//===- StackProtector.cpp - Stack Protector Insertion ---------------------===// // // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. // See https://llvm.org/LICENSE.txt for license information. // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception // //===----------------------------------------------------------------------===// // // This pass inserts stack protectors into functions which need them. A variable // with a random value in it is stored onto the stack before the local variables // are allocated. Upon exiting the block, the stored value is checked. If it's // changed, then there was some sort of violation and the program aborts. // //===----------------------------------------------------------------------===// #include "llvm/CodeGen/StackProtector.h" #include "llvm/ADT/SmallPtrSet.h" #include "llvm/ADT/Statistic.h" #include "llvm/Analysis/BranchProbabilityInfo.h" #include "llvm/Analysis/EHPersonalities.h" #include "llvm/Analysis/MemoryLocation.h" #include "llvm/Analysis/OptimizationRemarkEmitter.h" #include "llvm/CodeGen/Passes.h" #include "llvm/CodeGen/TargetLowering.h" #include "llvm/CodeGen/TargetPassConfig.h" #include "llvm/CodeGen/TargetSubtargetInfo.h" #include "llvm/IR/Attributes.h" #include "llvm/IR/BasicBlock.h" #include "llvm/IR/Constants.h" #include "llvm/IR/DataLayout.h" #include "llvm/IR/DebugInfo.h" #include "llvm/IR/DebugLoc.h" #include "llvm/IR/DerivedTypes.h" #include "llvm/IR/Dominators.h" #include "llvm/IR/Function.h" #include "llvm/IR/IRBuilder.h" #include "llvm/IR/Instruction.h" #include "llvm/IR/Instructions.h" #include "llvm/IR/IntrinsicInst.h" #include "llvm/IR/Intrinsics.h" #include "llvm/IR/MDBuilder.h" #include "llvm/IR/Module.h" #include "llvm/IR/Type.h" #include "llvm/IR/User.h" #include "llvm/InitializePasses.h" #include "llvm/Pass.h" #include "llvm/Support/Casting.h" #include "llvm/Support/CommandLine.h" #include "llvm/Target/TargetMachine.h" #include "llvm/Target/TargetOptions.h" #include <utility> using namespace llvm; #define DEBUG_TYPE "stack-protector" STATISTIC(NumFunProtected, "Number of functions protected"); STATISTIC(NumAddrTaken, "Number of local variables that have their address" " taken."); static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp", cl::init(true), cl::Hidden); char StackProtector::ID = 0; StackProtector::StackProtector() : FunctionPass(ID), SSPBufferSize(8) { initializeStackProtectorPass(*PassRegistry::getPassRegistry()); } INITIALIZE_PASS_BEGIN(StackProtector, DEBUG_TYPE, "Insert stack protectors", false, true) INITIALIZE_PASS_DEPENDENCY(TargetPassConfig) INITIALIZE_PASS_END(StackProtector, DEBUG_TYPE, "Insert stack protectors", false, true) FunctionPass *llvm::createStackProtectorPass() { return new StackProtector(); } void StackProtector::getAnalysisUsage(AnalysisUsage &AU) const { AU.addRequired<TargetPassConfig>(); AU.addPreserved<DominatorTreeWrapperPass>(); } bool StackProtector::runOnFunction(Function &Fn) { F = &Fn; M = F->getParent(); DominatorTreeWrapperPass *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>(); DT = DTWP ? &DTWP->getDomTree() : nullptr; TM = &getAnalysis<TargetPassConfig>().getTM<TargetMachine>(); Trip = TM->getTargetTriple(); TLI = TM->getSubtargetImpl(Fn)->getTargetLowering(); HasPrologue = false; HasIRCheck = false; Attribute Attr = Fn.getFnAttribute("stack-protector-buffer-size"); if (Attr.isStringAttribute() && Attr.getValueAsString().getAsInteger(10, SSPBufferSize)) return false; // Invalid integer string if (!RequiresStackProtector()) return false; // TODO(etienneb): Functions with funclets are not correctly supported now. // Do nothing if this is funclet-based personality. if (Fn.hasPersonalityFn()) { EHPersonality Personality = classifyEHPersonality(Fn.getPersonalityFn()); if (isFuncletEHPersonality(Personality)) return false; } ++NumFunProtected; return InsertStackProtectors(); } /// \param [out] IsLarge is set to true if a protectable array is found and /// it is "large" ( >= ssp-buffer-size). In the case of a structure with /// multiple arrays, this gets set if any of them is large. bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge, bool Strong, bool InStruct) const { if (!Ty) return false; if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) { if (!AT->getElementType()->isIntegerTy(8)) { // If we're on a non-Darwin platform or we're inside of a structure, don't // add stack protectors unless the array is a character array. // However, in strong mode any array, regardless of type and size, // triggers a protector. if (!Strong && (InStruct || !Trip.isOSDarwin())) return false; } // If an array has more than SSPBufferSize bytes of allocated space, then we // emit stack protectors. if (SSPBufferSize <= M->getDataLayout().getTypeAllocSize(AT)) { IsLarge = true; return true; } if (Strong) // Require a protector for all arrays in strong mode return true; } const StructType *ST = dyn_cast<StructType>(Ty); if (!ST) return false; bool NeedsProtector = false; for (StructType::element_iterator I = ST->element_begin(), E = ST->element_end(); I != E; ++I) if (ContainsProtectableArray(*I, IsLarge, Strong, true)) { // If the element is a protectable array and is large (>= SSPBufferSize) // then we are done. If the protectable array is not large, then // keep looking in case a subsequent element is a large array. if (IsLarge) return true; NeedsProtector = true; } return NeedsProtector; } bool StackProtector::HasAddressTaken(const Instruction *AI, uint64_t AllocSize) { const DataLayout &DL = M->getDataLayout(); for (const User *U : AI->users()) { const auto *I = cast<Instruction>(U); // If this instruction accesses memory make sure it doesn't access beyond // the bounds of the allocated object. Optional<MemoryLocation> MemLoc = MemoryLocation::getOrNone(I); if (MemLoc.hasValue() && MemLoc->Size.getValue() > AllocSize) return true; switch (I->getOpcode()) { case Instruction::Store: if (AI == cast<StoreInst>(I)->getValueOperand()) return true; break; case Instruction::AtomicCmpXchg: // cmpxchg conceptually includes both a load and store from the same // location. So, like store, the value being stored is what matters. if (AI == cast<AtomicCmpXchgInst>(I)->getNewValOperand()) return true; break; case Instruction::PtrToInt: if (AI == cast<PtrToIntInst>(I)->getOperand(0)) return true; break; case Instruction::Call: { // Ignore intrinsics that do not become real instructions. // TODO: Narrow this to intrinsics that have store-like effects. const auto *CI = cast<CallInst>(I); if (!isa<DbgInfoIntrinsic>(CI) && !CI->isLifetimeStartOrEnd()) return true; break; } case Instruction::Invoke: return true; case Instruction::GetElementPtr: { // If the GEP offset is out-of-bounds, or is non-constant and so has to be // assumed to be potentially out-of-bounds, then any memory access that // would use it could also be out-of-bounds meaning stack protection is // required. const GetElementPtrInst *GEP = cast<GetElementPtrInst>(I); unsigned TypeSize = DL.getIndexTypeSizeInBits(I->getType()); APInt Offset(TypeSize, 0); APInt MaxOffset(TypeSize, AllocSize); if (!GEP->accumulateConstantOffset(DL, Offset) || Offset.ugt(MaxOffset)) return true; // Adjust AllocSize to be the space remaining after this offset. if (HasAddressTaken(I, AllocSize - Offset.getLimitedValue())) return true; break; } case Instruction::BitCast: case Instruction::Select: case Instruction::AddrSpaceCast: if (HasAddressTaken(I, AllocSize)) return true; break; case Instruction::PHI: { // Keep track of what PHI nodes we have already visited to ensure // they are only visited once. const auto *PN = cast<PHINode>(I); if (VisitedPHIs.insert(PN).second) if (HasAddressTaken(PN, AllocSize)) return true; break; } case Instruction::Load: case Instruction::AtomicRMW: case Instruction::Ret: // These instructions take an address operand, but have load-like or // other innocuous behavior that should not trigger a stack protector. // atomicrmw conceptually has both load and store semantics, but the // value being stored must be integer; so if a pointer is being stored, // we'll catch it in the PtrToInt case above. break; default: // Conservatively return true for any instruction that takes an address // operand, but is not handled above. return true; } } return false; } /// Search for the first call to the llvm.stackprotector intrinsic and return it /// if present. static const CallInst *findStackProtectorIntrinsic(Function &F) { for (const BasicBlock &BB : F) for (const Instruction &I : BB) if (const CallInst *CI = dyn_cast<CallInst>(&I)) if (CI->getCalledFunction() == Intrinsic::getDeclaration(F.getParent(), Intrinsic::stackprotector)) return CI; return nullptr; } /// Check whether or not this function needs a stack protector based /// upon the stack protector level. /// /// We use two heuristics: a standard (ssp) and strong (sspstrong). /// The standard heuristic which will add a guard variable to functions that /// call alloca with a either a variable size or a size >= SSPBufferSize, /// functions with character buffers larger than SSPBufferSize, and functions /// with aggregates containing character buffers larger than SSPBufferSize. The /// strong heuristic will add a guard variables to functions that call alloca /// regardless of size, functions with any buffer regardless of type and size, /// functions with aggregates that contain any buffer regardless of type and /// size, and functions that contain stack-based variables that have had their /// address taken. bool StackProtector::RequiresStackProtector() { bool Strong = false; bool NeedsProtector = false; HasPrologue = findStackProtectorIntrinsic(*F); if (F->hasFnAttribute(Attribute::SafeStack)) return false; // We are constructing the OptimizationRemarkEmitter on the fly rather than // using the analysis pass to avoid building DominatorTree and LoopInfo which // are not available this late in the IR pipeline. OptimizationRemarkEmitter ORE(F); if (F->hasFnAttribute(Attribute::StackProtectReq)) { ORE.emit([&]() { return OptimizationRemark(DEBUG_TYPE, "StackProtectorRequested", F) << "Stack protection applied to function " << ore::NV("Function", F) << " due to a function attribute or command-line switch"; }); NeedsProtector = true; Strong = true; // Use the same heuristic as strong to determine SSPLayout } else if (F->hasFnAttribute(Attribute::StackProtectStrong)) Strong = true; else if (HasPrologue) NeedsProtector = true; else if (!F->hasFnAttribute(Attribute::StackProtect)) return false; for (const BasicBlock &BB : *F) { for (const Instruction &I : BB) { if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) { if (AI->isArrayAllocation()) { auto RemarkBuilder = [&]() { return OptimizationRemark(DEBUG_TYPE, "StackProtectorAllocaOrArray", &I) << "Stack protection applied to function " << ore::NV("Function", F) << " due to a call to alloca or use of a variable length " "array"; }; if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) { if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) { // A call to alloca with size >= SSPBufferSize requires // stack protectors. Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_LargeArray)); ORE.emit(RemarkBuilder); NeedsProtector = true; } else if (Strong) { // Require protectors for all alloca calls in strong mode. Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_SmallArray)); ORE.emit(RemarkBuilder); NeedsProtector = true; } } else { // A call to alloca with a variable size requires protectors. Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_LargeArray)); ORE.emit(RemarkBuilder); NeedsProtector = true; } continue; } bool IsLarge = false; if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) { Layout.insert(std::make_pair(AI, IsLarge ? MachineFrameInfo::SSPLK_LargeArray : MachineFrameInfo::SSPLK_SmallArray)); ORE.emit([&]() { return OptimizationRemark(DEBUG_TYPE, "StackProtectorBuffer", &I) << "Stack protection applied to function " << ore::NV("Function", F) << " due to a stack allocated buffer or struct containing a " "buffer"; }); NeedsProtector = true; continue; } if (Strong && HasAddressTaken(AI, M->getDataLayout().getTypeAllocSize( AI->getAllocatedType()))) { ++NumAddrTaken; Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_AddrOf)); ORE.emit([&]() { return OptimizationRemark(DEBUG_TYPE, "StackProtectorAddressTaken", &I) << "Stack protection applied to function " << ore::NV("Function", F) << " due to the address of a local variable being taken"; }); NeedsProtector = true; } // Clear any PHIs that we visited, to make sure we examine all uses of // any subsequent allocas that we look at. VisitedPHIs.clear(); } } } return NeedsProtector; } /// Create a stack guard loading and populate whether SelectionDAG SSP is /// supported. static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M, IRBuilder<> &B, bool *SupportsSelectionDAGSP = nullptr) { if (Value *Guard = TLI->getIRStackGuard(B)) return B.CreateLoad(B.getInt8PtrTy(), Guard, true, "StackGuard"); // Use SelectionDAG SSP handling, since there isn't an IR guard. // // This is more or less weird, since we optionally output whether we // should perform a SelectionDAG SP here. The reason is that it's strictly // defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also // mutating. There is no way to get this bit without mutating the IR, so // getting this bit has to happen in this right time. // // We could have define a new function TLI::supportsSelectionDAGSP(), but that // will put more burden on the backends' overriding work, especially when it // actually conveys the same information getIRStackGuard() already gives. if (SupportsSelectionDAGSP) *SupportsSelectionDAGSP = true; TLI->insertSSPDeclarations(*M); return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard)); } /// Insert code into the entry block that stores the stack guard /// variable onto the stack: /// /// entry: /// StackGuardSlot = alloca i8* /// StackGuard = <stack guard> /// call void @llvm.stackprotector(StackGuard, StackGuardSlot) /// /// Returns true if the platform/triple supports the stackprotectorcreate pseudo /// node. static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI, const TargetLoweringBase *TLI, AllocaInst *&AI) { bool SupportsSelectionDAGSP = false; IRBuilder<> B(&F->getEntryBlock().front()); PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext()); AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot"); Value *GuardSlot = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP); B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), {GuardSlot, AI}); return SupportsSelectionDAGSP; } /// InsertStackProtectors - Insert code into the prologue and epilogue of the /// function. /// /// - The prologue code loads and stores the stack guard onto the stack. /// - The epilogue checks the value stored in the prologue against the original /// value. It calls __stack_chk_fail if they differ. bool StackProtector::InsertStackProtectors() { // If the target wants to XOR the frame pointer into the guard value, it's // impossible to emit the check in IR, so the target *must* support stack // protection in SDAG. bool SupportsSelectionDAGSP = TLI->useStackGuardXorFP() || (EnableSelectionDAGSP && !TM->Options.EnableFastISel && !TM->Options.EnableGlobalISel); AllocaInst *AI = nullptr; // Place on stack that stores the stack guard. for (Function::iterator I = F->begin(), E = F->end(); I != E;) { BasicBlock *BB = &*I++; ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()); if (!RI) continue; // Generate prologue instrumentation if not already generated. if (!HasPrologue) { HasPrologue = true; SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI); } // SelectionDAG based code generation. Nothing else needs to be done here. // The epilogue instrumentation is postponed to SelectionDAG. if (SupportsSelectionDAGSP) break; // Find the stack guard slot if the prologue was not created by this pass // itself via a previous call to CreatePrologue(). if (!AI) { const CallInst *SPCall = findStackProtectorIntrinsic(*F); assert(SPCall && "Call to llvm.stackprotector is missing"); AI = cast<AllocaInst>(SPCall->getArgOperand(1)); } // Set HasIRCheck to true, so that SelectionDAG will not generate its own // version. SelectionDAG called 'shouldEmitSDCheck' to check whether // instrumentation has already been generated. HasIRCheck = true; // Generate epilogue instrumentation. The epilogue intrumentation can be // function-based or inlined depending on which mechanism the target is // providing. if (Function *GuardCheck = TLI->getSSPStackGuardCheck(*M)) { // Generate the function-based epilogue instrumentation. // The target provides a guard check function, generate a call to it. IRBuilder<> B(RI); LoadInst *Guard = B.CreateLoad(B.getInt8PtrTy(), AI, true, "Guard"); CallInst *Call = B.CreateCall(GuardCheck, {Guard}); Call->setAttributes(GuardCheck->getAttributes()); Call->setCallingConv(GuardCheck->getCallingConv()); } else { // Generate the epilogue with inline instrumentation. // If we do not support SelectionDAG based tail calls, generate IR level // tail calls. // // For each block with a return instruction, convert this: // // return: // ... // ret ... // // into this: // // return: // ... // %1 = <stack guard> // %2 = load StackGuardSlot // %3 = cmp i1 %1, %2 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk // // SP_return: // ret ... // // CallStackCheckFailBlk: // call void @__stack_chk_fail() // unreachable // Create the FailBB. We duplicate the BB every time since the MI tail // merge pass will merge together all of the various BB into one including // fail BB generated by the stack protector pseudo instruction. BasicBlock *FailBB = CreateFailBB(); // Split the basic block before the return instruction. BasicBlock *NewBB = BB->splitBasicBlock(RI->getIterator(), "SP_return"); // Update the dominator tree if we need to. if (DT && DT->isReachableFromEntry(BB)) { DT->addNewBlock(NewBB, BB); DT->addNewBlock(FailBB, BB); } // Remove default branch instruction to the new BB. BB->getTerminator()->eraseFromParent(); // Move the newly created basic block to the point right after the old // basic block so that it's in the "fall through" position. NewBB->moveAfter(BB); // Generate the stack protector instructions in the old basic block. IRBuilder<> B(BB); Value *Guard = getStackGuard(TLI, M, B); LoadInst *LI2 = B.CreateLoad(B.getInt8PtrTy(), AI, true); Value *Cmp = B.CreateICmpEQ(Guard, LI2); auto SuccessProb = BranchProbabilityInfo::getBranchProbStackProtector(true); auto FailureProb = BranchProbabilityInfo::getBranchProbStackProtector(false); MDNode *Weights = MDBuilder(F->getContext()) .createBranchWeights(SuccessProb.getNumerator(), FailureProb.getNumerator()); B.CreateCondBr(Cmp, NewBB, FailBB, Weights); } } // Return if we didn't modify any basic blocks. i.e., there are no return // statements in the function. return HasPrologue; } /// CreateFailBB - Create a basic block to jump to when the stack protector /// check fails. BasicBlock *StackProtector::CreateFailBB() { LLVMContext &Context = F->getContext(); BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F); IRBuilder<> B(FailBB); B.SetCurrentDebugLocation(DebugLoc::get(0, 0, F->getSubprogram())); if (Trip.isOSOpenBSD()) { FunctionCallee StackChkFail = M->getOrInsertFunction( "__stack_smash_handler", Type::getVoidTy(Context), Type::getInt8PtrTy(Context)); B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH")); } else { FunctionCallee StackChkFail = M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context)); B.CreateCall(StackChkFail, {}); } B.CreateUnreachable(); return FailBB; } bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const { return HasPrologue && !HasIRCheck && isa<ReturnInst>(BB.getTerminator()); } void StackProtector::copyToMachineFrameInfo(MachineFrameInfo &MFI) const { if (Layout.empty()) return; for (int I = 0, E = MFI.getObjectIndexEnd(); I != E; ++I) { if (MFI.isDeadObjectIndex(I)) continue; const AllocaInst *AI = MFI.getObjectAllocation(I); if (!AI) continue; SSPLayoutMap::const_iterator LI = Layout.find(AI); if (LI == Layout.end()) continue; MFI.setObjectSSPLayout(I, LI->second); } }