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