1 //===- StackProtector.cpp - Stack Protector Insertion ---------------------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This pass inserts stack protectors into functions which need them. A variable 11 // with a random value in it is stored onto the stack before the local variables 12 // are allocated. Upon exiting the block, the stored value is checked. If it's 13 // changed, then there was some sort of violation and the program aborts. 14 // 15 //===----------------------------------------------------------------------===// 16 17 #include "llvm/CodeGen/StackProtector.h" 18 #include "llvm/ADT/SmallPtrSet.h" 19 #include "llvm/ADT/Statistic.h" 20 #include "llvm/Analysis/BranchProbabilityInfo.h" 21 #include "llvm/Analysis/EHPersonalities.h" 22 #include "llvm/Analysis/OptimizationRemarkEmitter.h" 23 #include "llvm/CodeGen/Passes.h" 24 #include "llvm/CodeGen/TargetLowering.h" 25 #include "llvm/CodeGen/TargetPassConfig.h" 26 #include "llvm/CodeGen/TargetSubtargetInfo.h" 27 #include "llvm/IR/Attributes.h" 28 #include "llvm/IR/BasicBlock.h" 29 #include "llvm/IR/Constants.h" 30 #include "llvm/IR/DataLayout.h" 31 #include "llvm/IR/DebugInfo.h" 32 #include "llvm/IR/DebugLoc.h" 33 #include "llvm/IR/DerivedTypes.h" 34 #include "llvm/IR/Dominators.h" 35 #include "llvm/IR/Function.h" 36 #include "llvm/IR/IRBuilder.h" 37 #include "llvm/IR/Instruction.h" 38 #include "llvm/IR/Instructions.h" 39 #include "llvm/IR/IntrinsicInst.h" 40 #include "llvm/IR/Intrinsics.h" 41 #include "llvm/IR/MDBuilder.h" 42 #include "llvm/IR/Module.h" 43 #include "llvm/IR/Type.h" 44 #include "llvm/IR/User.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 INITIALIZE_PASS_BEGIN(StackProtector, DEBUG_TYPE, 66 "Insert stack protectors", false, true) 67 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig) 68 INITIALIZE_PASS_END(StackProtector, DEBUG_TYPE, 69 "Insert stack protectors", false, true) 70 71 FunctionPass *llvm::createStackProtectorPass() { return new StackProtector(); } 72 73 void StackProtector::getAnalysisUsage(AnalysisUsage &AU) const { 74 AU.addRequired<TargetPassConfig>(); 75 AU.addPreserved<DominatorTreeWrapperPass>(); 76 } 77 78 bool StackProtector::runOnFunction(Function &Fn) { 79 F = &Fn; 80 M = F->getParent(); 81 DominatorTreeWrapperPass *DTWP = 82 getAnalysisIfAvailable<DominatorTreeWrapperPass>(); 83 DT = DTWP ? &DTWP->getDomTree() : nullptr; 84 TM = &getAnalysis<TargetPassConfig>().getTM<TargetMachine>(); 85 Trip = TM->getTargetTriple(); 86 TLI = TM->getSubtargetImpl(Fn)->getTargetLowering(); 87 HasPrologue = false; 88 HasIRCheck = false; 89 90 Attribute Attr = Fn.getFnAttribute("stack-protector-buffer-size"); 91 if (Attr.isStringAttribute() && 92 Attr.getValueAsString().getAsInteger(10, SSPBufferSize)) 93 return false; // Invalid integer string 94 95 if (!RequiresStackProtector()) 96 return false; 97 98 // TODO(etienneb): Functions with funclets are not correctly supported now. 99 // Do nothing if this is funclet-based personality. 100 if (Fn.hasPersonalityFn()) { 101 EHPersonality Personality = classifyEHPersonality(Fn.getPersonalityFn()); 102 if (isFuncletEHPersonality(Personality)) 103 return false; 104 } 105 106 ++NumFunProtected; 107 return InsertStackProtectors(); 108 } 109 110 /// \param [out] IsLarge is set to true if a protectable array is found and 111 /// it is "large" ( >= ssp-buffer-size). In the case of a structure with 112 /// multiple arrays, this gets set if any of them is large. 113 bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge, 114 bool Strong, 115 bool InStruct) const { 116 if (!Ty) 117 return false; 118 if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) { 119 if (!AT->getElementType()->isIntegerTy(8)) { 120 // If we're on a non-Darwin platform or we're inside of a structure, don't 121 // add stack protectors unless the array is a character array. 122 // However, in strong mode any array, regardless of type and size, 123 // triggers a protector. 124 if (!Strong && (InStruct || !Trip.isOSDarwin())) 125 return false; 126 } 127 128 // If an array has more than SSPBufferSize bytes of allocated space, then we 129 // emit stack protectors. 130 if (SSPBufferSize <= M->getDataLayout().getTypeAllocSize(AT)) { 131 IsLarge = true; 132 return true; 133 } 134 135 if (Strong) 136 // Require a protector for all arrays in strong mode 137 return true; 138 } 139 140 const StructType *ST = dyn_cast<StructType>(Ty); 141 if (!ST) 142 return false; 143 144 bool NeedsProtector = false; 145 for (StructType::element_iterator I = ST->element_begin(), 146 E = ST->element_end(); 147 I != E; ++I) 148 if (ContainsProtectableArray(*I, IsLarge, Strong, true)) { 149 // If the element is a protectable array and is large (>= SSPBufferSize) 150 // then we are done. If the protectable array is not large, then 151 // keep looking in case a subsequent element is a large array. 152 if (IsLarge) 153 return true; 154 NeedsProtector = true; 155 } 156 157 return NeedsProtector; 158 } 159 160 static bool isLifetimeInst(const Instruction *I) { 161 if (const auto Intrinsic = dyn_cast<IntrinsicInst>(I)) { 162 const auto Id = Intrinsic->getIntrinsicID(); 163 return Id == Intrinsic::lifetime_start || Id == Intrinsic::lifetime_end; 164 } 165 return false; 166 } 167 168 bool StackProtector::HasAddressTaken(const Instruction *AI) { 169 for (const User *U : AI->users()) { 170 if (const StoreInst *SI = dyn_cast<StoreInst>(U)) { 171 if (AI == SI->getValueOperand()) 172 return true; 173 } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) { 174 if (AI == SI->getOperand(0)) 175 return true; 176 } else if (const CallInst *CI = dyn_cast<CallInst>(U)) { 177 // Ignore intrinsics that are not calls. TODO: Use isLoweredToCall(). 178 if (!isa<DbgInfoIntrinsic>(CI) && !isLifetimeInst(CI)) 179 return true; 180 } else if (isa<InvokeInst>(U)) { 181 return true; 182 } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) { 183 if (HasAddressTaken(SI)) 184 return true; 185 } else if (const PHINode *PN = dyn_cast<PHINode>(U)) { 186 // Keep track of what PHI nodes we have already visited to ensure 187 // they are only visited once. 188 if (VisitedPHIs.insert(PN).second) 189 if (HasAddressTaken(PN)) 190 return true; 191 } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) { 192 if (HasAddressTaken(GEP)) 193 return true; 194 } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) { 195 if (HasAddressTaken(BI)) 196 return true; 197 } 198 } 199 return false; 200 } 201 202 /// Check whether or not this function needs a stack protector based 203 /// upon the stack protector level. 204 /// 205 /// We use two heuristics: a standard (ssp) and strong (sspstrong). 206 /// The standard heuristic which will add a guard variable to functions that 207 /// call alloca with a either a variable size or a size >= SSPBufferSize, 208 /// functions with character buffers larger than SSPBufferSize, and functions 209 /// with aggregates containing character buffers larger than SSPBufferSize. The 210 /// strong heuristic will add a guard variables to functions that call alloca 211 /// regardless of size, functions with any buffer regardless of type and size, 212 /// functions with aggregates that contain any buffer regardless of type and 213 /// size, and functions that contain stack-based variables that have had their 214 /// address taken. 215 bool StackProtector::RequiresStackProtector() { 216 bool Strong = false; 217 bool NeedsProtector = false; 218 for (const BasicBlock &BB : *F) 219 for (const Instruction &I : BB) 220 if (const CallInst *CI = dyn_cast<CallInst>(&I)) 221 if (CI->getCalledFunction() == 222 Intrinsic::getDeclaration(F->getParent(), 223 Intrinsic::stackprotector)) 224 HasPrologue = true; 225 226 if (F->hasFnAttribute(Attribute::SafeStack)) 227 return false; 228 229 // We are constructing the OptimizationRemarkEmitter on the fly rather than 230 // using the analysis pass to avoid building DominatorTree and LoopInfo which 231 // are not available this late in the IR pipeline. 232 OptimizationRemarkEmitter ORE(F); 233 234 if (F->hasFnAttribute(Attribute::StackProtectReq)) { 235 ORE.emit([&]() { 236 return OptimizationRemark(DEBUG_TYPE, "StackProtectorRequested", F) 237 << "Stack protection applied to function " 238 << ore::NV("Function", F) 239 << " due to a function attribute or command-line switch"; 240 }); 241 NeedsProtector = true; 242 Strong = true; // Use the same heuristic as strong to determine SSPLayout 243 } else if (F->hasFnAttribute(Attribute::StackProtectStrong)) 244 Strong = true; 245 else if (HasPrologue) 246 NeedsProtector = true; 247 else if (!F->hasFnAttribute(Attribute::StackProtect)) 248 return false; 249 250 for (const BasicBlock &BB : *F) { 251 for (const Instruction &I : BB) { 252 if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) { 253 if (AI->isArrayAllocation()) { 254 auto RemarkBuilder = [&]() { 255 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAllocaOrArray", 256 &I) 257 << "Stack protection applied to function " 258 << ore::NV("Function", F) 259 << " due to a call to alloca or use of a variable length " 260 "array"; 261 }; 262 if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) { 263 if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) { 264 // A call to alloca with size >= SSPBufferSize requires 265 // stack protectors. 266 Layout.insert(std::make_pair(AI, 267 MachineFrameInfo::SSPLK_LargeArray)); 268 ORE.emit(RemarkBuilder); 269 NeedsProtector = true; 270 } else if (Strong) { 271 // Require protectors for all alloca calls in strong mode. 272 Layout.insert(std::make_pair(AI, 273 MachineFrameInfo::SSPLK_SmallArray)); 274 ORE.emit(RemarkBuilder); 275 NeedsProtector = true; 276 } 277 } else { 278 // A call to alloca with a variable size requires protectors. 279 Layout.insert(std::make_pair(AI, 280 MachineFrameInfo::SSPLK_LargeArray)); 281 ORE.emit(RemarkBuilder); 282 NeedsProtector = true; 283 } 284 continue; 285 } 286 287 bool IsLarge = false; 288 if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) { 289 Layout.insert(std::make_pair(AI, IsLarge 290 ? MachineFrameInfo::SSPLK_LargeArray 291 : MachineFrameInfo::SSPLK_SmallArray)); 292 ORE.emit([&]() { 293 return OptimizationRemark(DEBUG_TYPE, "StackProtectorBuffer", &I) 294 << "Stack protection applied to function " 295 << ore::NV("Function", F) 296 << " due to a stack allocated buffer or struct containing a " 297 "buffer"; 298 }); 299 NeedsProtector = true; 300 continue; 301 } 302 303 if (Strong && HasAddressTaken(AI)) { 304 ++NumAddrTaken; 305 Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_AddrOf)); 306 ORE.emit([&]() { 307 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAddressTaken", 308 &I) 309 << "Stack protection applied to function " 310 << ore::NV("Function", F) 311 << " due to the address of a local variable being taken"; 312 }); 313 NeedsProtector = true; 314 } 315 } 316 } 317 } 318 319 return NeedsProtector; 320 } 321 322 /// Create a stack guard loading and populate whether SelectionDAG SSP is 323 /// supported. 324 static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M, 325 IRBuilder<> &B, 326 bool *SupportsSelectionDAGSP = nullptr) { 327 if (Value *Guard = TLI->getIRStackGuard(B)) 328 return B.CreateLoad(Guard, true, "StackGuard"); 329 330 // Use SelectionDAG SSP handling, since there isn't an IR guard. 331 // 332 // This is more or less weird, since we optionally output whether we 333 // should perform a SelectionDAG SP here. The reason is that it's strictly 334 // defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also 335 // mutating. There is no way to get this bit without mutating the IR, so 336 // getting this bit has to happen in this right time. 337 // 338 // We could have define a new function TLI::supportsSelectionDAGSP(), but that 339 // will put more burden on the backends' overriding work, especially when it 340 // actually conveys the same information getIRStackGuard() already gives. 341 if (SupportsSelectionDAGSP) 342 *SupportsSelectionDAGSP = true; 343 TLI->insertSSPDeclarations(*M); 344 return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard)); 345 } 346 347 /// Insert code into the entry block that stores the stack guard 348 /// variable onto the stack: 349 /// 350 /// entry: 351 /// StackGuardSlot = alloca i8* 352 /// StackGuard = <stack guard> 353 /// call void @llvm.stackprotector(StackGuard, StackGuardSlot) 354 /// 355 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo 356 /// node. 357 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI, 358 const TargetLoweringBase *TLI, AllocaInst *&AI) { 359 bool SupportsSelectionDAGSP = false; 360 IRBuilder<> B(&F->getEntryBlock().front()); 361 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext()); 362 AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot"); 363 364 Value *GuardSlot = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP); 365 B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), 366 {GuardSlot, AI}); 367 return SupportsSelectionDAGSP; 368 } 369 370 /// InsertStackProtectors - Insert code into the prologue and epilogue of the 371 /// function. 372 /// 373 /// - The prologue code loads and stores the stack guard onto the stack. 374 /// - The epilogue checks the value stored in the prologue against the original 375 /// value. It calls __stack_chk_fail if they differ. 376 bool StackProtector::InsertStackProtectors() { 377 // If the target wants to XOR the frame pointer into the guard value, it's 378 // impossible to emit the check in IR, so the target *must* support stack 379 // protection in SDAG. 380 bool SupportsSelectionDAGSP = 381 TLI->useStackGuardXorFP() || 382 (EnableSelectionDAGSP && !TM->Options.EnableFastISel); 383 AllocaInst *AI = nullptr; // Place on stack that stores the stack guard. 384 385 for (Function::iterator I = F->begin(), E = F->end(); I != E;) { 386 BasicBlock *BB = &*I++; 387 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()); 388 if (!RI) 389 continue; 390 391 // Generate prologue instrumentation if not already generated. 392 if (!HasPrologue) { 393 HasPrologue = true; 394 SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI); 395 } 396 397 // SelectionDAG based code generation. Nothing else needs to be done here. 398 // The epilogue instrumentation is postponed to SelectionDAG. 399 if (SupportsSelectionDAGSP) 400 break; 401 402 // Set HasIRCheck to true, so that SelectionDAG will not generate its own 403 // version. SelectionDAG called 'shouldEmitSDCheck' to check whether 404 // instrumentation has already been generated. 405 HasIRCheck = true; 406 407 // Generate epilogue instrumentation. The epilogue intrumentation can be 408 // function-based or inlined depending on which mechanism the target is 409 // providing. 410 if (Value* GuardCheck = TLI->getSSPStackGuardCheck(*M)) { 411 // Generate the function-based epilogue instrumentation. 412 // The target provides a guard check function, generate a call to it. 413 IRBuilder<> B(RI); 414 LoadInst *Guard = B.CreateLoad(AI, true, "Guard"); 415 CallInst *Call = B.CreateCall(GuardCheck, {Guard}); 416 llvm::Function *Function = cast<llvm::Function>(GuardCheck); 417 Call->setAttributes(Function->getAttributes()); 418 Call->setCallingConv(Function->getCallingConv()); 419 } else { 420 // Generate the epilogue with inline instrumentation. 421 // If we do not support SelectionDAG based tail calls, generate IR level 422 // tail calls. 423 // 424 // For each block with a return instruction, convert this: 425 // 426 // return: 427 // ... 428 // ret ... 429 // 430 // into this: 431 // 432 // return: 433 // ... 434 // %1 = <stack guard> 435 // %2 = load StackGuardSlot 436 // %3 = cmp i1 %1, %2 437 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk 438 // 439 // SP_return: 440 // ret ... 441 // 442 // CallStackCheckFailBlk: 443 // call void @__stack_chk_fail() 444 // unreachable 445 446 // Create the FailBB. We duplicate the BB every time since the MI tail 447 // merge pass will merge together all of the various BB into one including 448 // fail BB generated by the stack protector pseudo instruction. 449 BasicBlock *FailBB = CreateFailBB(); 450 451 // Split the basic block before the return instruction. 452 BasicBlock *NewBB = BB->splitBasicBlock(RI->getIterator(), "SP_return"); 453 454 // Update the dominator tree if we need to. 455 if (DT && DT->isReachableFromEntry(BB)) { 456 DT->addNewBlock(NewBB, BB); 457 DT->addNewBlock(FailBB, BB); 458 } 459 460 // Remove default branch instruction to the new BB. 461 BB->getTerminator()->eraseFromParent(); 462 463 // Move the newly created basic block to the point right after the old 464 // basic block so that it's in the "fall through" position. 465 NewBB->moveAfter(BB); 466 467 // Generate the stack protector instructions in the old basic block. 468 IRBuilder<> B(BB); 469 Value *Guard = getStackGuard(TLI, M, B); 470 LoadInst *LI2 = B.CreateLoad(AI, true); 471 Value *Cmp = B.CreateICmpEQ(Guard, LI2); 472 auto SuccessProb = 473 BranchProbabilityInfo::getBranchProbStackProtector(true); 474 auto FailureProb = 475 BranchProbabilityInfo::getBranchProbStackProtector(false); 476 MDNode *Weights = MDBuilder(F->getContext()) 477 .createBranchWeights(SuccessProb.getNumerator(), 478 FailureProb.getNumerator()); 479 B.CreateCondBr(Cmp, NewBB, FailBB, Weights); 480 } 481 } 482 483 // Return if we didn't modify any basic blocks. i.e., there are no return 484 // statements in the function. 485 return HasPrologue; 486 } 487 488 /// CreateFailBB - Create a basic block to jump to when the stack protector 489 /// check fails. 490 BasicBlock *StackProtector::CreateFailBB() { 491 LLVMContext &Context = F->getContext(); 492 BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F); 493 IRBuilder<> B(FailBB); 494 B.SetCurrentDebugLocation(DebugLoc::get(0, 0, F->getSubprogram())); 495 if (Trip.isOSOpenBSD()) { 496 Constant *StackChkFail = 497 M->getOrInsertFunction("__stack_smash_handler", 498 Type::getVoidTy(Context), 499 Type::getInt8PtrTy(Context)); 500 501 B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH")); 502 } else { 503 Constant *StackChkFail = 504 M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context)); 505 506 B.CreateCall(StackChkFail, {}); 507 } 508 B.CreateUnreachable(); 509 return FailBB; 510 } 511 512 bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const { 513 return HasPrologue && !HasIRCheck && dyn_cast<ReturnInst>(BB.getTerminator()); 514 } 515 516 void StackProtector::copyToMachineFrameInfo(MachineFrameInfo &MFI) const { 517 if (Layout.empty()) 518 return; 519 520 for (int I = 0, E = MFI.getObjectIndexEnd(); I != E; ++I) { 521 if (MFI.isDeadObjectIndex(I)) 522 continue; 523 524 const AllocaInst *AI = MFI.getObjectAllocation(I); 525 if (!AI) 526 continue; 527 528 SSPLayoutMap::const_iterator LI = Layout.find(AI); 529 if (LI == Layout.end()) 530 continue; 531 532 MFI.setObjectSSPLayout(I, LI->second); 533 } 534 } 535