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/Target/TargetMachine.h"
28 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
29
30 using namespace llvm;
31
32 #define DEBUG_TYPE "kcfi"
33
34 STATISTIC(NumKCFIChecks, "Number of kcfi operands transformed into checks");
35
36 namespace {
37 class DiagnosticInfoKCFI : public DiagnosticInfo {
38 const Twine &Msg;
39
40 public:
DiagnosticInfoKCFI(const Twine & DiagMsg,DiagnosticSeverity Severity=DS_Error)41 DiagnosticInfoKCFI(const Twine &DiagMsg,
42 DiagnosticSeverity Severity = DS_Error)
43 : DiagnosticInfo(DK_Linker, Severity), Msg(DiagMsg) {}
print(DiagnosticPrinter & DP) const44 void print(DiagnosticPrinter &DP) const override { DP << Msg; }
45 };
46 } // namespace
47
run(Function & F,FunctionAnalysisManager & AM)48 PreservedAnalyses KCFIPass::run(Function &F, FunctionAnalysisManager &AM) {
49 Module &M = *F.getParent();
50 if (!M.getModuleFlag("kcfi"))
51 return PreservedAnalyses::all();
52
53 // Find call instructions with KCFI operand bundles.
54 SmallVector<CallInst *> KCFICalls;
55 for (Instruction &I : instructions(F)) {
56 if (auto *CI = dyn_cast<CallInst>(&I))
57 if (CI->getOperandBundle(LLVMContext::OB_kcfi))
58 KCFICalls.push_back(CI);
59 }
60
61 if (KCFICalls.empty())
62 return PreservedAnalyses::all();
63
64 LLVMContext &Ctx = M.getContext();
65 // patchable-function-prefix emits nops between the KCFI type identifier
66 // and the function start. As we don't know the size of the emitted nops,
67 // don't allow this attribute with generic lowering.
68 if (F.hasFnAttribute("patchable-function-prefix"))
69 Ctx.diagnose(
70 DiagnosticInfoKCFI("-fpatchable-function-entry=N,M, where M>0 is not "
71 "compatible with -fsanitize=kcfi on this target"));
72
73 IntegerType *Int32Ty = Type::getInt32Ty(Ctx);
74 MDNode *VeryUnlikelyWeights = MDBuilder(Ctx).createUnlikelyBranchWeights();
75 Triple T(M.getTargetTriple());
76
77 for (CallInst *CI : KCFICalls) {
78 // Get the expected hash value.
79 const uint32_t ExpectedHash =
80 cast<ConstantInt>(CI->getOperandBundle(LLVMContext::OB_kcfi)->Inputs[0])
81 ->getZExtValue();
82
83 // Drop the KCFI operand bundle.
84 CallBase *Call = CallBase::removeOperandBundle(CI, LLVMContext::OB_kcfi,
85 CI->getIterator());
86 assert(Call != CI);
87 Call->copyMetadata(*CI);
88 CI->replaceAllUsesWith(Call);
89 CI->eraseFromParent();
90
91 if (!Call->isIndirectCall())
92 continue;
93
94 // Emit a check and trap if the target hash doesn't match.
95 IRBuilder<> Builder(Call);
96 Value *FuncPtr = Call->getCalledOperand();
97 // ARM uses the least significant bit of the function pointer to select
98 // between ARM and Thumb modes for the callee. Instructions are always
99 // at least 16-bit aligned, so clear the LSB before we compute the hash
100 // location.
101 if (T.isARM() || T.isThumb()) {
102 FuncPtr = Builder.CreateIntToPtr(
103 Builder.CreateAnd(Builder.CreatePtrToInt(FuncPtr, Int32Ty),
104 ConstantInt::get(Int32Ty, -2)),
105 FuncPtr->getType());
106 }
107 Value *HashPtr = Builder.CreateConstInBoundsGEP1_32(Int32Ty, FuncPtr, -1);
108 Value *Test = Builder.CreateICmpNE(Builder.CreateLoad(Int32Ty, HashPtr),
109 ConstantInt::get(Int32Ty, ExpectedHash));
110 Instruction *ThenTerm =
111 SplitBlockAndInsertIfThen(Test, Call, false, VeryUnlikelyWeights);
112 Builder.SetInsertPoint(ThenTerm);
113 Builder.CreateCall(Intrinsic::getDeclaration(&M, Intrinsic::debugtrap));
114 ++NumKCFIChecks;
115 }
116
117 return PreservedAnalyses::none();
118 }
119