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