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::PtrToInt: 168 if (AI == cast<PtrToIntInst>(I)->getOperand(0)) 169 return true; 170 break; 171 case Instruction::Call: { 172 // Ignore intrinsics that are not calls. TODO: Use isLoweredToCall(). 173 const auto *CI = cast<CallInst>(I); 174 if (!isa<DbgInfoIntrinsic>(CI) && !CI->isLifetimeStartOrEnd()) 175 return true; 176 break; 177 } 178 case Instruction::Invoke: 179 return true; 180 case Instruction::BitCast: 181 case Instruction::GetElementPtr: 182 case Instruction::Select: 183 if (HasAddressTaken(I)) 184 return true; 185 break; 186 case Instruction::PHI: { 187 // Keep track of what PHI nodes we have already visited to ensure 188 // they are only visited once. 189 const auto *PN = cast<PHINode>(I); 190 if (VisitedPHIs.insert(PN).second) 191 if (HasAddressTaken(PN)) 192 return true; 193 break; 194 } 195 default: 196 break; 197 } 198 } 199 return false; 200 } 201 202 /// Search for the first call to the llvm.stackprotector intrinsic and return it 203 /// if present. 204 static const CallInst *findStackProtectorIntrinsic(Function &F) { 205 for (const BasicBlock &BB : F) 206 for (const Instruction &I : BB) 207 if (const CallInst *CI = dyn_cast<CallInst>(&I)) 208 if (CI->getCalledFunction() == 209 Intrinsic::getDeclaration(F.getParent(), Intrinsic::stackprotector)) 210 return CI; 211 return nullptr; 212 } 213 214 /// Check whether or not this function needs a stack protector based 215 /// upon the stack protector level. 216 /// 217 /// We use two heuristics: a standard (ssp) and strong (sspstrong). 218 /// The standard heuristic which will add a guard variable to functions that 219 /// call alloca with a either a variable size or a size >= SSPBufferSize, 220 /// functions with character buffers larger than SSPBufferSize, and functions 221 /// with aggregates containing character buffers larger than SSPBufferSize. The 222 /// strong heuristic will add a guard variables to functions that call alloca 223 /// regardless of size, functions with any buffer regardless of type and size, 224 /// functions with aggregates that contain any buffer regardless of type and 225 /// size, and functions that contain stack-based variables that have had their 226 /// address taken. 227 bool StackProtector::RequiresStackProtector() { 228 bool Strong = false; 229 bool NeedsProtector = false; 230 HasPrologue = findStackProtectorIntrinsic(*F); 231 232 if (F->hasFnAttribute(Attribute::SafeStack)) 233 return false; 234 235 // We are constructing the OptimizationRemarkEmitter on the fly rather than 236 // using the analysis pass to avoid building DominatorTree and LoopInfo which 237 // are not available this late in the IR pipeline. 238 OptimizationRemarkEmitter ORE(F); 239 240 if (F->hasFnAttribute(Attribute::StackProtectReq)) { 241 ORE.emit([&]() { 242 return OptimizationRemark(DEBUG_TYPE, "StackProtectorRequested", F) 243 << "Stack protection applied to function " 244 << ore::NV("Function", F) 245 << " due to a function attribute or command-line switch"; 246 }); 247 NeedsProtector = true; 248 Strong = true; // Use the same heuristic as strong to determine SSPLayout 249 } else if (F->hasFnAttribute(Attribute::StackProtectStrong)) 250 Strong = true; 251 else if (HasPrologue) 252 NeedsProtector = true; 253 else if (!F->hasFnAttribute(Attribute::StackProtect)) 254 return false; 255 256 for (const BasicBlock &BB : *F) { 257 for (const Instruction &I : BB) { 258 if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) { 259 if (AI->isArrayAllocation()) { 260 auto RemarkBuilder = [&]() { 261 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAllocaOrArray", 262 &I) 263 << "Stack protection applied to function " 264 << ore::NV("Function", F) 265 << " due to a call to alloca or use of a variable length " 266 "array"; 267 }; 268 if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) { 269 if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) { 270 // A call to alloca with size >= SSPBufferSize requires 271 // stack protectors. 272 Layout.insert(std::make_pair(AI, 273 MachineFrameInfo::SSPLK_LargeArray)); 274 ORE.emit(RemarkBuilder); 275 NeedsProtector = true; 276 } else if (Strong) { 277 // Require protectors for all alloca calls in strong mode. 278 Layout.insert(std::make_pair(AI, 279 MachineFrameInfo::SSPLK_SmallArray)); 280 ORE.emit(RemarkBuilder); 281 NeedsProtector = true; 282 } 283 } else { 284 // A call to alloca with a variable size requires protectors. 285 Layout.insert(std::make_pair(AI, 286 MachineFrameInfo::SSPLK_LargeArray)); 287 ORE.emit(RemarkBuilder); 288 NeedsProtector = true; 289 } 290 continue; 291 } 292 293 bool IsLarge = false; 294 if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) { 295 Layout.insert(std::make_pair(AI, IsLarge 296 ? MachineFrameInfo::SSPLK_LargeArray 297 : MachineFrameInfo::SSPLK_SmallArray)); 298 ORE.emit([&]() { 299 return OptimizationRemark(DEBUG_TYPE, "StackProtectorBuffer", &I) 300 << "Stack protection applied to function " 301 << ore::NV("Function", F) 302 << " due to a stack allocated buffer or struct containing a " 303 "buffer"; 304 }); 305 NeedsProtector = true; 306 continue; 307 } 308 309 if (Strong && HasAddressTaken(AI)) { 310 ++NumAddrTaken; 311 Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_AddrOf)); 312 ORE.emit([&]() { 313 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAddressTaken", 314 &I) 315 << "Stack protection applied to function " 316 << ore::NV("Function", F) 317 << " due to the address of a local variable being taken"; 318 }); 319 NeedsProtector = true; 320 } 321 } 322 } 323 } 324 325 return NeedsProtector; 326 } 327 328 /// Create a stack guard loading and populate whether SelectionDAG SSP is 329 /// supported. 330 static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M, 331 IRBuilder<> &B, 332 bool *SupportsSelectionDAGSP = nullptr) { 333 if (Value *Guard = TLI->getIRStackGuard(B)) 334 return B.CreateLoad(B.getInt8PtrTy(), Guard, true, "StackGuard"); 335 336 // Use SelectionDAG SSP handling, since there isn't an IR guard. 337 // 338 // This is more or less weird, since we optionally output whether we 339 // should perform a SelectionDAG SP here. The reason is that it's strictly 340 // defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also 341 // mutating. There is no way to get this bit without mutating the IR, so 342 // getting this bit has to happen in this right time. 343 // 344 // We could have define a new function TLI::supportsSelectionDAGSP(), but that 345 // will put more burden on the backends' overriding work, especially when it 346 // actually conveys the same information getIRStackGuard() already gives. 347 if (SupportsSelectionDAGSP) 348 *SupportsSelectionDAGSP = true; 349 TLI->insertSSPDeclarations(*M); 350 return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard)); 351 } 352 353 /// Insert code into the entry block that stores the stack guard 354 /// variable onto the stack: 355 /// 356 /// entry: 357 /// StackGuardSlot = alloca i8* 358 /// StackGuard = <stack guard> 359 /// call void @llvm.stackprotector(StackGuard, StackGuardSlot) 360 /// 361 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo 362 /// node. 363 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI, 364 const TargetLoweringBase *TLI, AllocaInst *&AI) { 365 bool SupportsSelectionDAGSP = false; 366 IRBuilder<> B(&F->getEntryBlock().front()); 367 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext()); 368 AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot"); 369 370 Value *GuardSlot = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP); 371 B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), 372 {GuardSlot, AI}); 373 return SupportsSelectionDAGSP; 374 } 375 376 /// InsertStackProtectors - Insert code into the prologue and epilogue of the 377 /// function. 378 /// 379 /// - The prologue code loads and stores the stack guard onto the stack. 380 /// - The epilogue checks the value stored in the prologue against the original 381 /// value. It calls __stack_chk_fail if they differ. 382 bool StackProtector::InsertStackProtectors() { 383 // If the target wants to XOR the frame pointer into the guard value, it's 384 // impossible to emit the check in IR, so the target *must* support stack 385 // protection in SDAG. 386 bool SupportsSelectionDAGSP = 387 TLI->useStackGuardXorFP() || 388 (EnableSelectionDAGSP && !TM->Options.EnableFastISel && 389 !TM->Options.EnableGlobalISel); 390 AllocaInst *AI = nullptr; // Place on stack that stores the stack guard. 391 392 for (Function::iterator I = F->begin(), E = F->end(); I != E;) { 393 BasicBlock *BB = &*I++; 394 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()); 395 if (!RI) 396 continue; 397 398 // Generate prologue instrumentation if not already generated. 399 if (!HasPrologue) { 400 HasPrologue = true; 401 SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI); 402 } 403 404 // SelectionDAG based code generation. Nothing else needs to be done here. 405 // The epilogue instrumentation is postponed to SelectionDAG. 406 if (SupportsSelectionDAGSP) 407 break; 408 409 // Find the stack guard slot if the prologue was not created by this pass 410 // itself via a previous call to CreatePrologue(). 411 if (!AI) { 412 const CallInst *SPCall = findStackProtectorIntrinsic(*F); 413 assert(SPCall && "Call to llvm.stackprotector is missing"); 414 AI = cast<AllocaInst>(SPCall->getArgOperand(1)); 415 } 416 417 // Set HasIRCheck to true, so that SelectionDAG will not generate its own 418 // version. SelectionDAG called 'shouldEmitSDCheck' to check whether 419 // instrumentation has already been generated. 420 HasIRCheck = true; 421 422 // Generate epilogue instrumentation. The epilogue intrumentation can be 423 // function-based or inlined depending on which mechanism the target is 424 // providing. 425 if (Function *GuardCheck = TLI->getSSPStackGuardCheck(*M)) { 426 // Generate the function-based epilogue instrumentation. 427 // The target provides a guard check function, generate a call to it. 428 IRBuilder<> B(RI); 429 LoadInst *Guard = B.CreateLoad(B.getInt8PtrTy(), AI, true, "Guard"); 430 CallInst *Call = B.CreateCall(GuardCheck, {Guard}); 431 Call->setAttributes(GuardCheck->getAttributes()); 432 Call->setCallingConv(GuardCheck->getCallingConv()); 433 } else { 434 // Generate the epilogue with inline instrumentation. 435 // If we do not support SelectionDAG based tail calls, generate IR level 436 // tail calls. 437 // 438 // For each block with a return instruction, convert this: 439 // 440 // return: 441 // ... 442 // ret ... 443 // 444 // into this: 445 // 446 // return: 447 // ... 448 // %1 = <stack guard> 449 // %2 = load StackGuardSlot 450 // %3 = cmp i1 %1, %2 451 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk 452 // 453 // SP_return: 454 // ret ... 455 // 456 // CallStackCheckFailBlk: 457 // call void @__stack_chk_fail() 458 // unreachable 459 460 // Create the FailBB. We duplicate the BB every time since the MI tail 461 // merge pass will merge together all of the various BB into one including 462 // fail BB generated by the stack protector pseudo instruction. 463 BasicBlock *FailBB = CreateFailBB(); 464 465 // Split the basic block before the return instruction. 466 BasicBlock *NewBB = BB->splitBasicBlock(RI->getIterator(), "SP_return"); 467 468 // Update the dominator tree if we need to. 469 if (DT && DT->isReachableFromEntry(BB)) { 470 DT->addNewBlock(NewBB, BB); 471 DT->addNewBlock(FailBB, BB); 472 } 473 474 // Remove default branch instruction to the new BB. 475 BB->getTerminator()->eraseFromParent(); 476 477 // Move the newly created basic block to the point right after the old 478 // basic block so that it's in the "fall through" position. 479 NewBB->moveAfter(BB); 480 481 // Generate the stack protector instructions in the old basic block. 482 IRBuilder<> B(BB); 483 Value *Guard = getStackGuard(TLI, M, B); 484 LoadInst *LI2 = B.CreateLoad(B.getInt8PtrTy(), AI, true); 485 Value *Cmp = B.CreateICmpEQ(Guard, LI2); 486 auto SuccessProb = 487 BranchProbabilityInfo::getBranchProbStackProtector(true); 488 auto FailureProb = 489 BranchProbabilityInfo::getBranchProbStackProtector(false); 490 MDNode *Weights = MDBuilder(F->getContext()) 491 .createBranchWeights(SuccessProb.getNumerator(), 492 FailureProb.getNumerator()); 493 B.CreateCondBr(Cmp, NewBB, FailBB, Weights); 494 } 495 } 496 497 // Return if we didn't modify any basic blocks. i.e., there are no return 498 // statements in the function. 499 return HasPrologue; 500 } 501 502 /// CreateFailBB - Create a basic block to jump to when the stack protector 503 /// check fails. 504 BasicBlock *StackProtector::CreateFailBB() { 505 LLVMContext &Context = F->getContext(); 506 BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F); 507 IRBuilder<> B(FailBB); 508 B.SetCurrentDebugLocation(DebugLoc::get(0, 0, F->getSubprogram())); 509 if (Trip.isOSOpenBSD()) { 510 FunctionCallee StackChkFail = M->getOrInsertFunction( 511 "__stack_smash_handler", Type::getVoidTy(Context), 512 Type::getInt8PtrTy(Context)); 513 514 B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH")); 515 } else { 516 FunctionCallee StackChkFail = 517 M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context)); 518 519 B.CreateCall(StackChkFail, {}); 520 } 521 B.CreateUnreachable(); 522 return FailBB; 523 } 524 525 bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const { 526 return HasPrologue && !HasIRCheck && isa<ReturnInst>(BB.getTerminator()); 527 } 528 529 void StackProtector::copyToMachineFrameInfo(MachineFrameInfo &MFI) const { 530 if (Layout.empty()) 531 return; 532 533 for (int I = 0, E = MFI.getObjectIndexEnd(); I != E; ++I) { 534 if (MFI.isDeadObjectIndex(I)) 535 continue; 536 537 const AllocaInst *AI = MFI.getObjectAllocation(I); 538 if (!AI) 539 continue; 540 541 SSPLayoutMap::const_iterator LI = Layout.find(AI); 542 if (LI == Layout.end()) 543 continue; 544 545 MFI.setObjectSSPLayout(I, LI->second); 546 } 547 } 548