Pdfium – Out-of-Bounds Read with Shading Pattern Backed by Pattern Colorspace
Related to issue 1490 . When parsing ShadingPatterns; according to the specification they shouldn't be permitted to have a pattern colorspace as their base colorspace, but this is not validated, leading to out-of-bounds reads when rendering using the malformed shading pattern. bool CPDF_ShadingPattern::Load() { // ... snip ... CPDF_Object* pCSObj = pShadingDict->GetDirectObjectFor("ColorSpace"); if (!pCSObj) return false; // No validation here on the type of colorspace. // ... snip ... return true; } If we now look at the code called during rendering of this pattern, we call through DrawFreeGouraudShading (cpdf_renderstatus.cpp), which will call CPDF_MeshStream::ReadVertex for each vertex in the shading pattern, which will call CPDF_MeshStream::ReadColor. std::tuple<float, float, float> CPDF_MeshStream::ReadColor() { ASSERT(ShouldCheckBPC(m_type)); float color_value[kMaxComponents]; for (uint32_t i = 0; i < m_nComponents; ++i) { color_value[i] = m_ColorMin[i] + m_BitStream->GetBits(m_nComponentBits) * (m_ColorMax[i] - m_ColorMin[i]) / m_ComponentMax; } // NB: color_value has only been initialised for the first m_nComponents elements float r = 0.0; float g = 0.0; float b = 0.0; if (m_funcs.empty()) { m_pCS->GetRGB(color_value, &r, &g, &b); // <-- we're interested in this call here return std::tuple<float, float, float>(r, g, b); } // ... snip ... } This call to GetRGB will be into the pattern cs bool CPDF_PatternCS::GetRGB(float* pBuf, float* R, float* G, float* B) const { if (m_pBaseCS) { ASSERT(m_pBaseCS->GetFamily() != PDFCS_PATTERN); PatternValue* pvalue = (PatternValue*)pBuf; // pvalue->m_Comps is now pointing 5 dwords into an 8 dword sized buffer, and p_pBaseCS expects to be able to read 8 dwords from it. if (m_pBaseCS->GetRGB(pvalue->m_Comps, R, G, B)) return true; } *R = 0.75f; *G = 0.75f; *B = 0.75f; return false; } Originally reported without 90 day deadline as https://bugs.chromium.org/p/chromium/issues/detail?id=795251, since it wasn't clear that there was an easy way to use the oob-read to leak information in a way that was useful, deadline applied as of 15/12 after working out how to use this as an information leak for issue 1489 . Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/44083.zip |