1 //===-- SafeStack.cpp - Safe Stack 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 splits the stack into the safe stack (kept as-is for LLVM backend)
11 // and the unsafe stack (explicitly allocated and managed through the runtime
12 // support library).
13 //
14 // http://clang.llvm.org/docs/SafeStack.html
15 //
16 //===----------------------------------------------------------------------===//
17
18 #include "llvm/Transforms/Instrumentation.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/ADT/Triple.h"
21 #include "llvm/Analysis/ScalarEvolution.h"
22 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
23 #include "llvm/CodeGen/Passes.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/DataLayout.h"
26 #include "llvm/IR/DerivedTypes.h"
27 #include "llvm/IR/DIBuilder.h"
28 #include "llvm/IR/Function.h"
29 #include "llvm/IR/InstIterator.h"
30 #include "llvm/IR/Instructions.h"
31 #include "llvm/IR/IntrinsicInst.h"
32 #include "llvm/IR/Intrinsics.h"
33 #include "llvm/IR/IRBuilder.h"
34 #include "llvm/IR/Module.h"
35 #include "llvm/Pass.h"
36 #include "llvm/Support/CommandLine.h"
37 #include "llvm/Support/Debug.h"
38 #include "llvm/Support/Format.h"
39 #include "llvm/Support/MathExtras.h"
40 #include "llvm/Support/raw_os_ostream.h"
41 #include "llvm/Target/TargetLowering.h"
42 #include "llvm/Target/TargetSubtargetInfo.h"
43 #include "llvm/Transforms/Utils/Local.h"
44 #include "llvm/Transforms/Utils/ModuleUtils.h"
45
46 using namespace llvm;
47
48 #define DEBUG_TYPE "safestack"
49
50 enum UnsafeStackPtrStorageVal { ThreadLocalUSP, SingleThreadUSP };
51
52 static cl::opt<UnsafeStackPtrStorageVal> USPStorage("safe-stack-usp-storage",
53 cl::Hidden, cl::init(ThreadLocalUSP),
54 cl::desc("Type of storage for the unsafe stack pointer"),
55 cl::values(clEnumValN(ThreadLocalUSP, "thread-local",
56 "Thread-local storage"),
57 clEnumValN(SingleThreadUSP, "single-thread",
58 "Non-thread-local storage"),
59 clEnumValEnd));
60
61 namespace llvm {
62
63 STATISTIC(NumFunctions, "Total number of functions");
64 STATISTIC(NumUnsafeStackFunctions, "Number of functions with unsafe stack");
65 STATISTIC(NumUnsafeStackRestorePointsFunctions,
66 "Number of functions that use setjmp or exceptions");
67
68 STATISTIC(NumAllocas, "Total number of allocas");
69 STATISTIC(NumUnsafeStaticAllocas, "Number of unsafe static allocas");
70 STATISTIC(NumUnsafeDynamicAllocas, "Number of unsafe dynamic allocas");
71 STATISTIC(NumUnsafeByValArguments, "Number of unsafe byval arguments");
72 STATISTIC(NumUnsafeStackRestorePoints, "Number of setjmps and landingpads");
73
74 } // namespace llvm
75
76 namespace {
77
78 /// Rewrite an SCEV expression for a memory access address to an expression that
79 /// represents offset from the given alloca.
80 ///
81 /// The implementation simply replaces all mentions of the alloca with zero.
82 class AllocaOffsetRewriter : public SCEVRewriteVisitor<AllocaOffsetRewriter> {
83 const Value *AllocaPtr;
84
85 public:
AllocaOffsetRewriter(ScalarEvolution & SE,const Value * AllocaPtr)86 AllocaOffsetRewriter(ScalarEvolution &SE, const Value *AllocaPtr)
87 : SCEVRewriteVisitor(SE), AllocaPtr(AllocaPtr) {}
88
visitUnknown(const SCEVUnknown * Expr)89 const SCEV *visitUnknown(const SCEVUnknown *Expr) {
90 if (Expr->getValue() == AllocaPtr)
91 return SE.getZero(Expr->getType());
92 return Expr;
93 }
94 };
95
96 /// The SafeStack pass splits the stack of each function into the safe
97 /// stack, which is only accessed through memory safe dereferences (as
98 /// determined statically), and the unsafe stack, which contains all
99 /// local variables that are accessed in ways that we can't prove to
100 /// be safe.
101 class SafeStack : public FunctionPass {
102 const TargetMachine *TM;
103 const TargetLoweringBase *TL;
104 const DataLayout *DL;
105 ScalarEvolution *SE;
106
107 Type *StackPtrTy;
108 Type *IntPtrTy;
109 Type *Int32Ty;
110 Type *Int8Ty;
111
112 Value *UnsafeStackPtr = nullptr;
113
114 /// Unsafe stack alignment. Each stack frame must ensure that the stack is
115 /// aligned to this value. We need to re-align the unsafe stack if the
116 /// alignment of any object on the stack exceeds this value.
117 ///
118 /// 16 seems like a reasonable upper bound on the alignment of objects that we
119 /// might expect to appear on the stack on most common targets.
120 enum { StackAlignment = 16 };
121
122 /// \brief Build a value representing a pointer to the unsafe stack pointer.
123 Value *getOrCreateUnsafeStackPtr(IRBuilder<> &IRB, Function &F);
124
125 /// \brief Find all static allocas, dynamic allocas, return instructions and
126 /// stack restore points (exception unwind blocks and setjmp calls) in the
127 /// given function and append them to the respective vectors.
128 void findInsts(Function &F, SmallVectorImpl<AllocaInst *> &StaticAllocas,
129 SmallVectorImpl<AllocaInst *> &DynamicAllocas,
130 SmallVectorImpl<Argument *> &ByValArguments,
131 SmallVectorImpl<ReturnInst *> &Returns,
132 SmallVectorImpl<Instruction *> &StackRestorePoints);
133
134 /// \brief Calculate the allocation size of a given alloca. Returns 0 if the
135 /// size can not be statically determined.
136 uint64_t getStaticAllocaAllocationSize(const AllocaInst* AI);
137
138 /// \brief Allocate space for all static allocas in \p StaticAllocas,
139 /// replace allocas with pointers into the unsafe stack and generate code to
140 /// restore the stack pointer before all return instructions in \p Returns.
141 ///
142 /// \returns A pointer to the top of the unsafe stack after all unsafe static
143 /// allocas are allocated.
144 Value *moveStaticAllocasToUnsafeStack(IRBuilder<> &IRB, Function &F,
145 ArrayRef<AllocaInst *> StaticAllocas,
146 ArrayRef<Argument *> ByValArguments,
147 ArrayRef<ReturnInst *> Returns);
148
149 /// \brief Generate code to restore the stack after all stack restore points
150 /// in \p StackRestorePoints.
151 ///
152 /// \returns A local variable in which to maintain the dynamic top of the
153 /// unsafe stack if needed.
154 AllocaInst *
155 createStackRestorePoints(IRBuilder<> &IRB, Function &F,
156 ArrayRef<Instruction *> StackRestorePoints,
157 Value *StaticTop, bool NeedDynamicTop);
158
159 /// \brief Replace all allocas in \p DynamicAllocas with code to allocate
160 /// space dynamically on the unsafe stack and store the dynamic unsafe stack
161 /// top to \p DynamicTop if non-null.
162 void moveDynamicAllocasToUnsafeStack(Function &F, Value *UnsafeStackPtr,
163 AllocaInst *DynamicTop,
164 ArrayRef<AllocaInst *> DynamicAllocas);
165
166 bool IsSafeStackAlloca(const Value *AllocaPtr, uint64_t AllocaSize);
167
168 bool IsMemIntrinsicSafe(const MemIntrinsic *MI, const Use &U,
169 const Value *AllocaPtr, uint64_t AllocaSize);
170 bool IsAccessSafe(Value *Addr, uint64_t Size, const Value *AllocaPtr,
171 uint64_t AllocaSize);
172
173 public:
174 static char ID; // Pass identification, replacement for typeid.
SafeStack(const TargetMachine * TM)175 SafeStack(const TargetMachine *TM)
176 : FunctionPass(ID), TM(TM), TL(nullptr), DL(nullptr) {
177 initializeSafeStackPass(*PassRegistry::getPassRegistry());
178 }
SafeStack()179 SafeStack() : SafeStack(nullptr) {}
180
getAnalysisUsage(AnalysisUsage & AU) const181 void getAnalysisUsage(AnalysisUsage &AU) const override {
182 AU.addRequired<ScalarEvolutionWrapperPass>();
183 }
184
doInitialization(Module & M)185 bool doInitialization(Module &M) override {
186 DL = &M.getDataLayout();
187
188 StackPtrTy = Type::getInt8PtrTy(M.getContext());
189 IntPtrTy = DL->getIntPtrType(M.getContext());
190 Int32Ty = Type::getInt32Ty(M.getContext());
191 Int8Ty = Type::getInt8Ty(M.getContext());
192
193 return false;
194 }
195
196 bool runOnFunction(Function &F) override;
197 }; // class SafeStack
198
getStaticAllocaAllocationSize(const AllocaInst * AI)199 uint64_t SafeStack::getStaticAllocaAllocationSize(const AllocaInst* AI) {
200 uint64_t Size = DL->getTypeAllocSize(AI->getAllocatedType());
201 if (AI->isArrayAllocation()) {
202 auto C = dyn_cast<ConstantInt>(AI->getArraySize());
203 if (!C)
204 return 0;
205 Size *= C->getZExtValue();
206 }
207 return Size;
208 }
209
IsAccessSafe(Value * Addr,uint64_t AccessSize,const Value * AllocaPtr,uint64_t AllocaSize)210 bool SafeStack::IsAccessSafe(Value *Addr, uint64_t AccessSize,
211 const Value *AllocaPtr, uint64_t AllocaSize) {
212 AllocaOffsetRewriter Rewriter(*SE, AllocaPtr);
213 const SCEV *Expr = Rewriter.visit(SE->getSCEV(Addr));
214
215 uint64_t BitWidth = SE->getTypeSizeInBits(Expr->getType());
216 ConstantRange AccessStartRange = SE->getUnsignedRange(Expr);
217 ConstantRange SizeRange =
218 ConstantRange(APInt(BitWidth, 0), APInt(BitWidth, AccessSize));
219 ConstantRange AccessRange = AccessStartRange.add(SizeRange);
220 ConstantRange AllocaRange =
221 ConstantRange(APInt(BitWidth, 0), APInt(BitWidth, AllocaSize));
222 bool Safe = AllocaRange.contains(AccessRange);
223
224 DEBUG(dbgs() << "[SafeStack] "
225 << (isa<AllocaInst>(AllocaPtr) ? "Alloca " : "ByValArgument ")
226 << *AllocaPtr << "\n"
227 << " Access " << *Addr << "\n"
228 << " SCEV " << *Expr
229 << " U: " << SE->getUnsignedRange(Expr)
230 << ", S: " << SE->getSignedRange(Expr) << "\n"
231 << " Range " << AccessRange << "\n"
232 << " AllocaRange " << AllocaRange << "\n"
233 << " " << (Safe ? "safe" : "unsafe") << "\n");
234
235 return Safe;
236 }
237
IsMemIntrinsicSafe(const MemIntrinsic * MI,const Use & U,const Value * AllocaPtr,uint64_t AllocaSize)238 bool SafeStack::IsMemIntrinsicSafe(const MemIntrinsic *MI, const Use &U,
239 const Value *AllocaPtr,
240 uint64_t AllocaSize) {
241 // All MemIntrinsics have destination address in Arg0 and size in Arg2.
242 if (MI->getRawDest() != U) return true;
243 const auto *Len = dyn_cast<ConstantInt>(MI->getLength());
244 // Non-constant size => unsafe. FIXME: try SCEV getRange.
245 if (!Len) return false;
246 return IsAccessSafe(U, Len->getZExtValue(), AllocaPtr, AllocaSize);
247 }
248
249 /// Check whether a given allocation must be put on the safe
250 /// stack or not. The function analyzes all uses of AI and checks whether it is
251 /// only accessed in a memory safe way (as decided statically).
IsSafeStackAlloca(const Value * AllocaPtr,uint64_t AllocaSize)252 bool SafeStack::IsSafeStackAlloca(const Value *AllocaPtr, uint64_t AllocaSize) {
253 // Go through all uses of this alloca and check whether all accesses to the
254 // allocated object are statically known to be memory safe and, hence, the
255 // object can be placed on the safe stack.
256 SmallPtrSet<const Value *, 16> Visited;
257 SmallVector<const Value *, 8> WorkList;
258 WorkList.push_back(AllocaPtr);
259
260 // A DFS search through all uses of the alloca in bitcasts/PHI/GEPs/etc.
261 while (!WorkList.empty()) {
262 const Value *V = WorkList.pop_back_val();
263 for (const Use &UI : V->uses()) {
264 auto I = cast<const Instruction>(UI.getUser());
265 assert(V == UI.get());
266
267 switch (I->getOpcode()) {
268 case Instruction::Load: {
269 if (!IsAccessSafe(UI, DL->getTypeStoreSize(I->getType()), AllocaPtr,
270 AllocaSize))
271 return false;
272 break;
273 }
274 case Instruction::VAArg:
275 // "va-arg" from a pointer is safe.
276 break;
277 case Instruction::Store: {
278 if (V == I->getOperand(0)) {
279 // Stored the pointer - conservatively assume it may be unsafe.
280 DEBUG(dbgs() << "[SafeStack] Unsafe alloca: " << *AllocaPtr
281 << "\n store of address: " << *I << "\n");
282 return false;
283 }
284
285 if (!IsAccessSafe(UI, DL->getTypeStoreSize(I->getOperand(0)->getType()),
286 AllocaPtr, AllocaSize))
287 return false;
288 break;
289 }
290 case Instruction::Ret: {
291 // Information leak.
292 return false;
293 }
294
295 case Instruction::Call:
296 case Instruction::Invoke: {
297 ImmutableCallSite CS(I);
298
299 if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
300 if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
301 II->getIntrinsicID() == Intrinsic::lifetime_end)
302 continue;
303 }
304
305 if (const MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
306 if (!IsMemIntrinsicSafe(MI, UI, AllocaPtr, AllocaSize)) {
307 DEBUG(dbgs() << "[SafeStack] Unsafe alloca: " << *AllocaPtr
308 << "\n unsafe memintrinsic: " << *I
309 << "\n");
310 return false;
311 }
312 continue;
313 }
314
315 // LLVM 'nocapture' attribute is only set for arguments whose address
316 // is not stored, passed around, or used in any other non-trivial way.
317 // We assume that passing a pointer to an object as a 'nocapture
318 // readnone' argument is safe.
319 // FIXME: a more precise solution would require an interprocedural
320 // analysis here, which would look at all uses of an argument inside
321 // the function being called.
322 ImmutableCallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
323 for (ImmutableCallSite::arg_iterator A = B; A != E; ++A)
324 if (A->get() == V)
325 if (!(CS.doesNotCapture(A - B) && (CS.doesNotAccessMemory(A - B) ||
326 CS.doesNotAccessMemory()))) {
327 DEBUG(dbgs() << "[SafeStack] Unsafe alloca: " << *AllocaPtr
328 << "\n unsafe call: " << *I << "\n");
329 return false;
330 }
331 continue;
332 }
333
334 default:
335 if (Visited.insert(I).second)
336 WorkList.push_back(cast<const Instruction>(I));
337 }
338 }
339 }
340
341 // All uses of the alloca are safe, we can place it on the safe stack.
342 return true;
343 }
344
getOrCreateUnsafeStackPtr(IRBuilder<> & IRB,Function & F)345 Value *SafeStack::getOrCreateUnsafeStackPtr(IRBuilder<> &IRB, Function &F) {
346 // Check if there is a target-specific location for the unsafe stack pointer.
347 if (TL)
348 if (Value *V = TL->getSafeStackPointerLocation(IRB))
349 return V;
350
351 // Otherwise, assume the target links with compiler-rt, which provides a
352 // thread-local variable with a magic name.
353 Module &M = *F.getParent();
354 const char *UnsafeStackPtrVar = "__safestack_unsafe_stack_ptr";
355 auto UnsafeStackPtr =
356 dyn_cast_or_null<GlobalVariable>(M.getNamedValue(UnsafeStackPtrVar));
357
358 bool UseTLS = USPStorage == ThreadLocalUSP;
359
360 if (!UnsafeStackPtr) {
361 auto TLSModel = UseTLS ?
362 GlobalValue::InitialExecTLSModel :
363 GlobalValue::NotThreadLocal;
364 // The global variable is not defined yet, define it ourselves.
365 // We use the initial-exec TLS model because we do not support the
366 // variable living anywhere other than in the main executable.
367 UnsafeStackPtr = new GlobalVariable(
368 M, StackPtrTy, false, GlobalValue::ExternalLinkage, nullptr,
369 UnsafeStackPtrVar, nullptr, TLSModel);
370 } else {
371 // The variable exists, check its type and attributes.
372 if (UnsafeStackPtr->getValueType() != StackPtrTy)
373 report_fatal_error(Twine(UnsafeStackPtrVar) + " must have void* type");
374 if (UseTLS != UnsafeStackPtr->isThreadLocal())
375 report_fatal_error(Twine(UnsafeStackPtrVar) + " must " +
376 (UseTLS ? "" : "not ") + "be thread-local");
377 }
378 return UnsafeStackPtr;
379 }
380
findInsts(Function & F,SmallVectorImpl<AllocaInst * > & StaticAllocas,SmallVectorImpl<AllocaInst * > & DynamicAllocas,SmallVectorImpl<Argument * > & ByValArguments,SmallVectorImpl<ReturnInst * > & Returns,SmallVectorImpl<Instruction * > & StackRestorePoints)381 void SafeStack::findInsts(Function &F,
382 SmallVectorImpl<AllocaInst *> &StaticAllocas,
383 SmallVectorImpl<AllocaInst *> &DynamicAllocas,
384 SmallVectorImpl<Argument *> &ByValArguments,
385 SmallVectorImpl<ReturnInst *> &Returns,
386 SmallVectorImpl<Instruction *> &StackRestorePoints) {
387 for (Instruction &I : instructions(&F)) {
388 if (auto AI = dyn_cast<AllocaInst>(&I)) {
389 ++NumAllocas;
390
391 uint64_t Size = getStaticAllocaAllocationSize(AI);
392 if (IsSafeStackAlloca(AI, Size))
393 continue;
394
395 if (AI->isStaticAlloca()) {
396 ++NumUnsafeStaticAllocas;
397 StaticAllocas.push_back(AI);
398 } else {
399 ++NumUnsafeDynamicAllocas;
400 DynamicAllocas.push_back(AI);
401 }
402 } else if (auto RI = dyn_cast<ReturnInst>(&I)) {
403 Returns.push_back(RI);
404 } else if (auto CI = dyn_cast<CallInst>(&I)) {
405 // setjmps require stack restore.
406 if (CI->getCalledFunction() && CI->canReturnTwice())
407 StackRestorePoints.push_back(CI);
408 } else if (auto LP = dyn_cast<LandingPadInst>(&I)) {
409 // Exception landing pads require stack restore.
410 StackRestorePoints.push_back(LP);
411 } else if (auto II = dyn_cast<IntrinsicInst>(&I)) {
412 if (II->getIntrinsicID() == Intrinsic::gcroot)
413 llvm::report_fatal_error(
414 "gcroot intrinsic not compatible with safestack attribute");
415 }
416 }
417 for (Argument &Arg : F.args()) {
418 if (!Arg.hasByValAttr())
419 continue;
420 uint64_t Size =
421 DL->getTypeStoreSize(Arg.getType()->getPointerElementType());
422 if (IsSafeStackAlloca(&Arg, Size))
423 continue;
424
425 ++NumUnsafeByValArguments;
426 ByValArguments.push_back(&Arg);
427 }
428 }
429
430 AllocaInst *
createStackRestorePoints(IRBuilder<> & IRB,Function & F,ArrayRef<Instruction * > StackRestorePoints,Value * StaticTop,bool NeedDynamicTop)431 SafeStack::createStackRestorePoints(IRBuilder<> &IRB, Function &F,
432 ArrayRef<Instruction *> StackRestorePoints,
433 Value *StaticTop, bool NeedDynamicTop) {
434 if (StackRestorePoints.empty())
435 return nullptr;
436
437 // We need the current value of the shadow stack pointer to restore
438 // after longjmp or exception catching.
439
440 // FIXME: On some platforms this could be handled by the longjmp/exception
441 // runtime itself.
442
443 AllocaInst *DynamicTop = nullptr;
444 if (NeedDynamicTop)
445 // If we also have dynamic alloca's, the stack pointer value changes
446 // throughout the function. For now we store it in an alloca.
447 DynamicTop = IRB.CreateAlloca(StackPtrTy, /*ArraySize=*/nullptr,
448 "unsafe_stack_dynamic_ptr");
449
450 if (!StaticTop)
451 // We need the original unsafe stack pointer value, even if there are
452 // no unsafe static allocas.
453 StaticTop = IRB.CreateLoad(UnsafeStackPtr, false, "unsafe_stack_ptr");
454
455 if (NeedDynamicTop)
456 IRB.CreateStore(StaticTop, DynamicTop);
457
458 // Restore current stack pointer after longjmp/exception catch.
459 for (Instruction *I : StackRestorePoints) {
460 ++NumUnsafeStackRestorePoints;
461
462 IRB.SetInsertPoint(I->getNextNode());
463 Value *CurrentTop = DynamicTop ? IRB.CreateLoad(DynamicTop) : StaticTop;
464 IRB.CreateStore(CurrentTop, UnsafeStackPtr);
465 }
466
467 return DynamicTop;
468 }
469
moveStaticAllocasToUnsafeStack(IRBuilder<> & IRB,Function & F,ArrayRef<AllocaInst * > StaticAllocas,ArrayRef<Argument * > ByValArguments,ArrayRef<ReturnInst * > Returns)470 Value *SafeStack::moveStaticAllocasToUnsafeStack(
471 IRBuilder<> &IRB, Function &F, ArrayRef<AllocaInst *> StaticAllocas,
472 ArrayRef<Argument *> ByValArguments, ArrayRef<ReturnInst *> Returns) {
473 if (StaticAllocas.empty() && ByValArguments.empty())
474 return nullptr;
475
476 DIBuilder DIB(*F.getParent());
477
478 // We explicitly compute and set the unsafe stack layout for all unsafe
479 // static alloca instructions. We save the unsafe "base pointer" in the
480 // prologue into a local variable and restore it in the epilogue.
481
482 // Load the current stack pointer (we'll also use it as a base pointer).
483 // FIXME: use a dedicated register for it ?
484 Instruction *BasePointer =
485 IRB.CreateLoad(UnsafeStackPtr, false, "unsafe_stack_ptr");
486 assert(BasePointer->getType() == StackPtrTy);
487
488 for (ReturnInst *RI : Returns) {
489 IRB.SetInsertPoint(RI);
490 IRB.CreateStore(BasePointer, UnsafeStackPtr);
491 }
492
493 // Compute maximum alignment among static objects on the unsafe stack.
494 unsigned MaxAlignment = 0;
495 for (Argument *Arg : ByValArguments) {
496 Type *Ty = Arg->getType()->getPointerElementType();
497 unsigned Align = std::max((unsigned)DL->getPrefTypeAlignment(Ty),
498 Arg->getParamAlignment());
499 if (Align > MaxAlignment)
500 MaxAlignment = Align;
501 }
502 for (AllocaInst *AI : StaticAllocas) {
503 Type *Ty = AI->getAllocatedType();
504 unsigned Align =
505 std::max((unsigned)DL->getPrefTypeAlignment(Ty), AI->getAlignment());
506 if (Align > MaxAlignment)
507 MaxAlignment = Align;
508 }
509
510 if (MaxAlignment > StackAlignment) {
511 // Re-align the base pointer according to the max requested alignment.
512 assert(isPowerOf2_32(MaxAlignment));
513 IRB.SetInsertPoint(BasePointer->getNextNode());
514 BasePointer = cast<Instruction>(IRB.CreateIntToPtr(
515 IRB.CreateAnd(IRB.CreatePtrToInt(BasePointer, IntPtrTy),
516 ConstantInt::get(IntPtrTy, ~uint64_t(MaxAlignment - 1))),
517 StackPtrTy));
518 }
519
520 int64_t StaticOffset = 0; // Current stack top.
521 IRB.SetInsertPoint(BasePointer->getNextNode());
522
523 for (Argument *Arg : ByValArguments) {
524 Type *Ty = Arg->getType()->getPointerElementType();
525
526 uint64_t Size = DL->getTypeStoreSize(Ty);
527 if (Size == 0)
528 Size = 1; // Don't create zero-sized stack objects.
529
530 // Ensure the object is properly aligned.
531 unsigned Align = std::max((unsigned)DL->getPrefTypeAlignment(Ty),
532 Arg->getParamAlignment());
533
534 // Add alignment.
535 // NOTE: we ensure that BasePointer itself is aligned to >= Align.
536 StaticOffset += Size;
537 StaticOffset = RoundUpToAlignment(StaticOffset, Align);
538
539 Value *Off = IRB.CreateGEP(BasePointer, // BasePointer is i8*
540 ConstantInt::get(Int32Ty, -StaticOffset));
541 Value *NewArg = IRB.CreateBitCast(Off, Arg->getType(),
542 Arg->getName() + ".unsafe-byval");
543
544 // Replace alloc with the new location.
545 replaceDbgDeclare(Arg, BasePointer, BasePointer->getNextNode(), DIB,
546 /*Deref=*/true, -StaticOffset);
547 Arg->replaceAllUsesWith(NewArg);
548 IRB.SetInsertPoint(cast<Instruction>(NewArg)->getNextNode());
549 IRB.CreateMemCpy(Off, Arg, Size, Arg->getParamAlignment());
550 }
551
552 // Allocate space for every unsafe static AllocaInst on the unsafe stack.
553 for (AllocaInst *AI : StaticAllocas) {
554 IRB.SetInsertPoint(AI);
555
556 Type *Ty = AI->getAllocatedType();
557 uint64_t Size = getStaticAllocaAllocationSize(AI);
558 if (Size == 0)
559 Size = 1; // Don't create zero-sized stack objects.
560
561 // Ensure the object is properly aligned.
562 unsigned Align =
563 std::max((unsigned)DL->getPrefTypeAlignment(Ty), AI->getAlignment());
564
565 // Add alignment.
566 // NOTE: we ensure that BasePointer itself is aligned to >= Align.
567 StaticOffset += Size;
568 StaticOffset = RoundUpToAlignment(StaticOffset, Align);
569
570 Value *Off = IRB.CreateGEP(BasePointer, // BasePointer is i8*
571 ConstantInt::get(Int32Ty, -StaticOffset));
572 Value *NewAI = IRB.CreateBitCast(Off, AI->getType(), AI->getName());
573 if (AI->hasName() && isa<Instruction>(NewAI))
574 cast<Instruction>(NewAI)->takeName(AI);
575
576 // Replace alloc with the new location.
577 replaceDbgDeclareForAlloca(AI, BasePointer, DIB, /*Deref=*/true, -StaticOffset);
578 AI->replaceAllUsesWith(NewAI);
579 AI->eraseFromParent();
580 }
581
582 // Re-align BasePointer so that our callees would see it aligned as
583 // expected.
584 // FIXME: no need to update BasePointer in leaf functions.
585 StaticOffset = RoundUpToAlignment(StaticOffset, StackAlignment);
586
587 // Update shadow stack pointer in the function epilogue.
588 IRB.SetInsertPoint(BasePointer->getNextNode());
589
590 Value *StaticTop =
591 IRB.CreateGEP(BasePointer, ConstantInt::get(Int32Ty, -StaticOffset),
592 "unsafe_stack_static_top");
593 IRB.CreateStore(StaticTop, UnsafeStackPtr);
594 return StaticTop;
595 }
596
moveDynamicAllocasToUnsafeStack(Function & F,Value * UnsafeStackPtr,AllocaInst * DynamicTop,ArrayRef<AllocaInst * > DynamicAllocas)597 void SafeStack::moveDynamicAllocasToUnsafeStack(
598 Function &F, Value *UnsafeStackPtr, AllocaInst *DynamicTop,
599 ArrayRef<AllocaInst *> DynamicAllocas) {
600 DIBuilder DIB(*F.getParent());
601
602 for (AllocaInst *AI : DynamicAllocas) {
603 IRBuilder<> IRB(AI);
604
605 // Compute the new SP value (after AI).
606 Value *ArraySize = AI->getArraySize();
607 if (ArraySize->getType() != IntPtrTy)
608 ArraySize = IRB.CreateIntCast(ArraySize, IntPtrTy, false);
609
610 Type *Ty = AI->getAllocatedType();
611 uint64_t TySize = DL->getTypeAllocSize(Ty);
612 Value *Size = IRB.CreateMul(ArraySize, ConstantInt::get(IntPtrTy, TySize));
613
614 Value *SP = IRB.CreatePtrToInt(IRB.CreateLoad(UnsafeStackPtr), IntPtrTy);
615 SP = IRB.CreateSub(SP, Size);
616
617 // Align the SP value to satisfy the AllocaInst, type and stack alignments.
618 unsigned Align = std::max(
619 std::max((unsigned)DL->getPrefTypeAlignment(Ty), AI->getAlignment()),
620 (unsigned)StackAlignment);
621
622 assert(isPowerOf2_32(Align));
623 Value *NewTop = IRB.CreateIntToPtr(
624 IRB.CreateAnd(SP, ConstantInt::get(IntPtrTy, ~uint64_t(Align - 1))),
625 StackPtrTy);
626
627 // Save the stack pointer.
628 IRB.CreateStore(NewTop, UnsafeStackPtr);
629 if (DynamicTop)
630 IRB.CreateStore(NewTop, DynamicTop);
631
632 Value *NewAI = IRB.CreatePointerCast(NewTop, AI->getType());
633 if (AI->hasName() && isa<Instruction>(NewAI))
634 NewAI->takeName(AI);
635
636 replaceDbgDeclareForAlloca(AI, NewAI, DIB, /*Deref=*/true);
637 AI->replaceAllUsesWith(NewAI);
638 AI->eraseFromParent();
639 }
640
641 if (!DynamicAllocas.empty()) {
642 // Now go through the instructions again, replacing stacksave/stackrestore.
643 for (inst_iterator It = inst_begin(&F), Ie = inst_end(&F); It != Ie;) {
644 Instruction *I = &*(It++);
645 auto II = dyn_cast<IntrinsicInst>(I);
646 if (!II)
647 continue;
648
649 if (II->getIntrinsicID() == Intrinsic::stacksave) {
650 IRBuilder<> IRB(II);
651 Instruction *LI = IRB.CreateLoad(UnsafeStackPtr);
652 LI->takeName(II);
653 II->replaceAllUsesWith(LI);
654 II->eraseFromParent();
655 } else if (II->getIntrinsicID() == Intrinsic::stackrestore) {
656 IRBuilder<> IRB(II);
657 Instruction *SI = IRB.CreateStore(II->getArgOperand(0), UnsafeStackPtr);
658 SI->takeName(II);
659 assert(II->use_empty());
660 II->eraseFromParent();
661 }
662 }
663 }
664 }
665
runOnFunction(Function & F)666 bool SafeStack::runOnFunction(Function &F) {
667 DEBUG(dbgs() << "[SafeStack] Function: " << F.getName() << "\n");
668
669 if (!F.hasFnAttribute(Attribute::SafeStack)) {
670 DEBUG(dbgs() << "[SafeStack] safestack is not requested"
671 " for this function\n");
672 return false;
673 }
674
675 if (F.isDeclaration()) {
676 DEBUG(dbgs() << "[SafeStack] function definition"
677 " is not available\n");
678 return false;
679 }
680
681 TL = TM ? TM->getSubtargetImpl(F)->getTargetLowering() : nullptr;
682 SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
683
684 {
685 // Make sure the regular stack protector won't run on this function
686 // (safestack attribute takes precedence).
687 AttrBuilder B;
688 B.addAttribute(Attribute::StackProtect)
689 .addAttribute(Attribute::StackProtectReq)
690 .addAttribute(Attribute::StackProtectStrong);
691 F.removeAttributes(
692 AttributeSet::FunctionIndex,
693 AttributeSet::get(F.getContext(), AttributeSet::FunctionIndex, B));
694 }
695
696 ++NumFunctions;
697
698 SmallVector<AllocaInst *, 16> StaticAllocas;
699 SmallVector<AllocaInst *, 4> DynamicAllocas;
700 SmallVector<Argument *, 4> ByValArguments;
701 SmallVector<ReturnInst *, 4> Returns;
702
703 // Collect all points where stack gets unwound and needs to be restored
704 // This is only necessary because the runtime (setjmp and unwind code) is
705 // not aware of the unsafe stack and won't unwind/restore it prorerly.
706 // To work around this problem without changing the runtime, we insert
707 // instrumentation to restore the unsafe stack pointer when necessary.
708 SmallVector<Instruction *, 4> StackRestorePoints;
709
710 // Find all static and dynamic alloca instructions that must be moved to the
711 // unsafe stack, all return instructions and stack restore points.
712 findInsts(F, StaticAllocas, DynamicAllocas, ByValArguments, Returns,
713 StackRestorePoints);
714
715 if (StaticAllocas.empty() && DynamicAllocas.empty() &&
716 ByValArguments.empty() && StackRestorePoints.empty())
717 return false; // Nothing to do in this function.
718
719 if (!StaticAllocas.empty() || !DynamicAllocas.empty() ||
720 !ByValArguments.empty())
721 ++NumUnsafeStackFunctions; // This function has the unsafe stack.
722
723 if (!StackRestorePoints.empty())
724 ++NumUnsafeStackRestorePointsFunctions;
725
726 IRBuilder<> IRB(&F.front(), F.begin()->getFirstInsertionPt());
727 UnsafeStackPtr = getOrCreateUnsafeStackPtr(IRB, F);
728
729 // The top of the unsafe stack after all unsafe static allocas are allocated.
730 Value *StaticTop = moveStaticAllocasToUnsafeStack(IRB, F, StaticAllocas,
731 ByValArguments, Returns);
732
733 // Safe stack object that stores the current unsafe stack top. It is updated
734 // as unsafe dynamic (non-constant-sized) allocas are allocated and freed.
735 // This is only needed if we need to restore stack pointer after longjmp
736 // or exceptions, and we have dynamic allocations.
737 // FIXME: a better alternative might be to store the unsafe stack pointer
738 // before setjmp / invoke instructions.
739 AllocaInst *DynamicTop = createStackRestorePoints(
740 IRB, F, StackRestorePoints, StaticTop, !DynamicAllocas.empty());
741
742 // Handle dynamic allocas.
743 moveDynamicAllocasToUnsafeStack(F, UnsafeStackPtr, DynamicTop,
744 DynamicAllocas);
745
746 DEBUG(dbgs() << "[SafeStack] safestack applied\n");
747 return true;
748 }
749
750 } // anonymous namespace
751
752 char SafeStack::ID = 0;
753 INITIALIZE_TM_PASS_BEGIN(SafeStack, "safe-stack",
754 "Safe Stack instrumentation pass", false, false)
755 INITIALIZE_TM_PASS_END(SafeStack, "safe-stack",
756 "Safe Stack instrumentation pass", false, false)
757
createSafeStackPass(const llvm::TargetMachine * TM)758 FunctionPass *llvm::createSafeStackPass(const llvm::TargetMachine *TM) {
759 return new SafeStack(TM);
760 }
761