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/CaptureTracking.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 /// Search for the first call to the llvm.stackprotector intrinsic and return it 161 /// if present. 162 static const CallInst *findStackProtectorIntrinsic(Function &F) { 163 for (const BasicBlock &BB : F) 164 for (const Instruction &I : BB) 165 if (const CallInst *CI = dyn_cast<CallInst>(&I)) 166 if (CI->getCalledFunction() == 167 Intrinsic::getDeclaration(F.getParent(), Intrinsic::stackprotector)) 168 return CI; 169 return nullptr; 170 } 171 172 /// Check whether or not this function needs a stack protector based 173 /// upon the stack protector level. 174 /// 175 /// We use two heuristics: a standard (ssp) and strong (sspstrong). 176 /// The standard heuristic which will add a guard variable to functions that 177 /// call alloca with a either a variable size or a size >= SSPBufferSize, 178 /// functions with character buffers larger than SSPBufferSize, and functions 179 /// with aggregates containing character buffers larger than SSPBufferSize. The 180 /// strong heuristic will add a guard variables to functions that call alloca 181 /// regardless of size, functions with any buffer regardless of type and size, 182 /// functions with aggregates that contain any buffer regardless of type and 183 /// size, and functions that contain stack-based variables that have had their 184 /// address taken. 185 bool StackProtector::RequiresStackProtector() { 186 bool Strong = false; 187 bool NeedsProtector = false; 188 HasPrologue = findStackProtectorIntrinsic(*F); 189 190 if (F->hasFnAttribute(Attribute::SafeStack)) 191 return false; 192 193 // We are constructing the OptimizationRemarkEmitter on the fly rather than 194 // using the analysis pass to avoid building DominatorTree and LoopInfo which 195 // are not available this late in the IR pipeline. 196 OptimizationRemarkEmitter ORE(F); 197 198 if (F->hasFnAttribute(Attribute::StackProtectReq)) { 199 ORE.emit([&]() { 200 return OptimizationRemark(DEBUG_TYPE, "StackProtectorRequested", F) 201 << "Stack protection applied to function " 202 << ore::NV("Function", F) 203 << " due to a function attribute or command-line switch"; 204 }); 205 NeedsProtector = true; 206 Strong = true; // Use the same heuristic as strong to determine SSPLayout 207 } else if (F->hasFnAttribute(Attribute::StackProtectStrong)) 208 Strong = true; 209 else if (HasPrologue) 210 NeedsProtector = true; 211 else if (!F->hasFnAttribute(Attribute::StackProtect)) 212 return false; 213 214 for (const BasicBlock &BB : *F) { 215 for (const Instruction &I : BB) { 216 if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) { 217 if (AI->isArrayAllocation()) { 218 auto RemarkBuilder = [&]() { 219 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAllocaOrArray", 220 &I) 221 << "Stack protection applied to function " 222 << ore::NV("Function", F) 223 << " due to a call to alloca or use of a variable length " 224 "array"; 225 }; 226 if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) { 227 if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) { 228 // A call to alloca with size >= SSPBufferSize requires 229 // stack protectors. 230 Layout.insert(std::make_pair(AI, 231 MachineFrameInfo::SSPLK_LargeArray)); 232 ORE.emit(RemarkBuilder); 233 NeedsProtector = true; 234 } else if (Strong) { 235 // Require protectors for all alloca calls in strong mode. 236 Layout.insert(std::make_pair(AI, 237 MachineFrameInfo::SSPLK_SmallArray)); 238 ORE.emit(RemarkBuilder); 239 NeedsProtector = true; 240 } 241 } else { 242 // A call to alloca with a variable size requires protectors. 243 Layout.insert(std::make_pair(AI, 244 MachineFrameInfo::SSPLK_LargeArray)); 245 ORE.emit(RemarkBuilder); 246 NeedsProtector = true; 247 } 248 continue; 249 } 250 251 bool IsLarge = false; 252 if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) { 253 Layout.insert(std::make_pair(AI, IsLarge 254 ? MachineFrameInfo::SSPLK_LargeArray 255 : MachineFrameInfo::SSPLK_SmallArray)); 256 ORE.emit([&]() { 257 return OptimizationRemark(DEBUG_TYPE, "StackProtectorBuffer", &I) 258 << "Stack protection applied to function " 259 << ore::NV("Function", F) 260 << " due to a stack allocated buffer or struct containing a " 261 "buffer"; 262 }); 263 NeedsProtector = true; 264 continue; 265 } 266 267 if (Strong && PointerMayBeCaptured(AI, 268 /* ReturnCaptures */ false, 269 /* StoreCaptures */ true)) { 270 ++NumAddrTaken; 271 Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_AddrOf)); 272 ORE.emit([&]() { 273 return OptimizationRemark(DEBUG_TYPE, "StackProtectorAddressTaken", 274 &I) 275 << "Stack protection applied to function " 276 << ore::NV("Function", F) 277 << " due to the address of a local variable being taken"; 278 }); 279 NeedsProtector = true; 280 } 281 } 282 } 283 } 284 285 return NeedsProtector; 286 } 287 288 /// Create a stack guard loading and populate whether SelectionDAG SSP is 289 /// supported. 290 static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M, 291 IRBuilder<> &B, 292 bool *SupportsSelectionDAGSP = nullptr) { 293 if (Value *Guard = TLI->getIRStackGuard(B)) 294 return B.CreateLoad(B.getInt8PtrTy(), Guard, true, "StackGuard"); 295 296 // Use SelectionDAG SSP handling, since there isn't an IR guard. 297 // 298 // This is more or less weird, since we optionally output whether we 299 // should perform a SelectionDAG SP here. The reason is that it's strictly 300 // defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also 301 // mutating. There is no way to get this bit without mutating the IR, so 302 // getting this bit has to happen in this right time. 303 // 304 // We could have define a new function TLI::supportsSelectionDAGSP(), but that 305 // will put more burden on the backends' overriding work, especially when it 306 // actually conveys the same information getIRStackGuard() already gives. 307 if (SupportsSelectionDAGSP) 308 *SupportsSelectionDAGSP = true; 309 TLI->insertSSPDeclarations(*M); 310 return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard)); 311 } 312 313 /// Insert code into the entry block that stores the stack guard 314 /// variable onto the stack: 315 /// 316 /// entry: 317 /// StackGuardSlot = alloca i8* 318 /// StackGuard = <stack guard> 319 /// call void @llvm.stackprotector(StackGuard, StackGuardSlot) 320 /// 321 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo 322 /// node. 323 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI, 324 const TargetLoweringBase *TLI, AllocaInst *&AI) { 325 bool SupportsSelectionDAGSP = false; 326 IRBuilder<> B(&F->getEntryBlock().front()); 327 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext()); 328 AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot"); 329 330 Value *GuardSlot = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP); 331 B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), 332 {GuardSlot, AI}); 333 return SupportsSelectionDAGSP; 334 } 335 336 /// InsertStackProtectors - Insert code into the prologue and epilogue of the 337 /// function. 338 /// 339 /// - The prologue code loads and stores the stack guard onto the stack. 340 /// - The epilogue checks the value stored in the prologue against the original 341 /// value. It calls __stack_chk_fail if they differ. 342 bool StackProtector::InsertStackProtectors() { 343 // If the target wants to XOR the frame pointer into the guard value, it's 344 // impossible to emit the check in IR, so the target *must* support stack 345 // protection in SDAG. 346 bool SupportsSelectionDAGSP = 347 TLI->useStackGuardXorFP() || 348 (EnableSelectionDAGSP && !TM->Options.EnableFastISel && 349 !TM->Options.EnableGlobalISel); 350 AllocaInst *AI = nullptr; // Place on stack that stores the stack guard. 351 352 for (Function::iterator I = F->begin(), E = F->end(); I != E;) { 353 BasicBlock *BB = &*I++; 354 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()); 355 if (!RI) 356 continue; 357 358 // Generate prologue instrumentation if not already generated. 359 if (!HasPrologue) { 360 HasPrologue = true; 361 SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI); 362 } 363 364 // SelectionDAG based code generation. Nothing else needs to be done here. 365 // The epilogue instrumentation is postponed to SelectionDAG. 366 if (SupportsSelectionDAGSP) 367 break; 368 369 // Find the stack guard slot if the prologue was not created by this pass 370 // itself via a previous call to CreatePrologue(). 371 if (!AI) { 372 const CallInst *SPCall = findStackProtectorIntrinsic(*F); 373 assert(SPCall && "Call to llvm.stackprotector is missing"); 374 AI = cast<AllocaInst>(SPCall->getArgOperand(1)); 375 } 376 377 // Set HasIRCheck to true, so that SelectionDAG will not generate its own 378 // version. SelectionDAG called 'shouldEmitSDCheck' to check whether 379 // instrumentation has already been generated. 380 HasIRCheck = true; 381 382 // Generate epilogue instrumentation. The epilogue intrumentation can be 383 // function-based or inlined depending on which mechanism the target is 384 // providing. 385 if (Function *GuardCheck = TLI->getSSPStackGuardCheck(*M)) { 386 // Generate the function-based epilogue instrumentation. 387 // The target provides a guard check function, generate a call to it. 388 IRBuilder<> B(RI); 389 LoadInst *Guard = B.CreateLoad(B.getInt8PtrTy(), AI, true, "Guard"); 390 CallInst *Call = B.CreateCall(GuardCheck, {Guard}); 391 Call->setAttributes(GuardCheck->getAttributes()); 392 Call->setCallingConv(GuardCheck->getCallingConv()); 393 } else { 394 // Generate the epilogue with inline instrumentation. 395 // If we do not support SelectionDAG based tail calls, generate IR level 396 // tail calls. 397 // 398 // For each block with a return instruction, convert this: 399 // 400 // return: 401 // ... 402 // ret ... 403 // 404 // into this: 405 // 406 // return: 407 // ... 408 // %1 = <stack guard> 409 // %2 = load StackGuardSlot 410 // %3 = cmp i1 %1, %2 411 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk 412 // 413 // SP_return: 414 // ret ... 415 // 416 // CallStackCheckFailBlk: 417 // call void @__stack_chk_fail() 418 // unreachable 419 420 // Create the FailBB. We duplicate the BB every time since the MI tail 421 // merge pass will merge together all of the various BB into one including 422 // fail BB generated by the stack protector pseudo instruction. 423 BasicBlock *FailBB = CreateFailBB(); 424 425 // Split the basic block before the return instruction. 426 BasicBlock *NewBB = BB->splitBasicBlock(RI->getIterator(), "SP_return"); 427 428 // Update the dominator tree if we need to. 429 if (DT && DT->isReachableFromEntry(BB)) { 430 DT->addNewBlock(NewBB, BB); 431 DT->addNewBlock(FailBB, BB); 432 } 433 434 // Remove default branch instruction to the new BB. 435 BB->getTerminator()->eraseFromParent(); 436 437 // Move the newly created basic block to the point right after the old 438 // basic block so that it's in the "fall through" position. 439 NewBB->moveAfter(BB); 440 441 // Generate the stack protector instructions in the old basic block. 442 IRBuilder<> B(BB); 443 Value *Guard = getStackGuard(TLI, M, B); 444 LoadInst *LI2 = B.CreateLoad(B.getInt8PtrTy(), AI, true); 445 Value *Cmp = B.CreateICmpEQ(Guard, LI2); 446 auto SuccessProb = 447 BranchProbabilityInfo::getBranchProbStackProtector(true); 448 auto FailureProb = 449 BranchProbabilityInfo::getBranchProbStackProtector(false); 450 MDNode *Weights = MDBuilder(F->getContext()) 451 .createBranchWeights(SuccessProb.getNumerator(), 452 FailureProb.getNumerator()); 453 B.CreateCondBr(Cmp, NewBB, FailBB, Weights); 454 } 455 } 456 457 // Return if we didn't modify any basic blocks. i.e., there are no return 458 // statements in the function. 459 return HasPrologue; 460 } 461 462 /// CreateFailBB - Create a basic block to jump to when the stack protector 463 /// check fails. 464 BasicBlock *StackProtector::CreateFailBB() { 465 LLVMContext &Context = F->getContext(); 466 BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F); 467 IRBuilder<> B(FailBB); 468 B.SetCurrentDebugLocation(DebugLoc::get(0, 0, F->getSubprogram())); 469 if (Trip.isOSOpenBSD()) { 470 FunctionCallee StackChkFail = M->getOrInsertFunction( 471 "__stack_smash_handler", Type::getVoidTy(Context), 472 Type::getInt8PtrTy(Context)); 473 474 B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH")); 475 } else { 476 FunctionCallee StackChkFail = 477 M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context)); 478 479 B.CreateCall(StackChkFail, {}); 480 } 481 B.CreateUnreachable(); 482 return FailBB; 483 } 484 485 bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const { 486 return HasPrologue && !HasIRCheck && isa<ReturnInst>(BB.getTerminator()); 487 } 488 489 void StackProtector::copyToMachineFrameInfo(MachineFrameInfo &MFI) const { 490 if (Layout.empty()) 491 return; 492 493 for (int I = 0, E = MFI.getObjectIndexEnd(); I != E; ++I) { 494 if (MFI.isDeadObjectIndex(I)) 495 continue; 496 497 const AllocaInst *AI = MFI.getObjectAllocation(I); 498 if (!AI) 499 continue; 500 501 SSPLayoutMap::const_iterator LI = Layout.find(AI); 502 if (LI == Layout.end()) 503 continue; 504 505 MFI.setObjectSSPLayout(I, LI->second); 506 } 507 } 508