1 //== CStringSyntaxChecker.cpp - CoreFoundation containers API *- 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 // An AST checker that looks for common pitfalls when using C string APIs. 10 // - Identifies erroneous patterns in the last argument to strncat - the number 11 // of bytes to copy. 12 // 13 //===----------------------------------------------------------------------===// 14 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h" 15 #include "clang/AST/Expr.h" 16 #include "clang/AST/OperationKinds.h" 17 #include "clang/AST/StmtVisitor.h" 18 #include "clang/Analysis/AnalysisDeclContext.h" 19 #include "clang/Basic/TargetInfo.h" 20 #include "clang/Basic/TypeTraits.h" 21 #include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h" 22 #include "clang/StaticAnalyzer/Core/Checker.h" 23 #include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h" 24 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h" 25 #include "llvm/ADT/SmallString.h" 26 #include "llvm/Support/raw_ostream.h" 27 28 using namespace clang; 29 using namespace ento; 30 31 namespace { 32 class WalkAST: public StmtVisitor<WalkAST> { 33 const CheckerBase *Checker; 34 BugReporter &BR; 35 AnalysisDeclContext* AC; 36 37 /// Check if two expressions refer to the same declaration. 38 bool sameDecl(const Expr *A1, const Expr *A2) { 39 if (const auto *D1 = dyn_cast<DeclRefExpr>(A1->IgnoreParenCasts())) 40 if (const auto *D2 = dyn_cast<DeclRefExpr>(A2->IgnoreParenCasts())) 41 return D1->getDecl() == D2->getDecl(); 42 return false; 43 } 44 45 /// Check if the expression E is a sizeof(WithArg). 46 bool isSizeof(const Expr *E, const Expr *WithArg) { 47 if (const auto *UE = dyn_cast<UnaryExprOrTypeTraitExpr>(E)) 48 if (UE->getKind() == UETT_SizeOf && !UE->isArgumentType()) 49 return sameDecl(UE->getArgumentExpr(), WithArg); 50 return false; 51 } 52 53 /// Check if the expression E is a strlen(WithArg). 54 bool isStrlen(const Expr *E, const Expr *WithArg) { 55 if (const auto *CE = dyn_cast<CallExpr>(E)) { 56 const FunctionDecl *FD = CE->getDirectCallee(); 57 if (!FD) 58 return false; 59 return (CheckerContext::isCLibraryFunction(FD, "strlen") && 60 sameDecl(CE->getArg(0), WithArg)); 61 } 62 return false; 63 } 64 65 /// Check if the expression is an integer literal with value 1. 66 bool isOne(const Expr *E) { 67 if (const auto *IL = dyn_cast<IntegerLiteral>(E)) 68 return (IL->getValue().isIntN(1)); 69 return false; 70 } 71 72 StringRef getPrintableName(const Expr *E) { 73 if (const auto *D = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts())) 74 return D->getDecl()->getName(); 75 return StringRef(); 76 } 77 78 /// Identify erroneous patterns in the last argument to strncat - the number 79 /// of bytes to copy. 80 bool containsBadStrncatPattern(const CallExpr *CE); 81 82 /// Identify erroneous patterns in the last argument to strlcpy - the number 83 /// of bytes to copy. 84 /// The bad pattern checked is when the size is known 85 /// to be larger than the destination can handle. 86 /// char dst[2]; 87 /// size_t cpy = 4; 88 /// strlcpy(dst, "abcd", sizeof("abcd") - 1); 89 /// strlcpy(dst, "abcd", 4); 90 /// strlcpy(dst + 3, "abcd", 2); 91 /// strlcpy(dst, "abcd", cpy); 92 /// Identify erroneous patterns in the last argument to strlcat - the number 93 /// of bytes to copy. 94 /// The bad pattern checked is when the last argument is basically 95 /// pointing to the destination buffer size or argument larger or 96 /// equal to. 97 /// char dst[2]; 98 /// strlcat(dst, src2, sizeof(dst)); 99 /// strlcat(dst, src2, 2); 100 /// strlcat(dst, src2, 10); 101 bool containsBadStrlcpyStrlcatPattern(const CallExpr *CE); 102 103 public: 104 WalkAST(const CheckerBase *Checker, BugReporter &BR, AnalysisDeclContext *AC) 105 : Checker(Checker), BR(BR), AC(AC) {} 106 107 // Statement visitor methods. 108 void VisitChildren(Stmt *S); 109 void VisitStmt(Stmt *S) { 110 VisitChildren(S); 111 } 112 void VisitCallExpr(CallExpr *CE); 113 }; 114 } // end anonymous namespace 115 116 // The correct size argument should look like following: 117 // strncat(dst, src, sizeof(dst) - strlen(dest) - 1); 118 // We look for the following anti-patterns: 119 // - strncat(dst, src, sizeof(dst) - strlen(dst)); 120 // - strncat(dst, src, sizeof(dst) - 1); 121 // - strncat(dst, src, sizeof(dst)); 122 bool WalkAST::containsBadStrncatPattern(const CallExpr *CE) { 123 if (CE->getNumArgs() != 3) 124 return false; 125 const Expr *DstArg = CE->getArg(0); 126 const Expr *SrcArg = CE->getArg(1); 127 const Expr *LenArg = CE->getArg(2); 128 129 // Identify wrong size expressions, which are commonly used instead. 130 if (const auto *BE = dyn_cast<BinaryOperator>(LenArg->IgnoreParenCasts())) { 131 // - sizeof(dst) - strlen(dst) 132 if (BE->getOpcode() == BO_Sub) { 133 const Expr *L = BE->getLHS(); 134 const Expr *R = BE->getRHS(); 135 if (isSizeof(L, DstArg) && isStrlen(R, DstArg)) 136 return true; 137 138 // - sizeof(dst) - 1 139 if (isSizeof(L, DstArg) && isOne(R->IgnoreParenCasts())) 140 return true; 141 } 142 } 143 // - sizeof(dst) 144 if (isSizeof(LenArg, DstArg)) 145 return true; 146 147 // - sizeof(src) 148 if (isSizeof(LenArg, SrcArg)) 149 return true; 150 return false; 151 } 152 153 bool WalkAST::containsBadStrlcpyStrlcatPattern(const CallExpr *CE) { 154 if (CE->getNumArgs() != 3) 155 return false; 156 const Expr *DstArg = CE->getArg(0); 157 const Expr *LenArg = CE->getArg(2); 158 159 const auto *DstArgDRE = dyn_cast<DeclRefExpr>(DstArg->IgnoreParenImpCasts()); 160 const auto *LenArgDRE = 161 dyn_cast<DeclRefExpr>(LenArg->IgnoreParenLValueCasts()); 162 uint64_t DstOff = 0; 163 if (isSizeof(LenArg, DstArg)) 164 return false; 165 166 // - size_t dstlen = sizeof(dst) 167 if (LenArgDRE) { 168 const auto *LenArgVal = dyn_cast<VarDecl>(LenArgDRE->getDecl()); 169 // If it's an EnumConstantDecl instead, then we're missing out on something. 170 if (!LenArgVal) { 171 assert(isa<EnumConstantDecl>(LenArgDRE->getDecl())); 172 return false; 173 } 174 if (LenArgVal->getInit()) 175 LenArg = LenArgVal->getInit(); 176 } 177 178 // - integral value 179 // We try to figure out if the last argument is possibly longer 180 // than the destination can possibly handle if its size can be defined. 181 if (const auto *IL = dyn_cast<IntegerLiteral>(LenArg->IgnoreParenImpCasts())) { 182 uint64_t ILRawVal = IL->getValue().getZExtValue(); 183 184 // Case when there is pointer arithmetic on the destination buffer 185 // especially when we offset from the base decreasing the 186 // buffer length accordingly. 187 if (!DstArgDRE) { 188 if (const auto *BE = 189 dyn_cast<BinaryOperator>(DstArg->IgnoreParenImpCasts())) { 190 DstArgDRE = dyn_cast<DeclRefExpr>(BE->getLHS()->IgnoreParenImpCasts()); 191 if (BE->getOpcode() == BO_Add) { 192 if ((IL = dyn_cast<IntegerLiteral>(BE->getRHS()->IgnoreParenImpCasts()))) { 193 DstOff = IL->getValue().getZExtValue(); 194 } 195 } 196 } 197 } 198 if (DstArgDRE) { 199 if (const auto *Buffer = 200 dyn_cast<ConstantArrayType>(DstArgDRE->getType())) { 201 ASTContext &C = BR.getContext(); 202 uint64_t BufferLen = C.getTypeSize(Buffer) / 8; 203 auto RemainingBufferLen = BufferLen - DstOff; 204 if (RemainingBufferLen < ILRawVal) 205 return true; 206 } 207 } 208 } 209 210 return false; 211 } 212 213 void WalkAST::VisitCallExpr(CallExpr *CE) { 214 const FunctionDecl *FD = CE->getDirectCallee(); 215 if (!FD) 216 return; 217 218 if (CheckerContext::isCLibraryFunction(FD, "strncat")) { 219 if (containsBadStrncatPattern(CE)) { 220 const Expr *DstArg = CE->getArg(0); 221 const Expr *LenArg = CE->getArg(2); 222 PathDiagnosticLocation Loc = 223 PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC); 224 225 StringRef DstName = getPrintableName(DstArg); 226 227 SmallString<256> S; 228 llvm::raw_svector_ostream os(S); 229 os << "Potential buffer overflow. "; 230 if (!DstName.empty()) { 231 os << "Replace with 'sizeof(" << DstName << ") " 232 "- strlen(" << DstName <<") - 1'"; 233 os << " or u"; 234 } else 235 os << "U"; 236 os << "se a safer 'strlcat' API"; 237 238 BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument", 239 "C String API", os.str(), Loc, 240 LenArg->getSourceRange()); 241 } 242 } else if (CheckerContext::isCLibraryFunction(FD, "strlcpy") || 243 CheckerContext::isCLibraryFunction(FD, "strlcat")) { 244 if (containsBadStrlcpyStrlcatPattern(CE)) { 245 const Expr *DstArg = CE->getArg(0); 246 const Expr *LenArg = CE->getArg(2); 247 PathDiagnosticLocation Loc = 248 PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC); 249 250 StringRef DstName = getPrintableName(DstArg); 251 252 SmallString<256> S; 253 llvm::raw_svector_ostream os(S); 254 os << "The third argument allows to potentially copy more bytes than it should. "; 255 os << "Replace with the value "; 256 if (!DstName.empty()) 257 os << "sizeof(" << DstName << ")"; 258 else 259 os << "sizeof(<destination buffer>)"; 260 os << " or lower"; 261 262 BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument", 263 "C String API", os.str(), Loc, 264 LenArg->getSourceRange()); 265 } 266 } 267 268 // Recurse and check children. 269 VisitChildren(CE); 270 } 271 272 void WalkAST::VisitChildren(Stmt *S) { 273 for (Stmt *Child : S->children()) 274 if (Child) 275 Visit(Child); 276 } 277 278 namespace { 279 class CStringSyntaxChecker: public Checker<check::ASTCodeBody> { 280 public: 281 282 void checkASTCodeBody(const Decl *D, AnalysisManager& Mgr, 283 BugReporter &BR) const { 284 WalkAST walker(this, BR, Mgr.getAnalysisDeclContext(D)); 285 walker.Visit(D->getBody()); 286 } 287 }; 288 } 289 290 void ento::registerCStringSyntaxChecker(CheckerManager &mgr) { 291 mgr.registerChecker<CStringSyntaxChecker>(); 292 } 293 294 bool ento::shouldRegisterCStringSyntaxChecker(const CheckerManager &mgr) { 295 return true; 296 } 297