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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/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)
INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)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 
getAnalysisUsage(AnalysisUsage & AU) const73 void StackProtector::getAnalysisUsage(AnalysisUsage &AU) const {
74   AU.addRequired<TargetPassConfig>();
75   AU.addPreserved<DominatorTreeWrapperPass>();
76 }
77 
runOnFunction(Function & Fn)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.
ContainsProtectableArray(Type * Ty,bool & IsLarge,bool Strong,bool InStruct) const113 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 
isLifetimeInst(const Instruction * I)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 
HasAddressTaken(const Instruction * AI)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.
RequiresStackProtector()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.
getStackGuard(const TargetLoweringBase * TLI,Module * M,IRBuilder<> & B,bool * SupportsSelectionDAGSP=nullptr)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.
CreatePrologue(Function * F,Module * M,ReturnInst * RI,const TargetLoweringBase * TLI,AllocaInst * & AI)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.
InsertStackProtectors()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.
CreateFailBB()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 
shouldEmitSDCheck(const BasicBlock & BB) const512 bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const {
513   return HasPrologue && !HasIRCheck && dyn_cast<ReturnInst>(BB.getTerminator());
514 }
515 
copyToMachineFrameInfo(MachineFrameInfo & MFI) const516 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