1 //===-- KCFI.cpp - Generic KCFI operand bundle lowering ---------*- C++ -*-===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This pass emits generic KCFI indirect call checks for targets that don't 10 // support lowering KCFI operand bundles in the back-end. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Transforms/Instrumentation/KCFI.h" 15 #include "llvm/ADT/Statistic.h" 16 #include "llvm/IR/Constants.h" 17 #include "llvm/IR/DiagnosticInfo.h" 18 #include "llvm/IR/DiagnosticPrinter.h" 19 #include "llvm/IR/Function.h" 20 #include "llvm/IR/GlobalObject.h" 21 #include "llvm/IR/IRBuilder.h" 22 #include "llvm/IR/InstIterator.h" 23 #include "llvm/IR/Instructions.h" 24 #include "llvm/IR/Intrinsics.h" 25 #include "llvm/IR/MDBuilder.h" 26 #include "llvm/IR/Module.h" 27 #include "llvm/InitializePasses.h" 28 #include "llvm/Pass.h" 29 #include "llvm/Target/TargetMachine.h" 30 #include "llvm/Transforms/Instrumentation.h" 31 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 32 33 using namespace llvm; 34 35 #define DEBUG_TYPE "kcfi" 36 37 STATISTIC(NumKCFIChecks, "Number of kcfi operands transformed into checks"); 38 39 namespace { 40 class DiagnosticInfoKCFI : public DiagnosticInfo { 41 const Twine &Msg; 42 43 public: 44 DiagnosticInfoKCFI(const Twine &DiagMsg, 45 DiagnosticSeverity Severity = DS_Error) 46 : DiagnosticInfo(DK_Linker, Severity), Msg(DiagMsg) {} 47 void print(DiagnosticPrinter &DP) const override { DP << Msg; } 48 }; 49 } // namespace 50 51 PreservedAnalyses KCFIPass::run(Function &F, FunctionAnalysisManager &AM) { 52 Module &M = *F.getParent(); 53 if (!M.getModuleFlag("kcfi")) 54 return PreservedAnalyses::all(); 55 56 // Find call instructions with KCFI operand bundles. 57 SmallVector<CallInst *> KCFICalls; 58 for (Instruction &I : instructions(F)) { 59 if (auto *CI = dyn_cast<CallInst>(&I)) 60 if (CI->getOperandBundle(LLVMContext::OB_kcfi)) 61 KCFICalls.push_back(CI); 62 } 63 64 if (KCFICalls.empty()) 65 return PreservedAnalyses::all(); 66 67 LLVMContext &Ctx = M.getContext(); 68 // patchable-function-prefix emits nops between the KCFI type identifier 69 // and the function start. As we don't know the size of the emitted nops, 70 // don't allow this attribute with generic lowering. 71 if (F.hasFnAttribute("patchable-function-prefix")) 72 Ctx.diagnose( 73 DiagnosticInfoKCFI("-fpatchable-function-entry=N,M, where M>0 is not " 74 "compatible with -fsanitize=kcfi on this target")); 75 76 IntegerType *Int32Ty = Type::getInt32Ty(Ctx); 77 MDNode *VeryUnlikelyWeights = 78 MDBuilder(Ctx).createBranchWeights(1, (1U << 20) - 1); 79 80 for (CallInst *CI : KCFICalls) { 81 // Get the expected hash value. 82 const uint32_t ExpectedHash = 83 cast<ConstantInt>(CI->getOperandBundle(LLVMContext::OB_kcfi)->Inputs[0]) 84 ->getZExtValue(); 85 86 // Drop the KCFI operand bundle. 87 CallBase *Call = 88 CallBase::removeOperandBundle(CI, LLVMContext::OB_kcfi, CI); 89 assert(Call != CI); 90 Call->copyMetadata(*CI); 91 CI->replaceAllUsesWith(Call); 92 CI->eraseFromParent(); 93 94 if (!Call->isIndirectCall()) 95 continue; 96 97 // Emit a check and trap if the target hash doesn't match. 98 IRBuilder<> Builder(Call); 99 Value *HashPtr = Builder.CreateConstInBoundsGEP1_32( 100 Int32Ty, Call->getCalledOperand(), -1); 101 Value *Test = Builder.CreateICmpNE(Builder.CreateLoad(Int32Ty, HashPtr), 102 ConstantInt::get(Int32Ty, ExpectedHash)); 103 Instruction *ThenTerm = 104 SplitBlockAndInsertIfThen(Test, Call, false, VeryUnlikelyWeights); 105 Builder.SetInsertPoint(ThenTerm); 106 Builder.CreateCall(Intrinsic::getDeclaration(&M, Intrinsic::trap)); 107 ++NumKCFIChecks; 108 } 109 110 return PreservedAnalyses::none(); 111 } 112