//===-- StackProtector.cpp - Stack Protector Insertion --------------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // 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/ValueTracking.h" #include "llvm/CodeGen/Passes.h" #include "llvm/IR/Attributes.h" #include "llvm/IR/Constants.h" #include "llvm/IR/DataLayout.h" #include "llvm/IR/DebugInfo.h" #include "llvm/IR/DerivedTypes.h" #include "llvm/IR/Function.h" #include "llvm/IR/GlobalValue.h" #include "llvm/IR/GlobalVariable.h" #include "llvm/IR/IRBuilder.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/Support/CommandLine.h" #include "llvm/Target/TargetSubtargetInfo.h" #include <cstdlib> 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; INITIALIZE_PASS(StackProtector, "stack-protector", "Insert stack protectors", false, true) FunctionPass *llvm::createStackProtectorPass(const TargetMachine *TM) { return new StackProtector(TM); } StackProtector::SSPLayoutKind StackProtector::getSSPLayout(const AllocaInst *AI) const { return AI ? Layout.lookup(AI) : SSPLK_None; } void StackProtector::adjustForColoring(const AllocaInst *From, const AllocaInst *To) { // When coloring replaces one alloca with another, transfer the SSPLayoutKind // tag from the remapped to the target alloca. The remapped alloca should // have a size smaller than or equal to the replacement alloca. SSPLayoutMap::iterator I = Layout.find(From); if (I != Layout.end()) { SSPLayoutKind Kind = I->second; Layout.erase(I); // Transfer the tag, but make sure that SSPLK_AddrOf does not overwrite // SSPLK_SmallArray or SSPLK_LargeArray, and make sure that // SSPLK_SmallArray does not overwrite SSPLK_LargeArray. I = Layout.find(To); if (I == Layout.end()) Layout.insert(std::make_pair(To, Kind)); else if (I->second != SSPLK_LargeArray && Kind != SSPLK_AddrOf) I->second = Kind; } } bool StackProtector::runOnFunction(Function &Fn) { F = &Fn; M = F->getParent(); DominatorTreeWrapperPass *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>(); DT = DTWP ? &DTWP->getDomTree() : nullptr; 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) { for (const User *U : AI->users()) { if (const StoreInst *SI = dyn_cast<StoreInst>(U)) { if (AI == SI->getValueOperand()) return true; } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) { if (AI == SI->getOperand(0)) return true; } else if (isa<CallInst>(U)) { return true; } else if (isa<InvokeInst>(U)) { return true; } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) { if (HasAddressTaken(SI)) return true; } else if (const PHINode *PN = dyn_cast<PHINode>(U)) { // Keep track of what PHI nodes we have already visited to ensure // they are only visited once. if (VisitedPHIs.insert(PN).second) if (HasAddressTaken(PN)) return true; } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) { if (HasAddressTaken(GEP)) return true; } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) { if (HasAddressTaken(BI)) return true; } } return false; } /// \brief 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; 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)) HasPrologue = true; if (F->hasFnAttribute(Attribute::SafeStack)) return false; if (F->hasFnAttribute(Attribute::StackProtectReq)) { 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()) { // SSP-Strong: Enable protectors for any call to alloca, regardless // of size. if (Strong) return true; 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, SSPLK_LargeArray)); NeedsProtector = true; } else if (Strong) { // Require protectors for all alloca calls in strong mode. Layout.insert(std::make_pair(AI, SSPLK_SmallArray)); NeedsProtector = true; } } else { // A call to alloca with a variable size requires protectors. Layout.insert(std::make_pair(AI, SSPLK_LargeArray)); NeedsProtector = true; } continue; } bool IsLarge = false; if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) { Layout.insert(std::make_pair(AI, IsLarge ? SSPLK_LargeArray : SSPLK_SmallArray)); NeedsProtector = true; continue; } if (Strong && HasAddressTaken(AI)) { ++NumAddrTaken; Layout.insert(std::make_pair(AI, SSPLK_AddrOf)); NeedsProtector = true; } } } } 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(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() { bool SupportsSelectionDAGSP = EnableSelectionDAGSP && !TM->Options.EnableFastISel; 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; // 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 (Value* 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(AI, true, "Guard"); CallInst *Call = B.CreateCall(GuardCheck, {Guard}); llvm::Function *Function = cast<llvm::Function>(GuardCheck); Call->setAttributes(Function->getAttributes()); Call->setCallingConv(Function->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(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()) { Constant *StackChkFail = M->getOrInsertFunction("__stack_smash_handler", Type::getVoidTy(Context), Type::getInt8PtrTy(Context), nullptr); B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH")); } else { Constant *StackChkFail = M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context), nullptr); B.CreateCall(StackChkFail, {}); } B.CreateUnreachable(); return FailBB; } bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const { return HasPrologue && !HasIRCheck && dyn_cast<ReturnInst>(BB.getTerminator()); }