b by <lcamtuf@google.com>
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[+] You have 2 CPU cores and 2 runnable tasks (utilization: 100%).
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[*] Checking CPU core loadout...
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[+] Found a free CPU core, binding to #0.
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[*] Checking core_pattern...
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[-] Hmm, your system is configured to send core dump notifications to an
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external utility. This will cause issues: there will be an extended delay
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between stumbling upon a crash and having this information relayed to the
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fuzzer via the standard waitpid() API.
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To avoid having crashes misinterpreted as timeouts, please log in as root
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and temporarily modify /proc/sys/kernel/core_pattern, like so:
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echo core >/proc/sys/kernel/core_pattern
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[-] PROGRAM ABORT : Pipe at the beginning of 'core_pattern'
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Location : check_crash_handling(), afl-fuzz.c:7275
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```
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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：
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```
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echo core >/proc/sys/kernel/core_pattern
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```
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2. 通用fuzz语法
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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：
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```
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$ ./afl-fuzz -i testcase_dir -o findings_dir / path / to / program [... params ...]
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```
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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：
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```
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$ ./afl-fuzz -i testcase_dir -o findings_dir / path / to / program @@
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```
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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
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```
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afl-fuzz 2.52b by <lcamtuf@google.com>
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[+] You have 2 CPU cores and 2 runnable tasks (utilization: 100%).
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[*] Checking CPU core loadout...
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[+] Found a free CPU core, binding to #0.
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[*] Checking core_pattern...
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[*] Setting up output directories...
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[+] Output directory exists but deemed OK to reuse.
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[*] Deleting old session data...
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[+] Output dir cleanup successful.
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[*] Scanning '../vuln/testcase/'...
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[+] No auto-generated dictionary tokens to reuse.
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[*] Creating hard links for all input files...
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[*] Validating target binary...
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[*] Attempting dry run with 'id:000000,orig:1'...
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[*] Spinning up the fork server...
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[+] All right - fork server is up.
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len = 3, map size = 1, exec speed = 295 us
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[*] Attempting dry run with 'id:000001,orig:2'...
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len = 23, map size = 1, exec speed = 125 us
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[!] WARNING: No new instrumentation output, test case may be useless.
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[+] All test cases processed.
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[!] WARNING: Some test cases look useless. Consider using a smaller set.
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[+] Here are some useful stats:
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Test case count : 1 favored, 0 variable, 2 total
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Bitmap range : 1 to 1 bits (average: 1.00 bits)
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Exec timing : 125 to 295 us (average: 210 us)
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[*] No -t option specified, so I'll use exec timeout of 20 ms.
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[+] All set and ready to roll!
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```
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3. 状态窗口
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我们可以看到afl很快就给我们制造了崩溃
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```
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american fuzzy lop 2.52b (v1-afl)
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┌─ process timing ─────────────────────────────────────┬─ overall results ─────┐
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│ run time : 0 days, 0 hrs, 4 min, 19 sec │ cycles done : 2477 │
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│ last new path : 0 days, 0 hrs, 2 min, 27 sec │ total paths : 3 │
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│ last uniq crash : 0 days, 0 hrs, 4 min, 19 sec │ uniq crashes : 1 │
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│ last uniq hang : 0 days, 0 hrs, 2 min, 12 sec │ uniq hangs : 1 │
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├─ cycle progress ────────────────────┬─ map coverage ─┴───────────────────────┤
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│ now processing : 2 (66.67%) │ map density : 0.00% / 0.00% │
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│ paths timed out : 0 (0.00%) │ count coverage : 1.00 bits/tuple │
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├─ stage progress ────────────────────┼─ findings in depth ────────────────────┤
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│ now trying : havoc │ favored paths : 1 (33.33%) │
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│ stage execs : 1433/1536 (93.29%) │ new edges on : 2 (66.67%) │
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│ total execs : 2.32M │ total crashes : 93.1k (1 unique) │
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│ exec speed : 0.00/sec (zzzz...) │ total tmouts : 8 (1 unique) │
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├─ fuzzing strategy yields ───────────┴───────────────┬─ path geometry ────────┤
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│ bit flips : 0/1152, 0/1149, 0/1143 │ levels : 2 │
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│ byte flips : 0/144, 0/14, 0/10 │ pending : 0 │
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│ arithmetics : 0/888, 0/25, 0/0 │ pend fav : 0 │
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│ known ints : 0/98, 0/390, 0/440 │ own finds : 1 │
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│ dictionary : 0/0, 0/0, 0/0 │ imported : n/a │
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│ havoc : 2/1.50M, 0/819k │ stability : 100.00% │
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│ trim : 11.88%/64, 80.00% ├────────────────────────┘
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└─────────────────────────────────────────────────────┘ [cpu000:102%] │
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│ stage execs : 1432/1536 (93.23%) │ new edges on : 2 (66.67%) │
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+++ Testing aborted by user +++ │ total crashes : 93.1k (1 unique) │
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[+] We're done here. Have a nice day! │ total tmouts : 8 (1 unique) │
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├─ fuzzing strategy yields ───────────┴───────────────┬─ path geometry ────────┤
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```
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由上面AFL状态窗口：
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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。
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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。
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② Overall results：Fuzzer当前状态的概述。
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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%
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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。
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⑤ Stage progress：Fuzzer现在正在执行的文件变异策略、执行次数和执行速度。
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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。
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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。
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⑧ Path geometry：有关Fuzzer找到的执行路径的信息。
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⑨ CPU load：CPU利用率
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## afl何时结束
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(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。
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(2) 距上一次发现新路径（或者崩溃）已经过去很长时间
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(3) 目标程序的代码几乎被测试用例完全覆盖
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## 处理输出结果
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> 确定造成这些crashes的bug是否可以利用，怎么利用？
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afl在fuzzing的过程中同时也产生了这些文件
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```
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$ tree ../vuln/out/
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../vuln/out/
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├── crashes
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│ ├── id:000000,sig:11,src:000000,op:havoc,rep:64
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│ └── README.txt
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├── fuzz_bitmap
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├── fuzzer_stats
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├── hangs
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├── plot_data
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└── queue
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├── id:000000,orig:1
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└── id:000001,orig:2
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3 directories, 7 files
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```
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在输出目录中创建了三个子目录并实时更新：
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* 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。
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* crashes： 导致被测程序接收致命信号的独特测试用例（例如，SIGSEGV，SIGILL，SIGABRT）。条目按接收信号分组。
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* 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。
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* 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。
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* fuzzer_stats：afl-fuzz的运行状态。
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* plot_data：用于afl-plot绘图。
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## 崩溃类型和可利用性
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1. triage_crashes
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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。
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```
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$ experimental/crash_triage/triage_crashes.sh ../vuln/out/ ../vuln/v1-afl 2>&1 | grep SIGNAL
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+++ ID 000000, SIGNAL 11 +++
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```
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2. crashwalk
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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：
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```
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$ apt-get install gdb golang
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$ mkdir tools
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$ cd tools
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$ git clone https://github.com/jfoote/exploitable.git
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$ mkdir go
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$ export GOPATH=~/tools/go
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$ export CW_EXPLOITABLE=~/tools/exploitable/exploitable/exploitable.py
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$ go get -u github.com/bnagy/crashwalk/cmd/...
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```
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- [ ] 这部分我好像还没完成
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3. afl-collect
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```
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$ afl-collect -d crashes.db -e gdb_script -j 8 -r ../vuln/out/ ../vuln/testcase -- ../vuln/v1-afl
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*** GDB+EXPLOITABLE SCRIPT OUTPUT ***
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[00001] out:id:000000,sig:11,src:000000,op:havoc,rep:64.................: EXPLOITABLE [ReturnAv (1/22)]
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*** ***************************** ***
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```
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-------------
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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。
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> 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。
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## 计算代码覆盖率
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**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
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1. gcc插桩
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**-fprofile-arcs -ftest-coverage**
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```
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$ gcc -fprofile-arcs -ftest-coverage ./v1.c -o v1-cov
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```
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2. afl-cov计算之前fuzzer的过程（结束后）
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```
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$ ../afl-2.52b/afl-cov/afl-cov -d ./out/ --enable-branch-coverage -c . -e "cat AFL_FILE | ./v1-cov AFL_FILE"
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Non-zero exit status '1' for CMD: /usr/bin/readelf -a cat
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*** Imported 2 new test cases from: ./out//queue
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[+] AFL test case: id:000000,orig:1 (0 / 2), cycle: 0
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lines......: 100.0% (6 of 6 lines)
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functions..: 100.0% (2 of 2 functions)
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branches...: no data found
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Coverage diff (init) id:000000,orig:1
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diff (init) -> id:000000,orig:1
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New src file: /home/han/ck/vuln/v1.c
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New 'function' coverage: main()
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New 'function' coverage: vulnerable_function()
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New 'line' coverage: 11
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New 'line' coverage: 12
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New 'line' coverage: 13
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New 'line' coverage: 6
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New 'line' coverage: 8
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New 'line' coverage: 9
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++++++ BEGIN - first exec output for CMD: cat ./out//queue/id:000000,orig:1 | ./v1-cov ./out//queue/id:000000,orig:1
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Hello, World
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++++++ END
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[+] AFL test case: id:000001,orig:2 (1 / 2), cycle: 0
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lines......: 100.0% (6 of 6 lines)
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functions..: 100.0% (2 of 2 functions)
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branches...: no data found
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[+] Processed 2 / 2 test cases.
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[+] Final zero coverage report: ./out//cov/zero-cov
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[+] Final positive coverage report: ./out//cov/pos-cov
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lines......: 100.0% (6 of 6 lines)
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functions..: 100.0% (2 of 2 functions)
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branches...: no data found
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[+] Final lcov web report: ./out//cov/web/index.html
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```
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3. LCOV展示
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------------------
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# 0x03 黑盒测试（使用qemu
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```
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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
|