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通过 libFuzzer 进行模糊测试

模糊测试(将可能无效的数据、异常数据或随机数据作为输入内容提供给程序)是在大型软件系统中查找错误的一种非常有效的方式,也是软件开发生命周期的重要组成部分。

Android 构建系统通过从 LLVM 编译器基础架构项目纳入 libFuzzer 来支持模糊测试。LibFuzzer 与被测库相关联,并处理在模糊测试会话期间出现的所有输入选择、变更和崩溃报告。LLVM 的排错程序用于协助内存损坏检测以及提供代码覆盖率指标。

本文介绍了 Android 上的 libFuzzer 以及如何执行插桩编译, 还介绍了如何编写、运行和自定义模糊测试工具。

设置和构建

为了确保在设备上运行正在运行的映像,您可以下载出厂映像并刷写设备。或者,您也可以下载 AOSP 源代码,然后按照下面的设置和构建示例进行操作。

设置示例

本例假设目标设备是 Pixel (taimen),并已准备好进行 USB 调试 (aosp_taimen-userdebug)。您可以从驱动程序二进制文件中下载其他 Pixel 二进制文件。

mkdir ~/bin
export PATH=~/bin:$PATH
curl https://storage.googleapis.com/git-repo-downloads/repo > ~/bin/repo
chmod a+x ~/bin/repo
repo init -u https://android.googlesource.com/platform/manifest -b master
repo sync -c -j8
wget https://dl.google.com/dl/android/aosp/google_devices-taimen-qq1a.191205.008-f4537f93.tgz
tar xvf google_devices-taimen-qq1a.191205.008-f4537f93.tgz
./extract-google_devices-taimen.sh
wget https://dl.google.com/dl/android/aosp/qcom-taimen-qq1a.191205.008-760afa6e.tgz
tar xvf qcom-taimen-qq1a.191205.008-760afa6e.tgz
./extract-qcom-taimen.sh
. build/envsetup.sh
lunch aosp_taimen-userdebug

编译示例

运行模糊测试目标的第一步是获取新的系统映像。我们建议至少使用最新的 Android 开发版本。

  1. 通过执行以下命令来执行初始构建:
    m
  2. 如需刷写设备,请在启动设备后使用相应的键组合进入 fastboot 模式。
  3. 解锁引导加载程序,并使用下列命令刷入新编译的映像
    fastboot oem unlock
    fastboot flashall
    

目标设备现在应该已经准备好进行 libFuzzer 模糊测试。

编写模糊测试工具

为了说明如何在 Android 中使用 libFuzzer 编写端到端的模糊测试工具,请将以下易受攻击的代码作为测试用例。 这样做有助于对模糊测试工具进行测试,确保一切运行正常,并说明崩溃数据是什么样的。

以下是测试函数。

#include <stdint.h>
#include <stddef.h>
bool FuzzMe(const char *Data, size_t DataSize) {
    return DataSize >= 3  &&
           Data[0] == 'F' &&
           Data[1] == 'U' &&
           Data[2] == 'Z' &&
           Data[3] == 'Z';  // ← Out of bounds access
}

要编译并运行此模糊测试工具,请执行以下操作:

  1. 模糊测试目标包含两个文件:一个构建文件和模糊测试目标源代码。使用要模糊的库旁边的位置创建文件。为模糊测试器命名,描述模糊器的作用。
  2. 使用 libFuzzer 编写模糊测试目标。模糊测试目标是一个函数,该函数可接收指定大小的 blob 数据,并将其传递给要接受模糊测试的函数。以下是针对易受攻击的测试函数的基本模糊测试工具:
    #include <stddef.h>
    #include <stdint.h>
    
    extern "C" int LLVMFuzzerTestOneInput(const char *data, size_t size) {
      // ...
      // Use the data to call the library you are fuzzing.
      // ...
      return FuzzMe(data, size);
    }
    
  3. 指示 Android 的编译系统创建模糊测试工具二进制文件。 要编译模糊测试工具,请将以下代码添加到 Android.bp 文件中:
    cc_fuzz {
      name: "fuzz_me_fuzzer",
      srcs: [
        "fuzz_me_fuzzer.cpp",
      ],
      // If the fuzzer has a dependent library, uncomment the following section and
      // include it.
      // static_libs: [
      //   "libfoo", // Dependent library
      // ],
      //
      // The advanced features below allow you to package your corpus and
      // dictionary files during building. You can find more information about
      // these features at:
      //  - Corpus: https://llvm.org/docs/LibFuzzer.html#corpus
      //  - Dictionaries: https://llvm.org/docs/LibFuzzer.html#dictionaries
      // These features are not required for fuzzing, but are highly recommended
      // to gain extra coverage.
      // To include a corpus folder, uncomment the following line.
      // corpus: ["corpus/*"],
      // To include a dictionary, uncomment the following line.
      // dictionary: "fuzz_me_fuzzer.dict",
    }
    
  4. 要使模糊测试工具在目标(设备)上运行,请执行以下操作:
    SANITIZE_TARGET=hwaddress m fuzz_me_fuzzer
    
  5. 要使模糊测试程序在主机上运行,请执行以下操作:
    SANITIZE_HOST=address m fuzz_me_fuzzer
    

为方便起见,请定义一些指向模糊测试目标的路径的 shell 变量和二进制文件的名称(来自您之前编写的构建文件)。

export FUZZER_NAME=your_fuzz_target

完成这些步骤之后,您便会得到一个编译好的模糊测试工具。 模糊测试工具的默认位置(本例中为 Pixel 版本):

  • $ANDROID_PRODUCT_OUT/data/fuzz/$TARGET_ARCH/$FUZZER_NAME/$FUZZER_NAME(针对设备)。
  • $ANDROID_HOST_OUT/fuzz/$TARGET_ARCH/$FUZZER_NAME/$FUZZER_NAME(适用于主机)。
  • 在主机上运行模糊测试工具

  • 添加到您的 Android.bp 构建文件中:
    host_supported: true,
    请注意,仅在您想要支持模糊测试的库受支持时,才能应用此政策。
  • 只需运行已构建的模糊器二进制文件,即可在主机上运行该模糊测试工具:
    $ANDROID_HOST_OUT/fuzz/x86_64/$FUZZER_NAME/$FUZZER_NAME
  • 在设备上运行模糊测试工具

    我们希望使用adb将这些内容复制到您的设备上。

    1. 要将这些文件上传到设备上的某个目录下,请运行以下命令:
      adb root
      adb sync data
       
    2. 使用以下命令在设备上运行测试模糊测试工具:
      adb shell /data/fuzz/$(get_build_var TARGET_ARCH)/$FUZZER_NAME/$FUZZER_NAME \
        /data/fuzz/$(get_build_var TARGET_ARCH)/$FUZZER_NAME/corpus

    执行此操作后,系统将输出类似于下方示例的内容。

    INFO: Seed: 913963180
    INFO: Loaded 2 modules   (16039 inline 8-bit counters): 16033 [0x7041769b88, 0x704176da29), 6 [0x60e00f4df0, 0x60e00f4df6),
    INFO: Loaded 2 PC tables (16039 PCs): 16033 [0x704176da30,0x70417ac440), 6 [0x60e00f4df8,0x60e00f4e58),
    INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
    INFO: A corpus is not provided, starting from an empty corpus
    #2	INITED cov: 5 ft: 5 corp: 1/1b exec/s: 0 rss: 24Mb
    #10	NEW    cov: 6 ft: 6 corp: 2/4b lim: 4 exec/s: 0 rss: 24Mb L: 3/3 MS: 3 CopyPart-ChangeByte-InsertByte-
    #712	NEW    cov: 7 ft: 7 corp: 3/9b lim: 8 exec/s: 0 rss: 24Mb L: 5/5 MS: 2 InsertByte-InsertByte-
    #744	REDUCE cov: 7 ft: 7 corp: 3/7b lim: 8 exec/s: 0 rss: 25Mb L: 3/3 MS: 2 ShuffleBytes-EraseBytes-
    #990	REDUCE cov: 8 ft: 8 corp: 4/10b lim: 8 exec/s: 0 rss: 25Mb L: 3/3 MS: 1 ChangeByte-
    ==18631==ERROR: HWAddressSanitizer: tag-mismatch on address 0x0041e00b4183 at pc 0x0060e00c5144
    READ of size 1 at 0x0041e00b4183 tags: f8/03 (ptr/mem) in thread T0
        #0 0x60e00c5140  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0xf140)
        #1 0x60e00ca130  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x14130)
        #2 0x60e00c9b8c  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x13b8c)
        #3 0x60e00cb188  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x15188)
        #4 0x60e00cbdec  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x15dec)
        #5 0x60e00d8fbc  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x22fbc)
        #6 0x60e00f0a98  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x3aa98)
        #7 0x7041b75d34  (/data/fuzz/arm64/lib/libc.so+0xa9d34)
    
    [0x0041e00b4180,0x0041e00b41a0) is a small allocated heap chunk; size: 32 offset: 3
    0x0041e00b4183 is located 0 bytes to the right of 3-byte region [0x0041e00b4180,0x0041e00b4183)
    allocated here:
        #0 0x70418392bc  (/data/fuzz/arm64/lib/libclang_rt.hwasan-aarch64-android.so+0x212bc)
        #1 0x60e00ca040  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x14040)
        #2 0x60e00c9b8c  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x13b8c)
        #3 0x60e00cb188  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x15188)
        #4 0x60e00cbdec  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x15dec)
        #5 0x60e00d8fbc  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x22fbc)
        #6 0x60e00f0a98  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x3aa98)
        #7 0x7041b75d34  (/data/fuzz/arm64/lib/libc.so+0xa9d34)
        #8 0x60e00c504c  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0xf04c)
        #9 0x70431aa9c4  (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0x519c4)
    
    Thread: T1 0x006700006000 stack: [0x007040c55000,0x007040d4ecc0) sz: 1023168 tls: [0x000000000000,0x000000000000)
    Thread: T0 0x006700002000 stack: [0x007fe51f3000,0x007fe59f3000) sz: 8388608 tls: [0x000000000000,0x000000000000)
    Memory tags around the buggy address (one tag corresponds to 16 bytes):
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       08  00  cf  08  dc  08  cd  08  b9  08  1a  1a  0b  00  04  3f
    => 27  00  08  00  bd  bd  2d  07 [03] 73  66  66  27  27  20  f6 <=
       5b  5b  87  87  03  00  01  00  4f  04  24  24  03  39  2c  2c
       05  00  04  00  be  be  85  85  04  00  4a  4a  05  05  5f  5f
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
       00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
    Tags for short granules around the buggy address (one tag corresponds to 16 bytes):
       04  ..  ..  cf  ..  dc  ..  cd  ..  b9  ..  ..  3f  ..  57  ..
    => ..  ..  21  ..  ..  ..  ..  2d [f8] ..  ..  ..  ..  ..  ..  .. <=
       ..  ..  ..  ..  9c  ..  e2  ..  ..  4f  ..  ..  99  ..  ..  ..
    See https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html#short-granules for a description of short granule tags
    Registers where the failure occurred (pc 0x0060e00c5144):
        x0  f8000041e00b4183  x1  000000000000005a  x2  0000000000000006  x3  000000704176d9c0
        x4  00000060e00f4df6  x5  0000000000000004  x6  0000000000000046  x7  000000000000005a
        x8  00000060e00f4df0  x9  0000006800000000  x10 0000000000000001  x11 00000060e0126a00
        x12 0000000000000001  x13 0000000000000231  x14 0000000000000000  x15 000e81434c909ede
        x16 0000007041838b14  x17 0000000000000003  x18 0000007042b80000  x19 f8000041e00b4180
        x20 0000006800000000  x21 000000000000005a  x22 24000056e00b4000  x23 00000060e00f5200
        x24 00000060e0128c88  x25 00000060e0128c20  x26 00000060e0128000  x27 00000060e0128000
        x28 0000007fe59f16e0  x29 0000007fe59f1400  x30 00000060e00c5144
    SUMMARY: HWAddressSanitizer: tag-mismatch (/data/fuzz/arm64/example_fuzzer/example_fuzzer+0xf140)
    MS: 1 ChangeByte-; base unit: e09f9c158989c56012ccd88111b82f778a816eae
    0x46,0x55,0x5a,
    FUZ
    artifact_prefix='./'; Test unit written to ./crash-0eb8e4ed029b774d80f2b66408203801cb982a60
    Base64: RlVa
    

    在示例输出中,崩溃是由第 10 行 fuzz_me_fuzzer.cpp 导致的:

          Data[3] == 'Z';  // :(
    

    如果数据长度为 3,会导致出界读取错误。

    运行模糊测试工具后,输出常常会导致崩溃,而导致问题的输入则会保存在语料库中,并提供一个 ID。在示例输出中,这是 crash-0eb8e4ed029b774d80f2b66408203801cb982a60

    要在在设备上进行模糊测试时检索崩溃信息,发出以下命令,指定您的崩溃 ID:

    adb pull /data/fuzz/arm64/fuzz_me_fuzzer/corpus/CRASH_ID
    请注意,如需将测试用例保存到正确目录,您可以使用正文文件夹(如上例所示)或使用 artifact_prefix 参数(例如 `-artifact_prefix=/data/fuzz/where/my/crashes/go`)。

    对主机进行模糊测试时,崩溃信息会出现在运行模糊测试的本地文件夹的崩溃文件夹中。

    如需详细了解 libFuzzer,请参阅上游文档