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 if (const StoreInst *SI = dyn_cast<StoreInst>(U)) { 162 if (AI == SI->getValueOperand()) 163 return true; 164 } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) { 165 if (AI == SI->getOperand(0)) 166 return true; 167 } else if (const CallInst *CI = dyn_cast<CallInst>(U)) { 168 // Ignore intrinsics that are not calls. TODO: Use isLoweredToCall(). 169 if (!isa<DbgInfoIntrinsic>(CI) && !CI->isLifetimeStartOrEnd()) 170 return true; 171 } else if (isa<InvokeInst>(U)) { 172 return true; 173 } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) { 174 if (HasAddressTaken(SI)) 175 return true; 176 } else if (const PHINode *PN = dyn_cast<PHINode>(U)) { 177 // Keep track of what PHI nodes we have already visited to ensure 178 // they are only visited once. 179 if (VisitedPHIs.insert(PN).second) 180 if (HasAddressTaken(PN)) 181 return true; 182 } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) { 183 if (HasAddressTaken(GEP)) 184 return true; 185 } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) { 186 if (HasAddressTaken(BI)) 187 return true; 188 } 189 } 190 return false; 191 } 192 193 /// Search for the first call to the llvm.stackprotector intrinsic and return it 194 /// if present. 195 static const CallInst *findStackProtectorIntrinsic(Function &F) { 196 for (const BasicBlock &BB : F) 197 for (const Instruction &I : BB) 198 if (const CallInst *CI = dyn_cast<CallInst>(&I)) 199 if (CI->getCalledFunction() == 200 Intrinsic::getDeclaration(F.getParent(), Intrinsic::stackprotector)) 201 return CI; 202 return nullptr; 203 } 204 205 /// Check whether or not this function needs a stack protector based 206 /// upon the stack protector level. 207 /// 208 /// We use two heuristics: a standard (ssp) and strong (sspstrong). 209 /// The standard heuristic which will add a guard variable to functions that 210 /// call alloca with a either a variable size or a size >= SSPBufferSize, 211 /// functions with character buffers larger than SSPBufferSize, and functions 212 /// with aggregates containing character buffers larger than SSPBufferSize. The 213 /// strong heuristic will add a guard variables to functions that call alloca 214 /// regardless of size, functions with any buffer regardless of type and size, 215 /// functions with aggregates that contain any buffer regardless of type and 216 /// size, and functions that contain stack-based variables that have had their 217 /// address taken. 218 bool StackProtector::RequiresStackProtector() { 219 bool Strong = false; 220 bool NeedsProtector = false; 221 HasPrologue = findStackProtectorIntrinsic(*F); 222 223 if (F->hasFnAttribute(Attribute::SafeStack)) 224 return false; 225 226 // We are constructing the OptimizationRemarkEmitter on the fly rather than 227 // using the analysis pass to avoid building DominatorTree and LoopInfo which 228 // are not available this late in the IR pipeline. 229 OptimizationRemarkEmitter ORE(F); 230 231 if (F->hasFnAttribute(Attribute::StackProtectReq)) { 232 ORE.emit([&]() { 233 return OptimizationRemark(DEBUG_TYPE, "StackProtectorRequested", F) 234 << "Stack protection applied to function " 235 << ore::NV("Function", F) 236 << " due to a function attribute or command-line switch"; 237 }); 238 NeedsProtector = true; 239 Strong = true; // Use the same heuristic as strong to determine SSPLayout 240 } else if (F->hasFnAttribute(Attribute::StackProtectStrong)) 241 Strong = true; 242 else if (HasPrologue) 243 NeedsProtector = true; 244 else if (!F->hasFnAttribute(Attribute::StackProtect)) 245 return false; 246 247 for (const BasicBlock &BB : *F) { 248 for (const Instruction &I : BB) { 249 if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) { 250 if (AI->isArrayAllocation()) { 251 auto RemarkBuilder = [&]() { 252 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAllocaOrArray", 253 &I) 254 << "Stack protection applied to function " 255 << ore::NV("Function", F) 256 << " due to a call to alloca or use of a variable length " 257 "array"; 258 }; 259 if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) { 260 if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) { 261 // A call to alloca with size >= SSPBufferSize requires 262 // stack protectors. 263 Layout.insert(std::make_pair(AI, 264 MachineFrameInfo::SSPLK_LargeArray)); 265 ORE.emit(RemarkBuilder); 266 NeedsProtector = true; 267 } else if (Strong) { 268 // Require protectors for all alloca calls in strong mode. 269 Layout.insert(std::make_pair(AI, 270 MachineFrameInfo::SSPLK_SmallArray)); 271 ORE.emit(RemarkBuilder); 272 NeedsProtector = true; 273 } 274 } else { 275 // A call to alloca with a variable size requires protectors. 276 Layout.insert(std::make_pair(AI, 277 MachineFrameInfo::SSPLK_LargeArray)); 278 ORE.emit(RemarkBuilder); 279 NeedsProtector = true; 280 } 281 continue; 282 } 283 284 bool IsLarge = false; 285 if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) { 286 Layout.insert(std::make_pair(AI, IsLarge 287 ? MachineFrameInfo::SSPLK_LargeArray 288 : MachineFrameInfo::SSPLK_SmallArray)); 289 ORE.emit([&]() { 290 return OptimizationRemark(DEBUG_TYPE, "StackProtectorBuffer", &I) 291 << "Stack protection applied to function " 292 << ore::NV("Function", F) 293 << " due to a stack allocated buffer or struct containing a " 294 "buffer"; 295 }); 296 NeedsProtector = true; 297 continue; 298 } 299 300 if (Strong && HasAddressTaken(AI)) { 301 ++NumAddrTaken; 302 Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_AddrOf)); 303 ORE.emit([&]() { 304 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAddressTaken", 305 &I) 306 << "Stack protection applied to function " 307 << ore::NV("Function", F) 308 << " due to the address of a local variable being taken"; 309 }); 310 NeedsProtector = true; 311 } 312 } 313 } 314 } 315 316 return NeedsProtector; 317 } 318 319 /// Create a stack guard loading and populate whether SelectionDAG SSP is 320 /// supported. 321 static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M, 322 IRBuilder<> &B, 323 bool *SupportsSelectionDAGSP = nullptr) { 324 if (Value *Guard = TLI->getIRStackGuard(B)) 325 return B.CreateLoad(B.getInt8PtrTy(), Guard, true, "StackGuard"); 326 327 // Use SelectionDAG SSP handling, since there isn't an IR guard. 328 // 329 // This is more or less weird, since we optionally output whether we 330 // should perform a SelectionDAG SP here. The reason is that it's strictly 331 // defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also 332 // mutating. There is no way to get this bit without mutating the IR, so 333 // getting this bit has to happen in this right time. 334 // 335 // We could have define a new function TLI::supportsSelectionDAGSP(), but that 336 // will put more burden on the backends' overriding work, especially when it 337 // actually conveys the same information getIRStackGuard() already gives. 338 if (SupportsSelectionDAGSP) 339 *SupportsSelectionDAGSP = true; 340 TLI->insertSSPDeclarations(*M); 341 return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard)); 342 } 343 344 /// Insert code into the entry block that stores the stack guard 345 /// variable onto the stack: 346 /// 347 /// entry: 348 /// StackGuardSlot = alloca i8* 349 /// StackGuard = <stack guard> 350 /// call void @llvm.stackprotector(StackGuard, StackGuardSlot) 351 /// 352 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo 353 /// node. 354 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI, 355 const TargetLoweringBase *TLI, AllocaInst *&AI) { 356 bool SupportsSelectionDAGSP = false; 357 IRBuilder<> B(&F->getEntryBlock().front()); 358 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext()); 359 AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot"); 360 361 Value *GuardSlot = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP); 362 B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), 363 {GuardSlot, AI}); 364 return SupportsSelectionDAGSP; 365 } 366 367 /// InsertStackProtectors - Insert code into the prologue and epilogue of the 368 /// function. 369 /// 370 /// - The prologue code loads and stores the stack guard onto the stack. 371 /// - The epilogue checks the value stored in the prologue against the original 372 /// value. It calls __stack_chk_fail if they differ. 373 bool StackProtector::InsertStackProtectors() { 374 // If the target wants to XOR the frame pointer into the guard value, it's 375 // impossible to emit the check in IR, so the target *must* support stack 376 // protection in SDAG. 377 bool SupportsSelectionDAGSP = 378 TLI->useStackGuardXorFP() || 379 (EnableSelectionDAGSP && !TM->Options.EnableFastISel && 380 !TM->Options.EnableGlobalISel); 381 AllocaInst *AI = nullptr; // Place on stack that stores the stack guard. 382 383 for (Function::iterator I = F->begin(), E = F->end(); I != E;) { 384 BasicBlock *BB = &*I++; 385 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()); 386 if (!RI) 387 continue; 388 389 // Generate prologue instrumentation if not already generated. 390 if (!HasPrologue) { 391 HasPrologue = true; 392 SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI); 393 } 394 395 // SelectionDAG based code generation. Nothing else needs to be done here. 396 // The epilogue instrumentation is postponed to SelectionDAG. 397 if (SupportsSelectionDAGSP) 398 break; 399 400 // Find the stack guard slot if the prologue was not created by this pass 401 // itself via a previous call to CreatePrologue(). 402 if (!AI) { 403 const CallInst *SPCall = findStackProtectorIntrinsic(*F); 404 assert(SPCall && "Call to llvm.stackprotector is missing"); 405 AI = cast<AllocaInst>(SPCall->getArgOperand(1)); 406 } 407 408 // Set HasIRCheck to true, so that SelectionDAG will not generate its own 409 // version. SelectionDAG called 'shouldEmitSDCheck' to check whether 410 // instrumentation has already been generated. 411 HasIRCheck = true; 412 413 // Generate epilogue instrumentation. The epilogue intrumentation can be 414 // function-based or inlined depending on which mechanism the target is 415 // providing. 416 if (Function *GuardCheck = TLI->getSSPStackGuardCheck(*M)) { 417 // Generate the function-based epilogue instrumentation. 418 // The target provides a guard check function, generate a call to it. 419 IRBuilder<> B(RI); 420 LoadInst *Guard = B.CreateLoad(B.getInt8PtrTy(), AI, true, "Guard"); 421 CallInst *Call = B.CreateCall(GuardCheck, {Guard}); 422 Call->setAttributes(GuardCheck->getAttributes()); 423 Call->setCallingConv(GuardCheck->getCallingConv()); 424 } else { 425 // Generate the epilogue with inline instrumentation. 426 // If we do not support SelectionDAG based tail calls, generate IR level 427 // tail calls. 428 // 429 // For each block with a return instruction, convert this: 430 // 431 // return: 432 // ... 433 // ret ... 434 // 435 // into this: 436 // 437 // return: 438 // ... 439 // %1 = <stack guard> 440 // %2 = load StackGuardSlot 441 // %3 = cmp i1 %1, %2 442 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk 443 // 444 // SP_return: 445 // ret ... 446 // 447 // CallStackCheckFailBlk: 448 // call void @__stack_chk_fail() 449 // unreachable 450 451 // Create the FailBB. We duplicate the BB every time since the MI tail 452 // merge pass will merge together all of the various BB into one including 453 // fail BB generated by the stack protector pseudo instruction. 454 BasicBlock *FailBB = CreateFailBB(); 455 456 // Split the basic block before the return instruction. 457 BasicBlock *NewBB = BB->splitBasicBlock(RI->getIterator(), "SP_return"); 458 459 // Update the dominator tree if we need to. 460 if (DT && DT->isReachableFromEntry(BB)) { 461 DT->addNewBlock(NewBB, BB); 462 DT->addNewBlock(FailBB, BB); 463 } 464 465 // Remove default branch instruction to the new BB. 466 BB->getTerminator()->eraseFromParent(); 467 468 // Move the newly created basic block to the point right after the old 469 // basic block so that it's in the "fall through" position. 470 NewBB->moveAfter(BB); 471 472 // Generate the stack protector instructions in the old basic block. 473 IRBuilder<> B(BB); 474 Value *Guard = getStackGuard(TLI, M, B); 475 LoadInst *LI2 = B.CreateLoad(B.getInt8PtrTy(), AI, true); 476 Value *Cmp = B.CreateICmpEQ(Guard, LI2); 477 auto SuccessProb = 478 BranchProbabilityInfo::getBranchProbStackProtector(true); 479 auto FailureProb = 480 BranchProbabilityInfo::getBranchProbStackProtector(false); 481 MDNode *Weights = MDBuilder(F->getContext()) 482 .createBranchWeights(SuccessProb.getNumerator(), 483 FailureProb.getNumerator()); 484 B.CreateCondBr(Cmp, NewBB, FailBB, Weights); 485 } 486 } 487 488 // Return if we didn't modify any basic blocks. i.e., there are no return 489 // statements in the function. 490 return HasPrologue; 491 } 492 493 /// CreateFailBB - Create a basic block to jump to when the stack protector 494 /// check fails. 495 BasicBlock *StackProtector::CreateFailBB() { 496 LLVMContext &Context = F->getContext(); 497 BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F); 498 IRBuilder<> B(FailBB); 499 B.SetCurrentDebugLocation(DebugLoc::get(0, 0, F->getSubprogram())); 500 if (Trip.isOSOpenBSD()) { 501 FunctionCallee StackChkFail = M->getOrInsertFunction( 502 "__stack_smash_handler", Type::getVoidTy(Context), 503 Type::getInt8PtrTy(Context)); 504 505 B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH")); 506 } else { 507 FunctionCallee StackChkFail = 508 M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context)); 509 510 B.CreateCall(StackChkFail, {}); 511 } 512 B.CreateUnreachable(); 513 return FailBB; 514 } 515 516 bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const { 517 return HasPrologue && !HasIRCheck && dyn_cast<ReturnInst>(BB.getTerminator()); 518 } 519 520 void StackProtector::copyToMachineFrameInfo(MachineFrameInfo &MFI) const { 521 if (Layout.empty()) 522 return; 523 524 for (int I = 0, E = MFI.getObjectIndexEnd(); I != E; ++I) { 525 if (MFI.isDeadObjectIndex(I)) 526 continue; 527 528 const AllocaInst *AI = MFI.getObjectAllocation(I); 529 if (!AI) 530 continue; 531 532 SSPLayoutMap::const_iterator LI = Layout.find(AI); 533 if (LI == Layout.end()) 534 continue; 535 536 MFI.setObjectSSPLayout(I, LI->second); 537 } 538 } 539