Windows DWrite 组件 RCE 漏洞 (CVE-2021-24093) 分析-白红宇

Windows DWrite 组件 RCE 漏洞 (CVE-2021-24093) 分析

阅读量：4112 次

发布时间：2019-05-25

本文共 15951 字，大约阅读时间需要 53 分钟。

聚焦源代码安全，网罗国内外最新资讯！

概述

Windows图形组件DWrite库存在数组越界写漏洞 (CVE-2021-24093)，可导致远程代码执行。

当DWrite库解析恶意构造的字体文件时，计算内存分配长度时出现错误，触发越界写，而且字体文件中的数据能够越界写入到任意位置，使攻击者可以做到RCE。

背景知识

TrueType (Free Library)字体通常包含在单个TrueType字体文件中，其文件后缀为.TTF。文件开头是 TableDirectory 结构，TTableDirectory 结构的最后一个字段是可变长度的 TableEntry 结构的数组。TrueType 字体中的所有数据都使用 big-endian 编码。

TrueType字体目录的c语言定义：

typedef sturct{  char tag[4];  ULONG checkSum;  ULONG offset;  ULONG length;}TableEntry;typedef struct{  Fixed sfntversion; //0x10000 for version 1.0  USHORT numTables;  USHORT searchRange;  USHORT entrySelector;  USHORT rangeShift;  TableEntry entries[1];//variable number of TableEntry}TableDirectory;

通过 010 Editor 编辑器打开poc.ttf文件，可以清楚的看到 ttf 文件的结构。

重点关注 maxp 表中的 maxPoints 字段和 maxCompositePoints，可以看到它们的值分别为0和3，它们相对于maxp表起始地址的偏移值分别为6和10。

复现

获取 PoC 后，将 poc.html 和 poc.ttf 放到同一目录，双击打开 poc.html 文件。

PoC 地址：

https://packetstormsecurity.com/files/161582/Microsoft-DirectWrite-fsg_ExecuteGlyph-Buffer-Overflow.html

点击OK，加载 poc.ttf 字体文件。浏览器渲染进程崩溃。

用 windbg 定位崩溃点。在第2步点击OK之前，将windbg工具attach到6768进程上。

如何确定进程6768是chrome当前渲染进程呢？可以借助procexp64.exe工具。

继续执行，在DWrite!fsg_ExecuteGlyph+0x72c位置触发内存访问违例，代码 “add word ptr [r8+56h],ax” 引用了一个非法地址。

此时的调用栈为：

# Child-SP          RetAddr               Call Site00 00000095`37bfa1c0 00007ffd`4c107e92     DWrite!fsg_ExecuteGlyph+0x72c01 00000095`37bfa2f0 00007ffd`4c107ac3     DWrite!fsg_CreateGlyphData+0x12e02 00000095`37bfa3b0 00007ffd`4c1072d9     DWrite!fsg_GridFit+0xbb03 00000095`37bfa440 00007ffd`4c105d72     DWrite!fs__Contour+0x1c104 00000095`37bfa530 00007ffd`4c105b57     DWrite!TrueTypeRasterizer::Implementation::RasterizeInternal+0xfa05 00000095`37bfa570 00007ffd`4c105aa4     DWrite!TrueTypeRasterizer::Implementation::GetBitmapInternal+0x2f06 00000095`37bfa5d0 00007ffd`4c1f0f2e     DWrite!TrueTypeRasterizer::Implementation::GetBitmap+0x3007 00000095`37bfa640 00007ffd`4c198122     DWrite!TrueTypeRasterizer::Implementation::GetMetrics+0x7a08 00000095`37bfa740 00007ffd`4c119e35     DWrite!TrueTypeFontMetricsBuilder::GetGlyphMetrics+0x6872209 00000095`37bfa820 00007ffd`4c1199a8     DWrite!DesignGlyphMetricsRasterizationState::RasterizeGlyph+0x450a 00000095`37bfa890 00007ffd`4c12f614     DWrite!GlyphDataElement
    
     ::AddGlyphs+0x1480b 00000095`37bfaa70 00007ffd`4c12f1ff     DWrite!GlyphDataElement
     
      ::GetGlyphData+0x2740c 00000095`37bfaba0 00007ffd`4c12dd20     DWrite!FontFace::GetDesignGlyphMetrics+0xaf0d 00000095`37bfac90 00007ffd`28bea88a     DWrite!DWriteFontFace::GetDesignGlyphMetrics+0xc00e 00000095`37bfad10 00007ffd`28bea4e6     chrome!ChromeMain+0x2f59ea0f 00000095`37bfadd0 00007ffd`28bea15f     chrome!ChromeMain+0x2f564610 00000095`37bfaea0 00007ffd`292f746e     chrome!ChromeMain+0x2f52bf11 00000095`37bfafc0 00007ffd`292f7975     chrome!ovly_debug_event+0x2cca5e12 00000095`37bfb030 00007ffd`28be9fdd     chrome!ovly_debug_event+0x2ccf6513 00000095`37bfb0c0 00007ffd`28be9bf8     chrome!ChromeMain+0x2f513d14 00000095`37bfb140 00007ffd`2e4b9f11     chrome!ChromeMain+0x2f4d5815 00000095`37bfb380 00007ffd`2d52b28c     chrome!VR_GetStringForHmdError+0x1c0bce116 00000095`37bfba60 00007ffd`2947abb9     chrome!VR_GetStringForHmdError+0xc7d05c17 00000095`37bfbaa0 00007ffd`28c34741     chrome!ovly_debug_event+0x4501a918 00000095`37bfbba0 00007ffd`28c27cbb     chrome!ChromeMain+0x33f8a119 00000095`37bfbc00 00007ffd`2e6d73b5     chrome!ChromeMain+0x332e1b1a 00000095`37bfbc70 00007ffd`2e6d89cd     chrome!VR_GetStringForHmdError+0x1e291851b 00000095`37bfbeb0 00007ffd`2d70ae8b     chrome!VR_GetStringForHmdError+0x1e2a79d1c 00000095`37bfbfd0 00007ffd`2d70a96f     chrome!VR_GetStringForHmdError+0xe5cc5b1d 00000095`37bfc3b0 00007ffd`2d70dad0     chrome!VR_GetStringForHmdError+0xe5c73f1e 00000095`37bfc420 00007ffd`2e521ab3     chrome!VR_GetStringForHmdError+0xe5f8a01f 00000095`37bfc480 00007ffd`2e522318     chrome!VR_GetStringForHmdError+0x1c7388320 00000095`37bfc4e0 00007ffd`2eef5404     chrome!VR_GetStringForHmdError+0x1c740e821 00000095`37bfc550 00007ffd`2e52380f     chrome!VR_GetStringForHmdError+0x26471d422 00000095`37bfc5e0 00007ffd`2e51dc86     chrome!VR_GetStringForHmdError+0x1c755df23 00000095`37bfc8e0 00007ffd`2eefb204     chrome!VR_GetStringForHmdError+0x1c6fa5624 00000095`37bfca90 00007ffd`2eef59e6     chrome!VR_GetStringForHmdError+0x264cfd425 00000095`37bfcbf0 00007ffd`2eef541b     chrome!VR_GetStringForHmdError+0x26477b626 00000095`37bfcdf0 00007ffd`2e52380f     chrome!VR_GetStringForHmdError+0x26471eb27 00000095`37bfce80 00007ffd`2e51dc86     chrome!VR_GetStringForHmdError+0x1c755df28 00000095`37bfd180 00007ffd`2eede763     chrome!VR_GetStringForHmdError+0x1c6fa5629 00000095`37bfd330 00007ffd`2eed8540     chrome!VR_GetStringForHmdError+0x26305332a 00000095`37bfd3c0 00007ffd`2d93e4e0     chrome!VR_GetStringForHmdError+0x262a3102b 00000095`37bfd420 00007ffd`2d95f7dd     chrome!VR_GetStringForHmdError+0x10902b02c 00000095`37bfd470 00007ffd`2d95ffa7     chrome!VR_GetStringForHmdError+0x10b15ad2d 00000095`37bfd4e0 00007ffd`2d95eb62     chrome!VR_GetStringForHmdError+0x10b1d772e 00000095`37bfd5e0 00007ffd`2d95d3f4     chrome!VR_GetStringForHmdError+0x10b09322f 00000095`37bfd6a0 00007ffd`2d95cbc1     chrome!VR_GetStringForHmdError+0x10af1c430 00000095`37bfd770 00007ffd`2cac07b4     chrome!VR_GetStringForHmdError+0x10ae99131 00000095`37bfd850 00007ffd`2d93e4e0     chrome!VR_GetStringForHmdError+0x21258432 00000095`37bfd8b0 00007ffd`2cac0bdd     chrome!VR_GetStringForHmdError+0x10902b033 00000095`37bfd900 00007ffd`2ca9b81c     chrome!VR_GetStringForHmdError+0x2129ad34 00000095`37bfd980 00007ffd`2ca9a106     chrome!VR_GetStringForHmdError+0x1ed5ec35 00000095`37bfda70 00007ffd`2c986353     chrome!VR_GetStringForHmdError+0x1ebed636 00000095`37bfdc30 00007ffd`2c986992     chrome!VR_GetStringForHmdError+0xd812337 00000095`37bfdc80 00007ffd`2c97824d     chrome!VR_GetStringForHmdError+0xd876238 00000095`37bfdd50 00007ffd`2e5830de     chrome!VR_GetStringForHmdError+0xca01d39 00000095`37bfdd80 00007ffd`2b197199     chrome!VR_GetStringForHmdError+0x1cd4eae3a 00000095`37bfde10 00007ffd`2b1a5fe8     chrome!CrashForExceptionInNonABICompliantCodeRange+0x58ad593b 00000095`37bfded0 00007ffd`2b1a5e4f     chrome!CrashForExceptionInNonABICompliantCodeRange+0x599ba83c 00000095`37bfdfb0 00007ffd`2b1b4c7d     chrome!CrashForExceptionInNonABICompliantCodeRange+0x599a0f3d 00000095`37bfdff0 00007ffd`2cc5eefc     chrome!CrashForExceptionInNonABICompliantCodeRange+0x5a883d3e 00000095`37bfe120 00007ffd`2cc564d2     chrome!VR_GetStringForHmdError+0x3b0ccc3f 00000095`37bfe300 00007ffd`2989b907     chrome!VR_GetStringForHmdError+0x3a82a240 00000095`37bfe4f0 00007ffd`28dad68a     chrome!ovly_debug_event+0x870ef741 00000095`37bfe5c0 00007ffd`2898d172     chrome!ChromeMain+0x4b87ea42 00000095`37bfe820 00007ffd`2898c651     chrome!ChromeMain+0x982d243 00000095`37bfe8d0 00007ffd`2898bed2     chrome!ChromeMain+0x977b144 00000095`37bfe9c0 00007ffd`2898b64f     chrome!ChromeMain+0x9703245 00000095`37bfebb0 00007ffd`28989b26     chrome!ChromeMain+0x967af46 00000095`37bfecb0 00007ffd`289897d1     chrome!ChromeMain+0x94c8647 00000095`37bfee30 00007ffd`289079fe     chrome!ChromeMain+0x9493148 00000095`37bfef10 00007ffd`2c0ca79d     chrome!ChromeMain+0x12b5e49 00000095`37bff020 00007ffd`2c0ca4cf     chrome!RelaunchChromeBrowserWithNewCommandLineIfNeeded+0xc72cd4a 00000095`37bff160 00007ffd`2890561b     chrome!RelaunchChromeBrowserWithNewCommandLineIfNeeded+0xc6fff4b 00000095`37bff1f0 00007ffd`289054ce     chrome!ChromeMain+0x1077b4c 00000095`37bff270 00007ffd`28904e1c     chrome!ChromeMain+0x1062e4d 00000095`37bff2d0 00007ffd`2bf8f794     chrome!ChromeMain+0xff7c4e 00000095`37bff3a0 00007ffd`28919241     chrome!IsSandboxedProcess+0x41e6f44f 00000095`37bff540 00007ffd`28900ddc     chrome!ChromeMain+0x243a150 00000095`37bff5e0 00007ffd`289007fe     chrome!ChromeMain+0xbf3c51 00000095`37bff8b0 00007ffd`288f4fb3     chrome!ChromeMain+0xb95e52 00000095`37bff940 00007ff6`da83259a     chrome!ChromeMain+0x11353 00000095`37bffa30 00007ff6`da831afd     chrome_exe!Ordinal0+0x259a54 00000095`37bffaf0 00007ff6`da96fdc2     chrome_exe!Ordinal0+0x1afd55 00000095`37bffef0 00007ffd`63627034     chrome_exe!GetHandleVerifier+0xe517256 00000095`37bfff30 00007ffd`655fd0d1     KERNEL32!BaseThreadInitThunk+0x1457 00000095`37bfff60 00000000`00000000     ntdll!RtlUserThreadStart+0x21

同时可以使用BrokenType/ttf-otf-dwrite-loader直接加载poc.ttf复现漏洞，崩溃地址不变，调用栈为：

# Child-SP          RetAddr               Call Site00 00000054`8d70e960 00007ffd`4c107e92     DWrite!fsg_ExecuteGlyph+0x72c01 00000054`8d70ea90 00007ffd`4c107ac3     DWrite!fsg_CreateGlyphData+0x12e02 00000054`8d70eb50 00007ffd`4c1072d9     DWrite!fsg_GridFit+0xbb03 00000054`8d70ebe0 00007ffd`4c105d72     DWrite!fs__Contour+0x1c104 00000054`8d70ecd0 00007ffd`4c105b57    DWrite!TrueTypeRasterizer::Implementation::RasterizeInternal+0xfa05 00000054`8d70ed10 00007ffd`4c105aa4     DWrite!TrueTypeRasterizer::Implementation::GetBitmapInternal+0x2f06 00000054`8d70ed70 00007ffd`4c1f0f2e    DWrite!TrueTypeRasterizer::Implementation::GetBitmap+0x3007 00000054`8d70ede0 00007ffd`4c198122    DWrite!TrueTypeRasterizer::Implementation::GetMetrics+0x7a08 00000054`8d70eee0 00007ffd`4c119e35    DWrite!TrueTypeFontMetricsBuilder::GetGlyphMetrics+0x6872209 00000054`8d70efc0 00007ffd`4c1199a8    DWrite!DesignGlyphMetricsRasterizationState::RasterizeGlyph+0x450a 00000054`8d70f030 00007ffd`4c12f614    DWrite!GlyphDataElement
    
     ::AddGlyphs+0x1480b 00000054`8d70f210 00007ffd`4c12f1ff     DWrite!GlyphDataElement
     
      ::GetGlyphData+0x2740c 00000054`8d70f340 00007ffd`4c12dd20    DWrite!FontFace::GetDesignGlyphMetrics+0xaf0d 00000054`8d70f430 00007ff7`9d301460     DWrite!DWriteFontFace::GetDesignGlyphMetrics+0xc00e 00000054`8d70f4b0 00007ff7`9d3018e4     ttf_otf_dwrite_loader+0x14600f 00000054`8d70f7a0 00007ffd`63627034     ttf_otf_dwrite_loader+0x18e410 00000054`8d70f7e0 00007ffd`655fd0d1     KERNEL32!BaseThreadInitThunk+0x1411 00000054`8d70f810 00000000`00000000     ntdll!RtlUserThreadStart+0x21

BrokenType/ttf-otf-dwrite-loader的下载地址为：

https://github.com/googleprojectzero/BrokenType/tree/master/ttf-otf-dwrite-loader

漏洞成因

字体加载后会调用 TrueTypeRasterizer::Implementation::Initialize 初始化函数，在 Initialize 内部调用 fs_NewSfnt 函数，fs_NewSfnt 又会调用fsg_WorkSpaceSetOffsets 函数，fsg_WorkSpaceSetOffsets 的作用是计算内存分配长度。查看 fsg_WorkSpaceSetOffsets函数的伪代码如下：

a1是指向maxp表的指针，v10 = *((WORD *)a1 + 5)取字段maxCompositePoints 的值，*((WORD *)a1 + 3取字段maxPoints，因为maxCompositePoints和maxPoints的偏移分别是10和6，同时a1被强制转化为(_WORD *) 类型，所以伪代码中偏移5和3就对应10和6。

这段代码逻辑是，比较maxPoints和maxCompositePoints的大小，取大值再与数值1比较大小，最后加上数值8后作为第一个参数传入fsg_GetOutlineSizeAndOffsets函数，最后返回一个计算好的数值。

内存分配操作在Initialize函数调用fs_NewSfnt之后不远处，可以看到fsg_WorkSpaceSetOffsets函数返回的数值最终影响内存分配的长度。

poc.ttf中，maxPoints和maxCompositePoints的值分别是0和3，是畸形数据。

由于fsg_WorkSpaceSetOffsets函数中没有恰当处理，会导致Initialize函数内存分配不足。

当字体渲染时，由于内存分配不足，数据结构fsg_WorkSpaceAddr的内容会覆盖结构体GlyphData。

typedef struct fsg_WorkSpaceAddr{    F26Dot6 *              pStack;                     /* Address of stack                  */    void *                 pGlyphOutlineBase;      /* Address of Glyph Outline Base     */    fnt_ElementType *    pGlyphElement;          /* Address of Glyph Element array    */    boolean *              pGlyphDataByteSet;      /* Address of ByteSet array          */    void *                 pvGlyphData;                /* Address of GlyphData array        */    void *                 pReusableMemoryMarker;  /* Address of reusable memory        */ } fsg_WorkSpaceAddr; struct GlyphData{   char        acIdent[2];             /* Identifier for GlyphData                         */   GlyphData * pSibling;               /* Pointer to siblings                              */   GlyphData * pChild;                 /* Pointer to children                              */   GlyphData * pParent;                /* Pointer to parent                                */   sfac_GHandle hGlyph;                /* Handle for font access                           */   GlyphTypes  GlyphType;              /* Type of glyph                                    */   uint16      usGlyphIndex;           /* Glyph Index                                      */   BBOX        bbox;                   /* Bounding box for glyph                           */   uint16      usNonScaledAW;          /* Nonscaled Advance Width                          */   int16       sNonScaledLSB;          /* Nonscaled Left Side Bearing                      */   uint16      usDepth;                /* Depth of Glyph in composite tree                 */   sfac_ComponentTypes MultiplexingIndicator;/* Flag for arguments of composites                */   boolean     bRoundXYToGrid;         /* Round composite offsets to grid                  */   int16       sXOffset;               /* X offset for composite (if supplied)             */   int16       sYOffset;               /* Y offset for composite (if supplied)             */   uint16      usAnchorPoint1;         /* Anchor Point 1 for composites (if not offsets)   */   uint16      usAnchorPoint2;         /* Anchor Point 2 for composites (if not offsets)   */   transMatrix mulT;                   /* Transformation matrix for composite              */   boolean     bUseChildMetrics;       /* Should use child metrics?                        */   boolean     bUseMyMetrics;          /* Is glyph USE_MY_METRICS?                         */   point       ptDevLSB;               /* Left Side Bearing Point                          */   point       ptDevRSB;               /* Right Side Bearing Point                         */   uint16      usScanType;             /* ScanType value for this glyph                    */   uint16      usSizeOfInstructions;   /* Size (in bytes) of glyph instructions            */   uint8 *     pbyInstructions;        /* Pointer to glyph instructions                    */   fnt_ElementType * pGlyphElement;    /* Current glyph element pointer                    */ };

在fsg_CreateGlyphData中，fsg_AllocateGlyphDataMemory负责计算GlyphData的起始地址，fsg_InitializeGlyphData函数将fsg_WorkSpaceAddr成员pGlyphElement的地址赋给了GlyphData的pGlyphElement。由于结构体fsg_WorkSpaceAddr的内容会覆盖到GlyphData，所以在后续函数fsg_ExecuteGlyph中GlyphData.pGlyphElement上的写入操作就会破坏GlyphData。

事实也是如此，当运行到函数fsg_ExecuteGlyph时，rsi+8（GlyphData+8）落在了memset_0准备设置的内存区间（pGlyphElement数组）内，从而触发越界写。

memset_0调用之后，rsi+8地址处的内容会被清零。

接着是__guard_dispatch_icall_fptr函数的调用，这是微软控制流保护机制，实际调用TrueTypeRasterizer::Implementation::ApplyOutlineVariation函数。ApplyOutlineVariation 会调用GlyphOutlineVariationInterpolator::ApplyVariation，作用是读取ttf文件中的数据，赋值给被memset_0清零的内存区间，rsi+8因为处在区间内，所以值会被改为ttf文件中的内容。