1 /* 2 * Copyright (C) 2016 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef CFI_SHADOW_H 18 #define CFI_SHADOW_H 19 20 #include <stdint.h> 21 22 #include "private/bionic_page.h" 23 #include "private/bionic_macros.h" 24 25 constexpr unsigned kLibraryAlignmentBits = 18; 26 constexpr size_t kLibraryAlignment = 1UL << kLibraryAlignmentBits; 27 28 // This class defines format of the shadow region for Control Flow Integrity support. 29 // See documentation in http://clang.llvm.org/docs/ControlFlowIntegrityDesign.html#shared-library-support. 30 // 31 // CFI shadow is effectively a very fast and specialized implementation of dladdr: given an address that 32 // belongs to a shared library or an executable, it can find the address of a specific export in that 33 // library (a function called "__cfi_check"). This is only guaranteed to work for 34 // addresses of possible CFI targets inside a library: indirectly called functions and virtual 35 // tables. A random address inside a library may not work in the future (but it does in the current 36 // implementation). 37 // 38 // Implementation is a sparse array of uint16_t where each element describes the location of 39 // __cfi_check for a 2**kShadowGranularity range of memory. Array elements (called "shadow values" 40 // below) are interpreted as follows. 41 // 42 // For an address P and corresponding shadow value V, the address of __cfi_check is calculated as 43 // align_up(P, 2**kShadowGranularity) - (V - 2) * (2 ** kCfiCheckGranularity) 44 // 45 // Special shadow values: 46 // 0 = kInvalidShadow, this memory range has no valid CFI targets. 47 // 1 = kUncheckedShadow, any address is this memory range is a valid CFI target 48 // 49 // Loader requirement: each aligned 2**kShadowGranularity region of address space may contain at 50 // most one DSO. 51 // Compiler requirement: __cfi_check is aligned at kCfiCheckGranularity. 52 // Compiler requirement: __cfi_check for a given DSO is located below any CFI target for that DSO. 53 class CFIShadow { 54 public: 55 static constexpr uintptr_t kShadowGranularity = kLibraryAlignmentBits; 56 static constexpr uintptr_t kCfiCheckGranularity = 12; 57 58 // Each uint16_t element of the shadow corresponds to this much application memory. 59 static constexpr uintptr_t kShadowAlign = 1UL << kShadowGranularity; 60 61 // Alignment of __cfi_check. 62 static constexpr uintptr_t kCfiCheckAlign = 1UL << kCfiCheckGranularity; // 4K 63 64 #if defined (__LP64__) 65 static constexpr uintptr_t kMaxTargetAddr = 0xffffffffffff; 66 #else 67 static constexpr uintptr_t kMaxTargetAddr = 0xffffffff; 68 #endif 69 70 // Shadow is 2 -> 2**kShadowGranularity. 71 static constexpr uintptr_t kShadowSize = 72 align_up((kMaxTargetAddr >> (kShadowGranularity - 1)), PAGE_SIZE); 73 74 // Returns offset inside the shadow region for an address. 75 static constexpr uintptr_t MemToShadowOffset(uintptr_t x) { 76 return (x >> kShadowGranularity) << 1; 77 } 78 79 typedef int (*CFICheckFn)(uint64_t, void *, void *); 80 81 public: 82 enum ShadowValues : uint16_t { 83 kInvalidShadow = 0, // Not a valid CFI target. 84 kUncheckedShadow = 1, // Unchecked, valid CFI target. 85 kRegularShadowMin = 2 // This and all higher values encode a negative offset to __cfi_check in 86 // the units of kCfiCheckGranularity, starting with 0 at 87 // kRegularShadowMin. 88 }; 89 }; 90 91 #endif // CFI_SHADOW_H 92
