1 //===- OSObjectCStyleCast.cpp ------------------------------------*- 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 file defines OSObjectCStyleCast checker, which checks for C-style casts
10 // of OSObjects. Such casts almost always indicate a code smell,
11 // as an explicit static or dynamic cast should be used instead.
12 //===----------------------------------------------------------------------===//
13 
14 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
15 #include "clang/ASTMatchers/ASTMatchFinder.h"
16 #include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
17 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
18 #include "clang/StaticAnalyzer/Core/Checker.h"
19 #include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
20 #include "llvm/Support/Debug.h"
21 
22 using namespace clang;
23 using namespace ento;
24 using namespace ast_matchers;
25 
26 namespace {
27 static constexpr const char *const WarnAtNode = "WarnAtNode";
28 static constexpr const char *const WarnRecordDecl = "WarnRecordDecl";
29 
30 class OSObjectCStyleCastChecker : public Checker<check::ASTCodeBody> {
31 public:
32   void checkASTCodeBody(const Decl *D, AnalysisManager &AM,
33                         BugReporter &BR) const;
34 };
35 } // namespace
36 
37 namespace clang {
38 namespace ast_matchers {
39 AST_MATCHER_P(StringLiteral, mentionsBoundType, std::string, BindingID) {
40   return Builder->removeBindings([this, &Node](const BoundNodesMap &Nodes) {
41     const auto &BN = Nodes.getNode(this->BindingID);
42     if (const auto *ND = BN.get<NamedDecl>()) {
43       return ND->getName() != Node.getString();
44     }
45     return true;
46   });
47 }
48 } // end namespace ast_matchers
49 } // end namespace clang
50 
51 static void emitDiagnostics(const BoundNodes &Nodes,
52                             BugReporter &BR,
53                             AnalysisDeclContext *ADC,
54                             const OSObjectCStyleCastChecker *Checker) {
55   const auto *CE = Nodes.getNodeAs<CastExpr>(WarnAtNode);
56   const CXXRecordDecl *RD = Nodes.getNodeAs<CXXRecordDecl>(WarnRecordDecl);
57   assert(CE && RD);
58 
59   std::string Diagnostics;
60   llvm::raw_string_ostream OS(Diagnostics);
61   OS << "C-style cast of an OSObject is prone to type confusion attacks; "
62      << "use 'OSRequiredCast' if the object is definitely of type '"
63      << RD->getNameAsString() << "', or 'OSDynamicCast' followed by "
64      << "a null check if unsure",
65 
66   BR.EmitBasicReport(
67     ADC->getDecl(),
68     Checker,
69     /*Name=*/"OSObject C-Style Cast",
70     categories::SecurityError,
71     OS.str(),
72     PathDiagnosticLocation::createBegin(CE, BR.getSourceManager(), ADC),
73     CE->getSourceRange());
74 }
75 
76 static decltype(auto) hasTypePointingTo(DeclarationMatcher DeclM) {
77   return hasType(pointerType(pointee(hasDeclaration(DeclM))));
78 }
79 
80 void OSObjectCStyleCastChecker::checkASTCodeBody(const Decl *D,
81                                                  AnalysisManager &AM,
82                                                  BugReporter &BR) const {
83 
84   AnalysisDeclContext *ADC = AM.getAnalysisDeclContext(D);
85 
86   auto DynamicCastM = callExpr(callee(functionDecl(hasName("safeMetaCast"))));
87   // 'allocClassWithName' allocates an object with the given type.
88   // The type is actually provided as a string argument (type's name).
89   // This makes the following pattern possible:
90   //
91   // Foo *object = (Foo *)allocClassWithName("Foo");
92   //
93   // While OSRequiredCast can be used here, it is still not a useful warning.
94   auto AllocClassWithNameM = callExpr(
95       callee(functionDecl(hasName("allocClassWithName"))),
96       // Here we want to make sure that the string argument matches the
97       // type in the cast expression.
98       hasArgument(0, stringLiteral(mentionsBoundType(WarnRecordDecl))));
99 
100   auto OSObjTypeM =
101       hasTypePointingTo(cxxRecordDecl(isDerivedFrom("OSMetaClassBase")));
102   auto OSObjSubclassM = hasTypePointingTo(
103       cxxRecordDecl(isDerivedFrom("OSObject")).bind(WarnRecordDecl));
104 
105   auto CastM =
106       cStyleCastExpr(
107           allOf(OSObjSubclassM,
108                 hasSourceExpression(
109                     allOf(OSObjTypeM,
110                           unless(anyOf(DynamicCastM, AllocClassWithNameM))))))
111           .bind(WarnAtNode);
112 
113   auto Matches =
114       match(stmt(forEachDescendant(CastM)), *D->getBody(), AM.getASTContext());
115   for (BoundNodes Match : Matches)
116     emitDiagnostics(Match, BR, ADC, this);
117 }
118 
119 void ento::registerOSObjectCStyleCast(CheckerManager &Mgr) {
120   Mgr.registerChecker<OSObjectCStyleCastChecker>();
121 }
122 
123 bool ento::shouldRegisterOSObjectCStyleCast(const CheckerManager &mgr) {
124   return true;
125 }
126