1 //=== VLASizeChecker.cpp - Undefined dereference checker --------*- C++ -*-===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This defines VLASizeChecker, a builtin check in ExprEngine that 10 // performs checks for declaration of VLA of undefined or zero size. 11 // In addition, VLASizeChecker is responsible for defining the extent 12 // of the MemRegion that represents a VLA. 13 // 14 //===----------------------------------------------------------------------===// 15 16 #include "clang/AST/CharUnits.h" 17 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h" 18 #include "clang/StaticAnalyzer/Checkers/Taint.h" 19 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h" 20 #include "clang/StaticAnalyzer/Core/Checker.h" 21 #include "clang/StaticAnalyzer/Core/CheckerManager.h" 22 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h" 23 #include "clang/StaticAnalyzer/Core/PathSensitive/DynamicExtent.h" 24 #include "llvm/ADT/STLExtras.h" 25 #include "llvm/ADT/SmallString.h" 26 #include "llvm/Support/raw_ostream.h" 27 #include <optional> 28 29 using namespace clang; 30 using namespace ento; 31 using namespace taint; 32 33 namespace { 34 class VLASizeChecker 35 : public Checker<check::PreStmt<DeclStmt>, 36 check::PreStmt<UnaryExprOrTypeTraitExpr>> { 37 const BugType BT{this, "Dangerous variable-length array (VLA) declaration"}; 38 const BugType TaintBT{this, 39 "Dangerous variable-length array (VLA) declaration", 40 categories::TaintedData}; 41 enum VLASize_Kind { VLA_Garbage, VLA_Zero, VLA_Negative, VLA_Overflow }; 42 43 /// Check a VLA for validity. 44 /// Every dimension of the array and the total size is checked for validity. 45 /// Returns null or a new state where the size is validated. 46 /// 'ArraySize' will contain SVal that refers to the total size (in char) 47 /// of the array. 48 ProgramStateRef checkVLA(CheckerContext &C, ProgramStateRef State, 49 const VariableArrayType *VLA, SVal &ArraySize) const; 50 /// Check a single VLA index size expression for validity. 51 ProgramStateRef checkVLAIndexSize(CheckerContext &C, ProgramStateRef State, 52 const Expr *SizeE) const; 53 54 void reportBug(VLASize_Kind Kind, const Expr *SizeE, ProgramStateRef State, 55 CheckerContext &C) const; 56 57 void reportTaintBug(const Expr *SizeE, ProgramStateRef State, 58 CheckerContext &C, SVal TaintedSVal) const; 59 60 public: 61 void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const; 62 void checkPreStmt(const UnaryExprOrTypeTraitExpr *UETTE, 63 CheckerContext &C) const; 64 }; 65 } // end anonymous namespace 66 67 ProgramStateRef VLASizeChecker::checkVLA(CheckerContext &C, 68 ProgramStateRef State, 69 const VariableArrayType *VLA, 70 SVal &ArraySize) const { 71 assert(VLA && "Function should be called with non-null VLA argument."); 72 73 const VariableArrayType *VLALast = nullptr; 74 llvm::SmallVector<const Expr *, 2> VLASizes; 75 76 // Walk over the VLAs for every dimension until a non-VLA is found. 77 // There is a VariableArrayType for every dimension (fixed or variable) until 78 // the most inner array that is variably modified. 79 // Dimension sizes are collected into 'VLASizes'. 'VLALast' is set to the 80 // innermost VLA that was encountered. 81 // In "int vla[x][2][y][3]" this will be the array for index "y" (with type 82 // int[3]). 'VLASizes' contains 'x', '2', and 'y'. 83 while (VLA) { 84 const Expr *SizeE = VLA->getSizeExpr(); 85 State = checkVLAIndexSize(C, State, SizeE); 86 if (!State) 87 return nullptr; 88 VLASizes.push_back(SizeE); 89 VLALast = VLA; 90 VLA = C.getASTContext().getAsVariableArrayType(VLA->getElementType()); 91 }; 92 assert(VLALast && 93 "Array should have at least one variably-modified dimension."); 94 95 ASTContext &Ctx = C.getASTContext(); 96 SValBuilder &SVB = C.getSValBuilder(); 97 CanQualType SizeTy = Ctx.getSizeType(); 98 uint64_t SizeMax = 99 SVB.getBasicValueFactory().getMaxValue(SizeTy).getZExtValue(); 100 101 // Get the element size. 102 CharUnits EleSize = Ctx.getTypeSizeInChars(VLALast->getElementType()); 103 NonLoc ArrSize = 104 SVB.makeIntVal(EleSize.getQuantity(), SizeTy).castAs<NonLoc>(); 105 106 // Try to calculate the known real size of the array in KnownSize. 107 uint64_t KnownSize = 0; 108 if (const llvm::APSInt *KV = SVB.getKnownValue(State, ArrSize)) 109 KnownSize = KV->getZExtValue(); 110 111 for (const Expr *SizeE : VLASizes) { 112 auto SizeD = C.getSVal(SizeE).castAs<DefinedSVal>(); 113 // Convert the array length to size_t. 114 NonLoc IndexLength = 115 SVB.evalCast(SizeD, SizeTy, SizeE->getType()).castAs<NonLoc>(); 116 // Multiply the array length by the element size. 117 SVal Mul = SVB.evalBinOpNN(State, BO_Mul, ArrSize, IndexLength, SizeTy); 118 if (auto MulNonLoc = Mul.getAs<NonLoc>()) 119 ArrSize = *MulNonLoc; 120 else 121 // Extent could not be determined. 122 return State; 123 124 if (const llvm::APSInt *IndexLVal = SVB.getKnownValue(State, IndexLength)) { 125 // Check if the array size will overflow. 126 // Size overflow check does not work with symbolic expressions because a 127 // overflow situation can not be detected easily. 128 uint64_t IndexL = IndexLVal->getZExtValue(); 129 // FIXME: See https://reviews.llvm.org/D80903 for discussion of 130 // some difference in assume and getKnownValue that leads to 131 // unexpected behavior. Just bail on IndexL == 0 at this point. 132 if (IndexL == 0) 133 return nullptr; 134 135 if (KnownSize <= SizeMax / IndexL) { 136 KnownSize *= IndexL; 137 } else { 138 // Array size does not fit into size_t. 139 reportBug(VLA_Overflow, SizeE, State, C); 140 return nullptr; 141 } 142 } else { 143 KnownSize = 0; 144 } 145 } 146 147 ArraySize = ArrSize; 148 149 return State; 150 } 151 152 ProgramStateRef VLASizeChecker::checkVLAIndexSize(CheckerContext &C, 153 ProgramStateRef State, 154 const Expr *SizeE) const { 155 SVal SizeV = C.getSVal(SizeE); 156 157 if (SizeV.isUndef()) { 158 reportBug(VLA_Garbage, SizeE, State, C); 159 return nullptr; 160 } 161 162 // See if the size value is known. It can't be undefined because we would have 163 // warned about that already. 164 if (SizeV.isUnknown()) 165 return nullptr; 166 167 // Check if the size is tainted. 168 if (isTainted(State, SizeV)) { 169 reportTaintBug(SizeE, State, C, SizeV); 170 return nullptr; 171 } 172 173 // Check if the size is zero. 174 DefinedSVal SizeD = SizeV.castAs<DefinedSVal>(); 175 176 ProgramStateRef StateNotZero, StateZero; 177 std::tie(StateNotZero, StateZero) = State->assume(SizeD); 178 179 if (StateZero && !StateNotZero) { 180 reportBug(VLA_Zero, SizeE, StateZero, C); 181 return nullptr; 182 } 183 184 // From this point on, assume that the size is not zero. 185 State = StateNotZero; 186 187 // Check if the size is negative. 188 SValBuilder &SVB = C.getSValBuilder(); 189 190 QualType SizeTy = SizeE->getType(); 191 DefinedOrUnknownSVal Zero = SVB.makeZeroVal(SizeTy); 192 193 SVal LessThanZeroVal = 194 SVB.evalBinOp(State, BO_LT, SizeD, Zero, SVB.getConditionType()); 195 if (std::optional<DefinedSVal> LessThanZeroDVal = 196 LessThanZeroVal.getAs<DefinedSVal>()) { 197 ConstraintManager &CM = C.getConstraintManager(); 198 ProgramStateRef StatePos, StateNeg; 199 200 std::tie(StateNeg, StatePos) = CM.assumeDual(State, *LessThanZeroDVal); 201 if (StateNeg && !StatePos) { 202 reportBug(VLA_Negative, SizeE, State, C); 203 return nullptr; 204 } 205 State = StatePos; 206 } 207 208 return State; 209 } 210 211 void VLASizeChecker::reportTaintBug(const Expr *SizeE, ProgramStateRef State, 212 CheckerContext &C, SVal TaintedSVal) const { 213 // Generate an error node. 214 ExplodedNode *N = C.generateErrorNode(State); 215 if (!N) 216 return; 217 218 SmallString<256> buf; 219 llvm::raw_svector_ostream os(buf); 220 os << "Declared variable-length array (VLA) "; 221 os << "has tainted size"; 222 223 auto report = std::make_unique<PathSensitiveBugReport>(TaintBT, os.str(), N); 224 report->addRange(SizeE->getSourceRange()); 225 bugreporter::trackExpressionValue(N, SizeE, *report); 226 // The vla size may be a complex expression where multiple memory locations 227 // are tainted. 228 for (auto Sym : getTaintedSymbols(State, TaintedSVal)) 229 report->markInteresting(Sym); 230 C.emitReport(std::move(report)); 231 } 232 233 void VLASizeChecker::reportBug(VLASize_Kind Kind, const Expr *SizeE, 234 ProgramStateRef State, CheckerContext &C) const { 235 // Generate an error node. 236 ExplodedNode *N = C.generateErrorNode(State); 237 if (!N) 238 return; 239 240 SmallString<256> buf; 241 llvm::raw_svector_ostream os(buf); 242 os << "Declared variable-length array (VLA) "; 243 switch (Kind) { 244 case VLA_Garbage: 245 os << "uses a garbage value as its size"; 246 break; 247 case VLA_Zero: 248 os << "has zero size"; 249 break; 250 case VLA_Negative: 251 os << "has negative size"; 252 break; 253 case VLA_Overflow: 254 os << "has too large size"; 255 break; 256 } 257 258 auto report = std::make_unique<PathSensitiveBugReport>(BT, os.str(), N); 259 report->addRange(SizeE->getSourceRange()); 260 bugreporter::trackExpressionValue(N, SizeE, *report); 261 C.emitReport(std::move(report)); 262 } 263 264 void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const { 265 if (!DS->isSingleDecl()) 266 return; 267 268 ASTContext &Ctx = C.getASTContext(); 269 SValBuilder &SVB = C.getSValBuilder(); 270 ProgramStateRef State = C.getState(); 271 QualType TypeToCheck; 272 273 const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl()); 274 275 if (VD) 276 TypeToCheck = VD->getType().getCanonicalType(); 277 else if (const auto *TND = dyn_cast<TypedefNameDecl>(DS->getSingleDecl())) 278 TypeToCheck = TND->getUnderlyingType().getCanonicalType(); 279 else 280 return; 281 282 const VariableArrayType *VLA = Ctx.getAsVariableArrayType(TypeToCheck); 283 if (!VLA) 284 return; 285 286 // Check the VLA sizes for validity. 287 288 SVal ArraySize; 289 290 State = checkVLA(C, State, VLA, ArraySize); 291 if (!State) 292 return; 293 294 if (!isa<NonLoc>(ArraySize)) { 295 // Array size could not be determined but state may contain new assumptions. 296 C.addTransition(State); 297 return; 298 } 299 300 // VLASizeChecker is responsible for defining the extent of the array. 301 if (VD) { 302 State = 303 setDynamicExtent(State, State->getRegion(VD, C.getLocationContext()), 304 ArraySize.castAs<NonLoc>(), SVB); 305 } 306 307 // Remember our assumptions! 308 C.addTransition(State); 309 } 310 311 void VLASizeChecker::checkPreStmt(const UnaryExprOrTypeTraitExpr *UETTE, 312 CheckerContext &C) const { 313 // Want to check for sizeof. 314 if (UETTE->getKind() != UETT_SizeOf) 315 return; 316 317 // Ensure a type argument. 318 if (!UETTE->isArgumentType()) 319 return; 320 321 const VariableArrayType *VLA = C.getASTContext().getAsVariableArrayType( 322 UETTE->getTypeOfArgument().getCanonicalType()); 323 // Ensure that the type is a VLA. 324 if (!VLA) 325 return; 326 327 ProgramStateRef State = C.getState(); 328 SVal ArraySize; 329 State = checkVLA(C, State, VLA, ArraySize); 330 if (!State) 331 return; 332 333 C.addTransition(State); 334 } 335 336 void ento::registerVLASizeChecker(CheckerManager &mgr) { 337 mgr.registerChecker<VLASizeChecker>(); 338 } 339 340 bool ento::shouldRegisterVLASizeChecker(const CheckerManager &mgr) { 341 return true; 342 } 343