1 //===- StackProtector.cpp - Stack Protector Insertion ---------------------===// 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 inserts stack protectors into functions which need them. A variable 10 // with a random value in it is stored onto the stack before the local variables 11 // are allocated. Upon exiting the block, the stored value is checked. If it's 12 // changed, then there was some sort of violation and the program aborts. 13 // 14 //===----------------------------------------------------------------------===// 15 16 #include "llvm/CodeGen/StackProtector.h" 17 #include "llvm/ADT/SmallPtrSet.h" 18 #include "llvm/ADT/Statistic.h" 19 #include "llvm/Analysis/BranchProbabilityInfo.h" 20 #include "llvm/Analysis/EHPersonalities.h" 21 #include "llvm/Analysis/OptimizationRemarkEmitter.h" 22 #include "llvm/CodeGen/Passes.h" 23 #include "llvm/CodeGen/TargetLowering.h" 24 #include "llvm/CodeGen/TargetPassConfig.h" 25 #include "llvm/CodeGen/TargetSubtargetInfo.h" 26 #include "llvm/IR/Attributes.h" 27 #include "llvm/IR/BasicBlock.h" 28 #include "llvm/IR/Constants.h" 29 #include "llvm/IR/DataLayout.h" 30 #include "llvm/IR/DebugInfo.h" 31 #include "llvm/IR/DebugLoc.h" 32 #include "llvm/IR/DerivedTypes.h" 33 #include "llvm/IR/Dominators.h" 34 #include "llvm/IR/Function.h" 35 #include "llvm/IR/IRBuilder.h" 36 #include "llvm/IR/Instruction.h" 37 #include "llvm/IR/Instructions.h" 38 #include "llvm/IR/IntrinsicInst.h" 39 #include "llvm/IR/Intrinsics.h" 40 #include "llvm/IR/MDBuilder.h" 41 #include "llvm/IR/Module.h" 42 #include "llvm/IR/Type.h" 43 #include "llvm/IR/User.h" 44 #include "llvm/InitializePasses.h" 45 #include "llvm/Pass.h" 46 #include "llvm/Support/Casting.h" 47 #include "llvm/Support/CommandLine.h" 48 #include "llvm/Target/TargetMachine.h" 49 #include "llvm/Target/TargetOptions.h" 50 #include <utility> 51 52 using namespace llvm; 53 54 #define DEBUG_TYPE "stack-protector" 55 56 STATISTIC(NumFunProtected, "Number of functions protected"); 57 STATISTIC(NumAddrTaken, "Number of local variables that have their address" 58 " taken."); 59 60 static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp", 61 cl::init(true), cl::Hidden); 62 63 char StackProtector::ID = 0; 64 65 StackProtector::StackProtector() : FunctionPass(ID), SSPBufferSize(8) { 66 initializeStackProtectorPass(*PassRegistry::getPassRegistry()); 67 } 68 69 INITIALIZE_PASS_BEGIN(StackProtector, DEBUG_TYPE, 70 "Insert stack protectors", false, true) 71 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig) 72 INITIALIZE_PASS_END(StackProtector, DEBUG_TYPE, 73 "Insert stack protectors", false, true) 74 75 FunctionPass *llvm::createStackProtectorPass() { return new StackProtector(); } 76 77 void StackProtector::getAnalysisUsage(AnalysisUsage &AU) const { 78 AU.addRequired<TargetPassConfig>(); 79 AU.addPreserved<DominatorTreeWrapperPass>(); 80 } 81 82 bool StackProtector::runOnFunction(Function &Fn) { 83 F = &Fn; 84 M = F->getParent(); 85 DominatorTreeWrapperPass *DTWP = 86 getAnalysisIfAvailable<DominatorTreeWrapperPass>(); 87 DT = DTWP ? &DTWP->getDomTree() : nullptr; 88 TM = &getAnalysis<TargetPassConfig>().getTM<TargetMachine>(); 89 Trip = TM->getTargetTriple(); 90 TLI = TM->getSubtargetImpl(Fn)->getTargetLowering(); 91 HasPrologue = false; 92 HasIRCheck = false; 93 94 Attribute Attr = Fn.getFnAttribute("stack-protector-buffer-size"); 95 if (Attr.isStringAttribute() && 96 Attr.getValueAsString().getAsInteger(10, SSPBufferSize)) 97 return false; // Invalid integer string 98 99 if (!RequiresStackProtector()) 100 return false; 101 102 // TODO(etienneb): Functions with funclets are not correctly supported now. 103 // Do nothing if this is funclet-based personality. 104 if (Fn.hasPersonalityFn()) { 105 EHPersonality Personality = classifyEHPersonality(Fn.getPersonalityFn()); 106 if (isFuncletEHPersonality(Personality)) 107 return false; 108 } 109 110 ++NumFunProtected; 111 return InsertStackProtectors(); 112 } 113 114 /// \param [out] IsLarge is set to true if a protectable array is found and 115 /// it is "large" ( >= ssp-buffer-size). In the case of a structure with 116 /// multiple arrays, this gets set if any of them is large. 117 bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge, 118 bool Strong, 119 bool InStruct) const { 120 if (!Ty) 121 return false; 122 if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) { 123 if (!AT->getElementType()->isIntegerTy(8)) { 124 // If we're on a non-Darwin platform or we're inside of a structure, don't 125 // add stack protectors unless the array is a character array. 126 // However, in strong mode any array, regardless of type and size, 127 // triggers a protector. 128 if (!Strong && (InStruct || !Trip.isOSDarwin())) 129 return false; 130 } 131 132 // If an array has more than SSPBufferSize bytes of allocated space, then we 133 // emit stack protectors. 134 if (SSPBufferSize <= M->getDataLayout().getTypeAllocSize(AT)) { 135 IsLarge = true; 136 return true; 137 } 138 139 if (Strong) 140 // Require a protector for all arrays in strong mode 141 return true; 142 } 143 144 const StructType *ST = dyn_cast<StructType>(Ty); 145 if (!ST) 146 return false; 147 148 bool NeedsProtector = false; 149 for (StructType::element_iterator I = ST->element_begin(), 150 E = ST->element_end(); 151 I != E; ++I) 152 if (ContainsProtectableArray(*I, IsLarge, Strong, true)) { 153 // If the element is a protectable array and is large (>= SSPBufferSize) 154 // then we are done. If the protectable array is not large, then 155 // keep looking in case a subsequent element is a large array. 156 if (IsLarge) 157 return true; 158 NeedsProtector = true; 159 } 160 161 return NeedsProtector; 162 } 163 164 bool StackProtector::HasAddressTaken(const Instruction *AI) { 165 for (const User *U : AI->users()) { 166 const auto *I = cast<Instruction>(U); 167 switch (I->getOpcode()) { 168 case Instruction::Store: 169 if (AI == cast<StoreInst>(I)->getValueOperand()) 170 return true; 171 break; 172 case Instruction::AtomicCmpXchg: 173 // cmpxchg conceptually includes both a load and store from the same 174 // location. So, like store, the value being stored is what matters. 175 if (AI == cast<AtomicCmpXchgInst>(I)->getNewValOperand()) 176 return true; 177 break; 178 case Instruction::PtrToInt: 179 if (AI == cast<PtrToIntInst>(I)->getOperand(0)) 180 return true; 181 break; 182 case Instruction::Call: { 183 // Ignore intrinsics that do not become real instructions. 184 // TODO: Narrow this to intrinsics that have store-like effects. 185 const auto *CI = cast<CallInst>(I); 186 if (!isa<DbgInfoIntrinsic>(CI) && !CI->isLifetimeStartOrEnd()) 187 return true; 188 break; 189 } 190 case Instruction::Invoke: 191 return true; 192 case Instruction::BitCast: 193 case Instruction::GetElementPtr: 194 case Instruction::Select: 195 case Instruction::AddrSpaceCast: 196 if (HasAddressTaken(I)) 197 return true; 198 break; 199 case Instruction::PHI: { 200 // Keep track of what PHI nodes we have already visited to ensure 201 // they are only visited once. 202 const auto *PN = cast<PHINode>(I); 203 if (VisitedPHIs.insert(PN).second) 204 if (HasAddressTaken(PN)) 205 return true; 206 break; 207 } 208 case Instruction::Load: 209 case Instruction::AtomicRMW: 210 case Instruction::Ret: 211 // These instructions take an address operand, but have load-like or 212 // other innocuous behavior that should not trigger a stack protector. 213 // atomicrmw conceptually has both load and store semantics, but the 214 // value being stored must be integer; so if a pointer is being stored, 215 // we'll catch it in the PtrToInt case above. 216 break; 217 default: 218 // Conservatively return true for any instruction that takes an address 219 // operand, but is not handled above. 220 return true; 221 } 222 } 223 return false; 224 } 225 226 /// Search for the first call to the llvm.stackprotector intrinsic and return it 227 /// if present. 228 static const CallInst *findStackProtectorIntrinsic(Function &F) { 229 for (const BasicBlock &BB : F) 230 for (const Instruction &I : BB) 231 if (const CallInst *CI = dyn_cast<CallInst>(&I)) 232 if (CI->getCalledFunction() == 233 Intrinsic::getDeclaration(F.getParent(), Intrinsic::stackprotector)) 234 return CI; 235 return nullptr; 236 } 237 238 /// Check whether or not this function needs a stack protector based 239 /// upon the stack protector level. 240 /// 241 /// We use two heuristics: a standard (ssp) and strong (sspstrong). 242 /// The standard heuristic which will add a guard variable to functions that 243 /// call alloca with a either a variable size or a size >= SSPBufferSize, 244 /// functions with character buffers larger than SSPBufferSize, and functions 245 /// with aggregates containing character buffers larger than SSPBufferSize. The 246 /// strong heuristic will add a guard variables to functions that call alloca 247 /// regardless of size, functions with any buffer regardless of type and size, 248 /// functions with aggregates that contain any buffer regardless of type and 249 /// size, and functions that contain stack-based variables that have had their 250 /// address taken. 251 bool StackProtector::RequiresStackProtector() { 252 bool Strong = false; 253 bool NeedsProtector = false; 254 HasPrologue = findStackProtectorIntrinsic(*F); 255 256 if (F->hasFnAttribute(Attribute::SafeStack)) 257 return false; 258 259 // We are constructing the OptimizationRemarkEmitter on the fly rather than 260 // using the analysis pass to avoid building DominatorTree and LoopInfo which 261 // are not available this late in the IR pipeline. 262 OptimizationRemarkEmitter ORE(F); 263 264 if (F->hasFnAttribute(Attribute::StackProtectReq)) { 265 ORE.emit([&]() { 266 return OptimizationRemark(DEBUG_TYPE, "StackProtectorRequested", F) 267 << "Stack protection applied to function " 268 << ore::NV("Function", F) 269 << " due to a function attribute or command-line switch"; 270 }); 271 NeedsProtector = true; 272 Strong = true; // Use the same heuristic as strong to determine SSPLayout 273 } else if (F->hasFnAttribute(Attribute::StackProtectStrong)) 274 Strong = true; 275 else if (HasPrologue) 276 NeedsProtector = true; 277 else if (!F->hasFnAttribute(Attribute::StackProtect)) 278 return false; 279 280 for (const BasicBlock &BB : *F) { 281 for (const Instruction &I : BB) { 282 if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) { 283 if (AI->isArrayAllocation()) { 284 auto RemarkBuilder = [&]() { 285 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAllocaOrArray", 286 &I) 287 << "Stack protection applied to function " 288 << ore::NV("Function", F) 289 << " due to a call to alloca or use of a variable length " 290 "array"; 291 }; 292 if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) { 293 if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) { 294 // A call to alloca with size >= SSPBufferSize requires 295 // stack protectors. 296 Layout.insert(std::make_pair(AI, 297 MachineFrameInfo::SSPLK_LargeArray)); 298 ORE.emit(RemarkBuilder); 299 NeedsProtector = true; 300 } else if (Strong) { 301 // Require protectors for all alloca calls in strong mode. 302 Layout.insert(std::make_pair(AI, 303 MachineFrameInfo::SSPLK_SmallArray)); 304 ORE.emit(RemarkBuilder); 305 NeedsProtector = true; 306 } 307 } else { 308 // A call to alloca with a variable size requires protectors. 309 Layout.insert(std::make_pair(AI, 310 MachineFrameInfo::SSPLK_LargeArray)); 311 ORE.emit(RemarkBuilder); 312 NeedsProtector = true; 313 } 314 continue; 315 } 316 317 bool IsLarge = false; 318 if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) { 319 Layout.insert(std::make_pair(AI, IsLarge 320 ? MachineFrameInfo::SSPLK_LargeArray 321 : MachineFrameInfo::SSPLK_SmallArray)); 322 ORE.emit([&]() { 323 return OptimizationRemark(DEBUG_TYPE, "StackProtectorBuffer", &I) 324 << "Stack protection applied to function " 325 << ore::NV("Function", F) 326 << " due to a stack allocated buffer or struct containing a " 327 "buffer"; 328 }); 329 NeedsProtector = true; 330 continue; 331 } 332 333 if (Strong && HasAddressTaken(AI)) { 334 ++NumAddrTaken; 335 Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_AddrOf)); 336 ORE.emit([&]() { 337 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAddressTaken", 338 &I) 339 << "Stack protection applied to function " 340 << ore::NV("Function", F) 341 << " due to the address of a local variable being taken"; 342 }); 343 NeedsProtector = true; 344 } 345 } 346 } 347 } 348 349 return NeedsProtector; 350 } 351 352 /// Create a stack guard loading and populate whether SelectionDAG SSP is 353 /// supported. 354 static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M, 355 IRBuilder<> &B, 356 bool *SupportsSelectionDAGSP = nullptr) { 357 if (Value *Guard = TLI->getIRStackGuard(B)) 358 return B.CreateLoad(B.getInt8PtrTy(), Guard, true, "StackGuard"); 359 360 // Use SelectionDAG SSP handling, since there isn't an IR guard. 361 // 362 // This is more or less weird, since we optionally output whether we 363 // should perform a SelectionDAG SP here. The reason is that it's strictly 364 // defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also 365 // mutating. There is no way to get this bit without mutating the IR, so 366 // getting this bit has to happen in this right time. 367 // 368 // We could have define a new function TLI::supportsSelectionDAGSP(), but that 369 // will put more burden on the backends' overriding work, especially when it 370 // actually conveys the same information getIRStackGuard() already gives. 371 if (SupportsSelectionDAGSP) 372 *SupportsSelectionDAGSP = true; 373 TLI->insertSSPDeclarations(*M); 374 return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard)); 375 } 376 377 /// Insert code into the entry block that stores the stack guard 378 /// variable onto the stack: 379 /// 380 /// entry: 381 /// StackGuardSlot = alloca i8* 382 /// StackGuard = <stack guard> 383 /// call void @llvm.stackprotector(StackGuard, StackGuardSlot) 384 /// 385 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo 386 /// node. 387 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI, 388 const TargetLoweringBase *TLI, AllocaInst *&AI) { 389 bool SupportsSelectionDAGSP = false; 390 IRBuilder<> B(&F->getEntryBlock().front()); 391 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext()); 392 AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot"); 393 394 Value *GuardSlot = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP); 395 B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), 396 {GuardSlot, AI}); 397 return SupportsSelectionDAGSP; 398 } 399 400 /// InsertStackProtectors - Insert code into the prologue and epilogue of the 401 /// function. 402 /// 403 /// - The prologue code loads and stores the stack guard onto the stack. 404 /// - The epilogue checks the value stored in the prologue against the original 405 /// value. It calls __stack_chk_fail if they differ. 406 bool StackProtector::InsertStackProtectors() { 407 // If the target wants to XOR the frame pointer into the guard value, it's 408 // impossible to emit the check in IR, so the target *must* support stack 409 // protection in SDAG. 410 bool SupportsSelectionDAGSP = 411 TLI->useStackGuardXorFP() || 412 (EnableSelectionDAGSP && !TM->Options.EnableFastISel && 413 !TM->Options.EnableGlobalISel); 414 AllocaInst *AI = nullptr; // Place on stack that stores the stack guard. 415 416 for (Function::iterator I = F->begin(), E = F->end(); I != E;) { 417 BasicBlock *BB = &*I++; 418 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()); 419 if (!RI) 420 continue; 421 422 // Generate prologue instrumentation if not already generated. 423 if (!HasPrologue) { 424 HasPrologue = true; 425 SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI); 426 } 427 428 // SelectionDAG based code generation. Nothing else needs to be done here. 429 // The epilogue instrumentation is postponed to SelectionDAG. 430 if (SupportsSelectionDAGSP) 431 break; 432 433 // Find the stack guard slot if the prologue was not created by this pass 434 // itself via a previous call to CreatePrologue(). 435 if (!AI) { 436 const CallInst *SPCall = findStackProtectorIntrinsic(*F); 437 assert(SPCall && "Call to llvm.stackprotector is missing"); 438 AI = cast<AllocaInst>(SPCall->getArgOperand(1)); 439 } 440 441 // Set HasIRCheck to true, so that SelectionDAG will not generate its own 442 // version. SelectionDAG called 'shouldEmitSDCheck' to check whether 443 // instrumentation has already been generated. 444 HasIRCheck = true; 445 446 // Generate epilogue instrumentation. The epilogue intrumentation can be 447 // function-based or inlined depending on which mechanism the target is 448 // providing. 449 if (Function *GuardCheck = TLI->getSSPStackGuardCheck(*M)) { 450 // Generate the function-based epilogue instrumentation. 451 // The target provides a guard check function, generate a call to it. 452 IRBuilder<> B(RI); 453 LoadInst *Guard = B.CreateLoad(B.getInt8PtrTy(), AI, true, "Guard"); 454 CallInst *Call = B.CreateCall(GuardCheck, {Guard}); 455 Call->setAttributes(GuardCheck->getAttributes()); 456 Call->setCallingConv(GuardCheck->getCallingConv()); 457 } else { 458 // Generate the epilogue with inline instrumentation. 459 // If we do not support SelectionDAG based tail calls, generate IR level 460 // tail calls. 461 // 462 // For each block with a return instruction, convert this: 463 // 464 // return: 465 // ... 466 // ret ... 467 // 468 // into this: 469 // 470 // return: 471 // ... 472 // %1 = <stack guard> 473 // %2 = load StackGuardSlot 474 // %3 = cmp i1 %1, %2 475 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk 476 // 477 // SP_return: 478 // ret ... 479 // 480 // CallStackCheckFailBlk: 481 // call void @__stack_chk_fail() 482 // unreachable 483 484 // Create the FailBB. We duplicate the BB every time since the MI tail 485 // merge pass will merge together all of the various BB into one including 486 // fail BB generated by the stack protector pseudo instruction. 487 BasicBlock *FailBB = CreateFailBB(); 488 489 // Split the basic block before the return instruction. 490 BasicBlock *NewBB = BB->splitBasicBlock(RI->getIterator(), "SP_return"); 491 492 // Update the dominator tree if we need to. 493 if (DT && DT->isReachableFromEntry(BB)) { 494 DT->addNewBlock(NewBB, BB); 495 DT->addNewBlock(FailBB, BB); 496 } 497 498 // Remove default branch instruction to the new BB. 499 BB->getTerminator()->eraseFromParent(); 500 501 // Move the newly created basic block to the point right after the old 502 // basic block so that it's in the "fall through" position. 503 NewBB->moveAfter(BB); 504 505 // Generate the stack protector instructions in the old basic block. 506 IRBuilder<> B(BB); 507 Value *Guard = getStackGuard(TLI, M, B); 508 LoadInst *LI2 = B.CreateLoad(B.getInt8PtrTy(), AI, true); 509 Value *Cmp = B.CreateICmpEQ(Guard, LI2); 510 auto SuccessProb = 511 BranchProbabilityInfo::getBranchProbStackProtector(true); 512 auto FailureProb = 513 BranchProbabilityInfo::getBranchProbStackProtector(false); 514 MDNode *Weights = MDBuilder(F->getContext()) 515 .createBranchWeights(SuccessProb.getNumerator(), 516 FailureProb.getNumerator()); 517 B.CreateCondBr(Cmp, NewBB, FailBB, Weights); 518 } 519 } 520 521 // Return if we didn't modify any basic blocks. i.e., there are no return 522 // statements in the function. 523 return HasPrologue; 524 } 525 526 /// CreateFailBB - Create a basic block to jump to when the stack protector 527 /// check fails. 528 BasicBlock *StackProtector::CreateFailBB() { 529 LLVMContext &Context = F->getContext(); 530 BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F); 531 IRBuilder<> B(FailBB); 532 B.SetCurrentDebugLocation(DebugLoc::get(0, 0, F->getSubprogram())); 533 if (Trip.isOSOpenBSD()) { 534 FunctionCallee StackChkFail = M->getOrInsertFunction( 535 "__stack_smash_handler", Type::getVoidTy(Context), 536 Type::getInt8PtrTy(Context)); 537 538 B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH")); 539 } else { 540 FunctionCallee StackChkFail = 541 M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context)); 542 543 B.CreateCall(StackChkFail, {}); 544 } 545 B.CreateUnreachable(); 546 return FailBB; 547 } 548 549 bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const { 550 return HasPrologue && !HasIRCheck && isa<ReturnInst>(BB.getTerminator()); 551 } 552 553 void StackProtector::copyToMachineFrameInfo(MachineFrameInfo &MFI) const { 554 if (Layout.empty()) 555 return; 556 557 for (int I = 0, E = MFI.getObjectIndexEnd(); I != E; ++I) { 558 if (MFI.isDeadObjectIndex(I)) 559 continue; 560 561 const AllocaInst *AI = MFI.getObjectAllocation(I); 562 if (!AI) 563 continue; 564 565 SSPLayoutMap::const_iterator LI = Layout.find(AI); 566 if (LI == Layout.end()) 567 continue; 568 569 MFI.setObjectSSPLayout(I, LI->second); 570 } 571 } 572