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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/ValueTracking.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/DebugInfo.h"
28 #include "llvm/IR/DerivedTypes.h"
29 #include "llvm/IR/Function.h"
30 #include "llvm/IR/GlobalValue.h"
31 #include "llvm/IR/GlobalVariable.h"
32 #include "llvm/IR/IRBuilder.h"
33 #include "llvm/IR/Instructions.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/IR/Intrinsics.h"
36 #include "llvm/IR/MDBuilder.h"
37 #include "llvm/IR/Module.h"
38 #include "llvm/Support/CommandLine.h"
39 #include "llvm/Target/TargetSubtargetInfo.h"
40 #include <cstdlib>
41 using namespace llvm;
42 
43 #define DEBUG_TYPE "stack-protector"
44 
45 STATISTIC(NumFunProtected, "Number of functions protected");
46 STATISTIC(NumAddrTaken, "Number of local variables that have their address"
47                         " taken.");
48 
49 static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp",
50                                           cl::init(true), cl::Hidden);
51 
52 char StackProtector::ID = 0;
53 INITIALIZE_PASS(StackProtector, "stack-protector", "Insert stack protectors",
54                 false, true)
55 
createStackProtectorPass(const TargetMachine * TM)56 FunctionPass *llvm::createStackProtectorPass(const TargetMachine *TM) {
57   return new StackProtector(TM);
58 }
59 
60 StackProtector::SSPLayoutKind
getSSPLayout(const AllocaInst * AI) const61 StackProtector::getSSPLayout(const AllocaInst *AI) const {
62   return AI ? Layout.lookup(AI) : SSPLK_None;
63 }
64 
adjustForColoring(const AllocaInst * From,const AllocaInst * To)65 void StackProtector::adjustForColoring(const AllocaInst *From,
66                                        const AllocaInst *To) {
67   // When coloring replaces one alloca with another, transfer the SSPLayoutKind
68   // tag from the remapped to the target alloca. The remapped alloca should
69   // have a size smaller than or equal to the replacement alloca.
70   SSPLayoutMap::iterator I = Layout.find(From);
71   if (I != Layout.end()) {
72     SSPLayoutKind Kind = I->second;
73     Layout.erase(I);
74 
75     // Transfer the tag, but make sure that SSPLK_AddrOf does not overwrite
76     // SSPLK_SmallArray or SSPLK_LargeArray, and make sure that
77     // SSPLK_SmallArray does not overwrite SSPLK_LargeArray.
78     I = Layout.find(To);
79     if (I == Layout.end())
80       Layout.insert(std::make_pair(To, Kind));
81     else if (I->second != SSPLK_LargeArray && Kind != SSPLK_AddrOf)
82       I->second = Kind;
83   }
84 }
85 
runOnFunction(Function & Fn)86 bool StackProtector::runOnFunction(Function &Fn) {
87   F = &Fn;
88   M = F->getParent();
89   DominatorTreeWrapperPass *DTWP =
90       getAnalysisIfAvailable<DominatorTreeWrapperPass>();
91   DT = DTWP ? &DTWP->getDomTree() : nullptr;
92   TLI = TM->getSubtargetImpl(Fn)->getTargetLowering();
93   HasPrologue = false;
94   HasIRCheck = false;
95 
96   Attribute Attr = Fn.getFnAttribute("stack-protector-buffer-size");
97   if (Attr.isStringAttribute() &&
98       Attr.getValueAsString().getAsInteger(10, SSPBufferSize))
99     return false; // Invalid integer string
100 
101   if (!RequiresStackProtector())
102     return false;
103 
104   // TODO(etienneb): Functions with funclets are not correctly supported now.
105   // Do nothing if this is funclet-based personality.
106   if (Fn.hasPersonalityFn()) {
107     EHPersonality Personality = classifyEHPersonality(Fn.getPersonalityFn());
108     if (isFuncletEHPersonality(Personality))
109       return false;
110   }
111 
112   ++NumFunProtected;
113   return InsertStackProtectors();
114 }
115 
116 /// \param [out] IsLarge is set to true if a protectable array is found and
117 /// it is "large" ( >= ssp-buffer-size).  In the case of a structure with
118 /// multiple arrays, this gets set if any of them is large.
ContainsProtectableArray(Type * Ty,bool & IsLarge,bool Strong,bool InStruct) const119 bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge,
120                                               bool Strong,
121                                               bool InStruct) const {
122   if (!Ty)
123     return false;
124   if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
125     if (!AT->getElementType()->isIntegerTy(8)) {
126       // If we're on a non-Darwin platform or we're inside of a structure, don't
127       // add stack protectors unless the array is a character array.
128       // However, in strong mode any array, regardless of type and size,
129       // triggers a protector.
130       if (!Strong && (InStruct || !Trip.isOSDarwin()))
131         return false;
132     }
133 
134     // If an array has more than SSPBufferSize bytes of allocated space, then we
135     // emit stack protectors.
136     if (SSPBufferSize <= M->getDataLayout().getTypeAllocSize(AT)) {
137       IsLarge = true;
138       return true;
139     }
140 
141     if (Strong)
142       // Require a protector for all arrays in strong mode
143       return true;
144   }
145 
146   const StructType *ST = dyn_cast<StructType>(Ty);
147   if (!ST)
148     return false;
149 
150   bool NeedsProtector = false;
151   for (StructType::element_iterator I = ST->element_begin(),
152                                     E = ST->element_end();
153        I != E; ++I)
154     if (ContainsProtectableArray(*I, IsLarge, Strong, true)) {
155       // If the element is a protectable array and is large (>= SSPBufferSize)
156       // then we are done.  If the protectable array is not large, then
157       // keep looking in case a subsequent element is a large array.
158       if (IsLarge)
159         return true;
160       NeedsProtector = true;
161     }
162 
163   return NeedsProtector;
164 }
165 
HasAddressTaken(const Instruction * AI)166 bool StackProtector::HasAddressTaken(const Instruction *AI) {
167   for (const User *U : AI->users()) {
168     if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
169       if (AI == SI->getValueOperand())
170         return true;
171     } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) {
172       if (AI == SI->getOperand(0))
173         return true;
174     } else if (isa<CallInst>(U)) {
175       return true;
176     } else if (isa<InvokeInst>(U)) {
177       return true;
178     } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) {
179       if (HasAddressTaken(SI))
180         return true;
181     } else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
182       // Keep track of what PHI nodes we have already visited to ensure
183       // they are only visited once.
184       if (VisitedPHIs.insert(PN).second)
185         if (HasAddressTaken(PN))
186           return true;
187     } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
188       if (HasAddressTaken(GEP))
189         return true;
190     } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) {
191       if (HasAddressTaken(BI))
192         return true;
193     }
194   }
195   return false;
196 }
197 
198 /// \brief Check whether or not this function needs a stack protector based
199 /// upon the stack protector level.
200 ///
201 /// We use two heuristics: a standard (ssp) and strong (sspstrong).
202 /// The standard heuristic which will add a guard variable to functions that
203 /// call alloca with a either a variable size or a size >= SSPBufferSize,
204 /// functions with character buffers larger than SSPBufferSize, and functions
205 /// with aggregates containing character buffers larger than SSPBufferSize. The
206 /// strong heuristic will add a guard variables to functions that call alloca
207 /// regardless of size, functions with any buffer regardless of type and size,
208 /// functions with aggregates that contain any buffer regardless of type and
209 /// size, and functions that contain stack-based variables that have had their
210 /// address taken.
RequiresStackProtector()211 bool StackProtector::RequiresStackProtector() {
212   bool Strong = false;
213   bool NeedsProtector = false;
214   for (const BasicBlock &BB : *F)
215     for (const Instruction &I : BB)
216       if (const CallInst *CI = dyn_cast<CallInst>(&I))
217         if (CI->getCalledFunction() ==
218             Intrinsic::getDeclaration(F->getParent(),
219                                       Intrinsic::stackprotector))
220           HasPrologue = true;
221 
222   if (F->hasFnAttribute(Attribute::SafeStack))
223     return false;
224 
225   if (F->hasFnAttribute(Attribute::StackProtectReq)) {
226     NeedsProtector = true;
227     Strong = true; // Use the same heuristic as strong to determine SSPLayout
228   } else if (F->hasFnAttribute(Attribute::StackProtectStrong))
229     Strong = true;
230   else if (HasPrologue)
231     NeedsProtector = true;
232   else if (!F->hasFnAttribute(Attribute::StackProtect))
233     return false;
234 
235   for (const BasicBlock &BB : *F) {
236     for (const Instruction &I : BB) {
237       if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
238         if (AI->isArrayAllocation()) {
239           // SSP-Strong: Enable protectors for any call to alloca, regardless
240           // of size.
241           if (Strong)
242             return true;
243 
244           if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) {
245             if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) {
246               // A call to alloca with size >= SSPBufferSize requires
247               // stack protectors.
248               Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
249               NeedsProtector = true;
250             } else if (Strong) {
251               // Require protectors for all alloca calls in strong mode.
252               Layout.insert(std::make_pair(AI, SSPLK_SmallArray));
253               NeedsProtector = true;
254             }
255           } else {
256             // A call to alloca with a variable size requires protectors.
257             Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
258             NeedsProtector = true;
259           }
260           continue;
261         }
262 
263         bool IsLarge = false;
264         if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) {
265           Layout.insert(std::make_pair(AI, IsLarge ? SSPLK_LargeArray
266                                                    : SSPLK_SmallArray));
267           NeedsProtector = true;
268           continue;
269         }
270 
271         if (Strong && HasAddressTaken(AI)) {
272           ++NumAddrTaken;
273           Layout.insert(std::make_pair(AI, SSPLK_AddrOf));
274           NeedsProtector = true;
275         }
276       }
277     }
278   }
279 
280   return NeedsProtector;
281 }
282 
283 /// Create a stack guard loading and populate whether SelectionDAG SSP is
284 /// supported.
getStackGuard(const TargetLoweringBase * TLI,Module * M,IRBuilder<> & B,bool * SupportsSelectionDAGSP=nullptr)285 static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M,
286                             IRBuilder<> &B,
287                             bool *SupportsSelectionDAGSP = nullptr) {
288   if (Value *Guard = TLI->getIRStackGuard(B))
289     return B.CreateLoad(Guard, true, "StackGuard");
290 
291   // Use SelectionDAG SSP handling, since there isn't an IR guard.
292   //
293   // This is more or less weird, since we optionally output whether we
294   // should perform a SelectionDAG SP here. The reason is that it's strictly
295   // defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also
296   // mutating. There is no way to get this bit without mutating the IR, so
297   // getting this bit has to happen in this right time.
298   //
299   // We could have define a new function TLI::supportsSelectionDAGSP(), but that
300   // will put more burden on the backends' overriding work, especially when it
301   // actually conveys the same information getIRStackGuard() already gives.
302   if (SupportsSelectionDAGSP)
303     *SupportsSelectionDAGSP = true;
304   TLI->insertSSPDeclarations(*M);
305   return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard));
306 }
307 
308 /// Insert code into the entry block that stores the stack guard
309 /// variable onto the stack:
310 ///
311 ///   entry:
312 ///     StackGuardSlot = alloca i8*
313 ///     StackGuard = <stack guard>
314 ///     call void @llvm.stackprotector(StackGuard, StackGuardSlot)
315 ///
316 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo
317 /// node.
CreatePrologue(Function * F,Module * M,ReturnInst * RI,const TargetLoweringBase * TLI,AllocaInst * & AI)318 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
319                            const TargetLoweringBase *TLI, AllocaInst *&AI) {
320   bool SupportsSelectionDAGSP = false;
321   IRBuilder<> B(&F->getEntryBlock().front());
322   PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
323   AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot");
324 
325   Value *GuardSlot = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP);
326   B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
327                {GuardSlot, AI});
328   return SupportsSelectionDAGSP;
329 }
330 
331 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
332 /// function.
333 ///
334 ///  - The prologue code loads and stores the stack guard onto the stack.
335 ///  - The epilogue checks the value stored in the prologue against the original
336 ///    value. It calls __stack_chk_fail if they differ.
InsertStackProtectors()337 bool StackProtector::InsertStackProtectors() {
338   bool SupportsSelectionDAGSP =
339       EnableSelectionDAGSP && !TM->Options.EnableFastISel;
340   AllocaInst *AI = nullptr;       // Place on stack that stores the stack guard.
341 
342   for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
343     BasicBlock *BB = &*I++;
344     ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
345     if (!RI)
346       continue;
347 
348     // Generate prologue instrumentation if not already generated.
349     if (!HasPrologue) {
350       HasPrologue = true;
351       SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI);
352     }
353 
354     // SelectionDAG based code generation. Nothing else needs to be done here.
355     // The epilogue instrumentation is postponed to SelectionDAG.
356     if (SupportsSelectionDAGSP)
357       break;
358 
359     // Set HasIRCheck to true, so that SelectionDAG will not generate its own
360     // version. SelectionDAG called 'shouldEmitSDCheck' to check whether
361     // instrumentation has already been generated.
362     HasIRCheck = true;
363 
364     // Generate epilogue instrumentation. The epilogue intrumentation can be
365     // function-based or inlined depending on which mechanism the target is
366     // providing.
367     if (Value* GuardCheck = TLI->getSSPStackGuardCheck(*M)) {
368       // Generate the function-based epilogue instrumentation.
369       // The target provides a guard check function, generate a call to it.
370       IRBuilder<> B(RI);
371       LoadInst *Guard = B.CreateLoad(AI, true, "Guard");
372       CallInst *Call = B.CreateCall(GuardCheck, {Guard});
373       llvm::Function *Function = cast<llvm::Function>(GuardCheck);
374       Call->setAttributes(Function->getAttributes());
375       Call->setCallingConv(Function->getCallingConv());
376     } else {
377       // Generate the epilogue with inline instrumentation.
378       // If we do not support SelectionDAG based tail calls, generate IR level
379       // tail calls.
380       //
381       // For each block with a return instruction, convert this:
382       //
383       //   return:
384       //     ...
385       //     ret ...
386       //
387       // into this:
388       //
389       //   return:
390       //     ...
391       //     %1 = <stack guard>
392       //     %2 = load StackGuardSlot
393       //     %3 = cmp i1 %1, %2
394       //     br i1 %3, label %SP_return, label %CallStackCheckFailBlk
395       //
396       //   SP_return:
397       //     ret ...
398       //
399       //   CallStackCheckFailBlk:
400       //     call void @__stack_chk_fail()
401       //     unreachable
402 
403       // Create the FailBB. We duplicate the BB every time since the MI tail
404       // merge pass will merge together all of the various BB into one including
405       // fail BB generated by the stack protector pseudo instruction.
406       BasicBlock *FailBB = CreateFailBB();
407 
408       // Split the basic block before the return instruction.
409       BasicBlock *NewBB = BB->splitBasicBlock(RI->getIterator(), "SP_return");
410 
411       // Update the dominator tree if we need to.
412       if (DT && DT->isReachableFromEntry(BB)) {
413         DT->addNewBlock(NewBB, BB);
414         DT->addNewBlock(FailBB, BB);
415       }
416 
417       // Remove default branch instruction to the new BB.
418       BB->getTerminator()->eraseFromParent();
419 
420       // Move the newly created basic block to the point right after the old
421       // basic block so that it's in the "fall through" position.
422       NewBB->moveAfter(BB);
423 
424       // Generate the stack protector instructions in the old basic block.
425       IRBuilder<> B(BB);
426       Value *Guard = getStackGuard(TLI, M, B);
427       LoadInst *LI2 = B.CreateLoad(AI, true);
428       Value *Cmp = B.CreateICmpEQ(Guard, LI2);
429       auto SuccessProb =
430           BranchProbabilityInfo::getBranchProbStackProtector(true);
431       auto FailureProb =
432           BranchProbabilityInfo::getBranchProbStackProtector(false);
433       MDNode *Weights = MDBuilder(F->getContext())
434                             .createBranchWeights(SuccessProb.getNumerator(),
435                                                  FailureProb.getNumerator());
436       B.CreateCondBr(Cmp, NewBB, FailBB, Weights);
437     }
438   }
439 
440   // Return if we didn't modify any basic blocks. i.e., there are no return
441   // statements in the function.
442   return HasPrologue;
443 }
444 
445 /// CreateFailBB - Create a basic block to jump to when the stack protector
446 /// check fails.
CreateFailBB()447 BasicBlock *StackProtector::CreateFailBB() {
448   LLVMContext &Context = F->getContext();
449   BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F);
450   IRBuilder<> B(FailBB);
451   B.SetCurrentDebugLocation(DebugLoc::get(0, 0, F->getSubprogram()));
452   if (Trip.isOSOpenBSD()) {
453     Constant *StackChkFail =
454         M->getOrInsertFunction("__stack_smash_handler",
455                                Type::getVoidTy(Context),
456                                Type::getInt8PtrTy(Context), nullptr);
457 
458     B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
459   } else {
460     Constant *StackChkFail =
461         M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context),
462                                nullptr);
463     B.CreateCall(StackChkFail, {});
464   }
465   B.CreateUnreachable();
466   return FailBB;
467 }
468 
shouldEmitSDCheck(const BasicBlock & BB) const469 bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const {
470   return HasPrologue && !HasIRCheck && dyn_cast<ReturnInst>(BB.getTerminator());
471 }
472