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      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/ValueTracking.h"
     22 #include "llvm/CodeGen/Analysis.h"
     23 #include "llvm/CodeGen/Passes.h"
     24 #include "llvm/IR/Attributes.h"
     25 #include "llvm/IR/Constants.h"
     26 #include "llvm/IR/DataLayout.h"
     27 #include "llvm/IR/DerivedTypes.h"
     28 #include "llvm/IR/Function.h"
     29 #include "llvm/IR/GlobalValue.h"
     30 #include "llvm/IR/GlobalVariable.h"
     31 #include "llvm/IR/IRBuilder.h"
     32 #include "llvm/IR/Instructions.h"
     33 #include "llvm/IR/IntrinsicInst.h"
     34 #include "llvm/IR/Intrinsics.h"
     35 #include "llvm/IR/MDBuilder.h"
     36 #include "llvm/IR/Module.h"
     37 #include "llvm/Support/CommandLine.h"
     38 #include "llvm/Target/TargetSubtargetInfo.h"
     39 #include <cstdlib>
     40 using namespace llvm;
     41 
     42 #define DEBUG_TYPE "stack-protector"
     43 
     44 STATISTIC(NumFunProtected, "Number of functions protected");
     45 STATISTIC(NumAddrTaken, "Number of local variables that have their address"
     46                         " taken.");
     47 
     48 static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp",
     49                                           cl::init(true), cl::Hidden);
     50 
     51 char StackProtector::ID = 0;
     52 INITIALIZE_PASS(StackProtector, "stack-protector", "Insert stack protectors",
     53                 false, true)
     54 
     55 FunctionPass *llvm::createStackProtectorPass(const TargetMachine *TM) {
     56   return new StackProtector(TM);
     57 }
     58 
     59 StackProtector::SSPLayoutKind
     60 StackProtector::getSSPLayout(const AllocaInst *AI) const {
     61   return AI ? Layout.lookup(AI) : SSPLK_None;
     62 }
     63 
     64 void StackProtector::adjustForColoring(const AllocaInst *From,
     65                                        const AllocaInst *To) {
     66   // When coloring replaces one alloca with another, transfer the SSPLayoutKind
     67   // tag from the remapped to the target alloca. The remapped alloca should
     68   // have a size smaller than or equal to the replacement alloca.
     69   SSPLayoutMap::iterator I = Layout.find(From);
     70   if (I != Layout.end()) {
     71     SSPLayoutKind Kind = I->second;
     72     Layout.erase(I);
     73 
     74     // Transfer the tag, but make sure that SSPLK_AddrOf does not overwrite
     75     // SSPLK_SmallArray or SSPLK_LargeArray, and make sure that
     76     // SSPLK_SmallArray does not overwrite SSPLK_LargeArray.
     77     I = Layout.find(To);
     78     if (I == Layout.end())
     79       Layout.insert(std::make_pair(To, Kind));
     80     else if (I->second != SSPLK_LargeArray && Kind != SSPLK_AddrOf)
     81       I->second = Kind;
     82   }
     83 }
     84 
     85 bool StackProtector::runOnFunction(Function &Fn) {
     86   F = &Fn;
     87   M = F->getParent();
     88   DominatorTreeWrapperPass *DTWP =
     89       getAnalysisIfAvailable<DominatorTreeWrapperPass>();
     90   DT = DTWP ? &DTWP->getDomTree() : nullptr;
     91   TLI = TM->getSubtargetImpl(Fn)->getTargetLowering();
     92 
     93   Attribute Attr = Fn.getFnAttribute("stack-protector-buffer-size");
     94   if (Attr.isStringAttribute() &&
     95       Attr.getValueAsString().getAsInteger(10, SSPBufferSize))
     96       return false; // Invalid integer string
     97 
     98   if (!RequiresStackProtector())
     99     return false;
    100 
    101   ++NumFunProtected;
    102   return InsertStackProtectors();
    103 }
    104 
    105 /// \param [out] IsLarge is set to true if a protectable array is found and
    106 /// it is "large" ( >= ssp-buffer-size).  In the case of a structure with
    107 /// multiple arrays, this gets set if any of them is large.
    108 bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge,
    109                                               bool Strong,
    110                                               bool InStruct) const {
    111   if (!Ty)
    112     return false;
    113   if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
    114     if (!AT->getElementType()->isIntegerTy(8)) {
    115       // If we're on a non-Darwin platform or we're inside of a structure, don't
    116       // add stack protectors unless the array is a character array.
    117       // However, in strong mode any array, regardless of type and size,
    118       // triggers a protector.
    119       if (!Strong && (InStruct || !Trip.isOSDarwin()))
    120         return false;
    121     }
    122 
    123     // If an array has more than SSPBufferSize bytes of allocated space, then we
    124     // emit stack protectors.
    125     if (SSPBufferSize <= M->getDataLayout().getTypeAllocSize(AT)) {
    126       IsLarge = true;
    127       return true;
    128     }
    129 
    130     if (Strong)
    131       // Require a protector for all arrays in strong mode
    132       return true;
    133   }
    134 
    135   const StructType *ST = dyn_cast<StructType>(Ty);
    136   if (!ST)
    137     return false;
    138 
    139   bool NeedsProtector = false;
    140   for (StructType::element_iterator I = ST->element_begin(),
    141                                     E = ST->element_end();
    142        I != E; ++I)
    143     if (ContainsProtectableArray(*I, IsLarge, Strong, true)) {
    144       // If the element is a protectable array and is large (>= SSPBufferSize)
    145       // then we are done.  If the protectable array is not large, then
    146       // keep looking in case a subsequent element is a large array.
    147       if (IsLarge)
    148         return true;
    149       NeedsProtector = true;
    150     }
    151 
    152   return NeedsProtector;
    153 }
    154 
    155 bool StackProtector::HasAddressTaken(const Instruction *AI) {
    156   for (const User *U : AI->users()) {
    157     if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
    158       if (AI == SI->getValueOperand())
    159         return true;
    160     } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) {
    161       if (AI == SI->getOperand(0))
    162         return true;
    163     } else if (isa<CallInst>(U)) {
    164       return true;
    165     } else if (isa<InvokeInst>(U)) {
    166       return true;
    167     } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) {
    168       if (HasAddressTaken(SI))
    169         return true;
    170     } else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
    171       // Keep track of what PHI nodes we have already visited to ensure
    172       // they are only visited once.
    173       if (VisitedPHIs.insert(PN).second)
    174         if (HasAddressTaken(PN))
    175           return true;
    176     } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
    177       if (HasAddressTaken(GEP))
    178         return true;
    179     } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) {
    180       if (HasAddressTaken(BI))
    181         return true;
    182     }
    183   }
    184   return false;
    185 }
    186 
    187 /// \brief Check whether or not this function needs a stack protector based
    188 /// upon the stack protector level.
    189 ///
    190 /// We use two heuristics: a standard (ssp) and strong (sspstrong).
    191 /// The standard heuristic which will add a guard variable to functions that
    192 /// call alloca with a either a variable size or a size >= SSPBufferSize,
    193 /// functions with character buffers larger than SSPBufferSize, and functions
    194 /// with aggregates containing character buffers larger than SSPBufferSize. The
    195 /// strong heuristic will add a guard variables to functions that call alloca
    196 /// regardless of size, functions with any buffer regardless of type and size,
    197 /// functions with aggregates that contain any buffer regardless of type and
    198 /// size, and functions that contain stack-based variables that have had their
    199 /// address taken.
    200 bool StackProtector::RequiresStackProtector() {
    201   bool Strong = false;
    202   bool NeedsProtector = false;
    203   if (F->hasFnAttribute(Attribute::StackProtectReq)) {
    204     NeedsProtector = true;
    205     Strong = true; // Use the same heuristic as strong to determine SSPLayout
    206   } else if (F->hasFnAttribute(Attribute::StackProtectStrong))
    207     Strong = true;
    208   else if (!F->hasFnAttribute(Attribute::StackProtect))
    209     return false;
    210 
    211   for (const BasicBlock &BB : *F) {
    212     for (const Instruction &I : BB) {
    213       if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
    214         if (AI->isArrayAllocation()) {
    215           // SSP-Strong: Enable protectors for any call to alloca, regardless
    216           // of size.
    217           if (Strong)
    218             return true;
    219 
    220           if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) {
    221             if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) {
    222               // A call to alloca with size >= SSPBufferSize requires
    223               // stack protectors.
    224               Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
    225               NeedsProtector = true;
    226             } else if (Strong) {
    227               // Require protectors for all alloca calls in strong mode.
    228               Layout.insert(std::make_pair(AI, SSPLK_SmallArray));
    229               NeedsProtector = true;
    230             }
    231           } else {
    232             // A call to alloca with a variable size requires protectors.
    233             Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
    234             NeedsProtector = true;
    235           }
    236           continue;
    237         }
    238 
    239         bool IsLarge = false;
    240         if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) {
    241           Layout.insert(std::make_pair(AI, IsLarge ? SSPLK_LargeArray
    242                                                    : SSPLK_SmallArray));
    243           NeedsProtector = true;
    244           continue;
    245         }
    246 
    247         if (Strong && HasAddressTaken(AI)) {
    248           ++NumAddrTaken;
    249           Layout.insert(std::make_pair(AI, SSPLK_AddrOf));
    250           NeedsProtector = true;
    251         }
    252       }
    253     }
    254   }
    255 
    256   return NeedsProtector;
    257 }
    258 
    259 static bool InstructionWillNotHaveChain(const Instruction *I) {
    260   return !I->mayHaveSideEffects() && !I->mayReadFromMemory() &&
    261          isSafeToSpeculativelyExecute(I);
    262 }
    263 
    264 /// Identify if RI has a previous instruction in the "Tail Position" and return
    265 /// it. Otherwise return 0.
    266 ///
    267 /// This is based off of the code in llvm::isInTailCallPosition. The difference
    268 /// is that it inverts the first part of llvm::isInTailCallPosition since
    269 /// isInTailCallPosition is checking if a call is in a tail call position, and
    270 /// we are searching for an unknown tail call that might be in the tail call
    271 /// position. Once we find the call though, the code uses the same refactored
    272 /// code, returnTypeIsEligibleForTailCall.
    273 static CallInst *FindPotentialTailCall(BasicBlock *BB, ReturnInst *RI,
    274                                        const TargetLoweringBase *TLI) {
    275   // Establish a reasonable upper bound on the maximum amount of instructions we
    276   // will look through to find a tail call.
    277   unsigned SearchCounter = 0;
    278   const unsigned MaxSearch = 4;
    279   bool NoInterposingChain = true;
    280 
    281   for (BasicBlock::reverse_iterator I = std::next(BB->rbegin()), E = BB->rend();
    282        I != E && SearchCounter < MaxSearch; ++I) {
    283     Instruction *Inst = &*I;
    284 
    285     // Skip over debug intrinsics and do not allow them to affect our MaxSearch
    286     // counter.
    287     if (isa<DbgInfoIntrinsic>(Inst))
    288       continue;
    289 
    290     // If we find a call and the following conditions are satisifed, then we
    291     // have found a tail call that satisfies at least the target independent
    292     // requirements of a tail call:
    293     //
    294     // 1. The call site has the tail marker.
    295     //
    296     // 2. The call site either will not cause the creation of a chain or if a
    297     // chain is necessary there are no instructions in between the callsite and
    298     // the call which would create an interposing chain.
    299     //
    300     // 3. The return type of the function does not impede tail call
    301     // optimization.
    302     if (CallInst *CI = dyn_cast<CallInst>(Inst)) {
    303       if (CI->isTailCall() &&
    304           (InstructionWillNotHaveChain(CI) || NoInterposingChain) &&
    305           returnTypeIsEligibleForTailCall(BB->getParent(), CI, RI, *TLI))
    306         return CI;
    307     }
    308 
    309     // If we did not find a call see if we have an instruction that may create
    310     // an interposing chain.
    311     NoInterposingChain =
    312         NoInterposingChain && InstructionWillNotHaveChain(Inst);
    313 
    314     // Increment max search.
    315     SearchCounter++;
    316   }
    317 
    318   return nullptr;
    319 }
    320 
    321 /// Insert code into the entry block that stores the __stack_chk_guard
    322 /// variable onto the stack:
    323 ///
    324 ///   entry:
    325 ///     StackGuardSlot = alloca i8*
    326 ///     StackGuard = load __stack_chk_guard
    327 ///     call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
    328 ///
    329 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo
    330 /// node.
    331 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
    332                            const TargetLoweringBase *TLI, const Triple &TT,
    333                            AllocaInst *&AI, Value *&StackGuardVar) {
    334   bool SupportsSelectionDAGSP = false;
    335   PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
    336   unsigned AddressSpace, Offset;
    337   if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
    338     Constant *OffsetVal =
    339         ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
    340 
    341     StackGuardVar =
    342         ConstantExpr::getIntToPtr(OffsetVal, PointerType::get(PtrTy,
    343                                                               AddressSpace));
    344   } else if (TT.isOSOpenBSD()) {
    345     StackGuardVar = M->getOrInsertGlobal("__guard_local", PtrTy);
    346     cast<GlobalValue>(StackGuardVar)
    347         ->setVisibility(GlobalValue::HiddenVisibility);
    348   } else {
    349     SupportsSelectionDAGSP = true;
    350     StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
    351   }
    352 
    353   IRBuilder<> B(&F->getEntryBlock().front());
    354   AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot");
    355   LoadInst *LI = B.CreateLoad(StackGuardVar, "StackGuard");
    356   B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
    357                {LI, AI});
    358 
    359   return SupportsSelectionDAGSP;
    360 }
    361 
    362 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
    363 /// function.
    364 ///
    365 ///  - The prologue code loads and stores the stack guard onto the stack.
    366 ///  - The epilogue checks the value stored in the prologue against the original
    367 ///    value. It calls __stack_chk_fail if they differ.
    368 bool StackProtector::InsertStackProtectors() {
    369   bool HasPrologue = false;
    370   bool SupportsSelectionDAGSP =
    371       EnableSelectionDAGSP && !TM->Options.EnableFastISel;
    372   AllocaInst *AI = nullptr;       // Place on stack that stores the stack guard.
    373   Value *StackGuardVar = nullptr; // The stack guard variable.
    374 
    375   for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
    376     BasicBlock *BB = &*I++;
    377     ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
    378     if (!RI)
    379       continue;
    380 
    381     if (!HasPrologue) {
    382       HasPrologue = true;
    383       SupportsSelectionDAGSP &=
    384           CreatePrologue(F, M, RI, TLI, Trip, AI, StackGuardVar);
    385     }
    386 
    387     if (SupportsSelectionDAGSP) {
    388       // Since we have a potential tail call, insert the special stack check
    389       // intrinsic.
    390       Instruction *InsertionPt = nullptr;
    391       if (CallInst *CI = FindPotentialTailCall(BB, RI, TLI)) {
    392         InsertionPt = CI;
    393       } else {
    394         InsertionPt = RI;
    395         // At this point we know that BB has a return statement so it *DOES*
    396         // have a terminator.
    397         assert(InsertionPt != nullptr &&
    398                "BB must have a terminator instruction at this point.");
    399       }
    400 
    401       Function *Intrinsic =
    402           Intrinsic::getDeclaration(M, Intrinsic::stackprotectorcheck);
    403       CallInst::Create(Intrinsic, StackGuardVar, "", InsertionPt);
    404     } else {
    405       // If we do not support SelectionDAG based tail calls, generate IR level
    406       // tail calls.
    407       //
    408       // For each block with a return instruction, convert this:
    409       //
    410       //   return:
    411       //     ...
    412       //     ret ...
    413       //
    414       // into this:
    415       //
    416       //   return:
    417       //     ...
    418       //     %1 = load __stack_chk_guard
    419       //     %2 = load StackGuardSlot
    420       //     %3 = cmp i1 %1, %2
    421       //     br i1 %3, label %SP_return, label %CallStackCheckFailBlk
    422       //
    423       //   SP_return:
    424       //     ret ...
    425       //
    426       //   CallStackCheckFailBlk:
    427       //     call void @__stack_chk_fail()
    428       //     unreachable
    429 
    430       // Create the FailBB. We duplicate the BB every time since the MI tail
    431       // merge pass will merge together all of the various BB into one including
    432       // fail BB generated by the stack protector pseudo instruction.
    433       BasicBlock *FailBB = CreateFailBB();
    434 
    435       // Split the basic block before the return instruction.
    436       BasicBlock *NewBB = BB->splitBasicBlock(RI->getIterator(), "SP_return");
    437 
    438       // Update the dominator tree if we need to.
    439       if (DT && DT->isReachableFromEntry(BB)) {
    440         DT->addNewBlock(NewBB, BB);
    441         DT->addNewBlock(FailBB, BB);
    442       }
    443 
    444       // Remove default branch instruction to the new BB.
    445       BB->getTerminator()->eraseFromParent();
    446 
    447       // Move the newly created basic block to the point right after the old
    448       // basic block so that it's in the "fall through" position.
    449       NewBB->moveAfter(BB);
    450 
    451       // Generate the stack protector instructions in the old basic block.
    452       IRBuilder<> B(BB);
    453       LoadInst *LI1 = B.CreateLoad(StackGuardVar);
    454       LoadInst *LI2 = B.CreateLoad(AI);
    455       Value *Cmp = B.CreateICmpEQ(LI1, LI2);
    456       unsigned SuccessWeight =
    457           BranchProbabilityInfo::getBranchWeightStackProtector(true);
    458       unsigned FailureWeight =
    459           BranchProbabilityInfo::getBranchWeightStackProtector(false);
    460       MDNode *Weights = MDBuilder(F->getContext())
    461                             .createBranchWeights(SuccessWeight, FailureWeight);
    462       B.CreateCondBr(Cmp, NewBB, FailBB, Weights);
    463     }
    464   }
    465 
    466   // Return if we didn't modify any basic blocks. i.e., there are no return
    467   // statements in the function.
    468   return HasPrologue;
    469 }
    470 
    471 /// CreateFailBB - Create a basic block to jump to when the stack protector
    472 /// check fails.
    473 BasicBlock *StackProtector::CreateFailBB() {
    474   LLVMContext &Context = F->getContext();
    475   BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F);
    476   IRBuilder<> B(FailBB);
    477   if (Trip.isOSOpenBSD()) {
    478     Constant *StackChkFail =
    479         M->getOrInsertFunction("__stack_smash_handler",
    480                                Type::getVoidTy(Context),
    481                                Type::getInt8PtrTy(Context), nullptr);
    482 
    483     B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
    484   } else {
    485     Constant *StackChkFail =
    486         M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context),
    487                                nullptr);
    488     B.CreateCall(StackChkFail, {});
    489   }
    490   B.CreateUnreachable();
    491   return FailBB;
    492 }
    493