1 //===- VforkChecker.cpp -------- Vfork usage checks --------------*- 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 vfork checker which checks for dangerous uses of vfork.
10 //  Vforked process shares memory (including stack) with parent so it's
11 //  range of actions is significantly limited: can't write variables,
12 //  can't call functions not in the allowed list, etc. For more details, see
13 //  http://man7.org/linux/man-pages/man2/vfork.2.html
14 //
15 //  This checker checks for prohibited constructs in vforked process.
16 //  The state transition diagram:
17 //  PARENT ---(vfork() == 0)--> CHILD
18 //                                   |
19 //                                   --(*p = ...)--> bug
20 //                                   |
21 //                                   --foo()--> bug
22 //                                   |
23 //                                   --return--> bug
24 //
25 //===----------------------------------------------------------------------===//
26 
27 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
28 #include "clang/StaticAnalyzer/Core/PathSensitive/CallEvent.h"
29 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
30 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerHelpers.h"
31 #include "clang/StaticAnalyzer/Core/PathSensitive/ProgramState.h"
32 #include "clang/StaticAnalyzer/Core/PathSensitive/ProgramStateTrait.h"
33 #include "clang/StaticAnalyzer/Core/PathSensitive/SymbolManager.h"
34 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
35 #include "clang/StaticAnalyzer/Core/Checker.h"
36 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
37 #include "clang/AST/ParentMap.h"
38 
39 using namespace clang;
40 using namespace ento;
41 
42 namespace {
43 
44 class VforkChecker : public Checker<check::PreCall, check::PostCall,
45                                     check::Bind, check::PreStmt<ReturnStmt>> {
46   mutable std::unique_ptr<BuiltinBug> BT;
47   mutable llvm::SmallSet<const IdentifierInfo *, 10> VforkAllowlist;
48   mutable const IdentifierInfo *II_vfork;
49 
50   static bool isChildProcess(const ProgramStateRef State);
51 
52   bool isVforkCall(const Decl *D, CheckerContext &C) const;
53   bool isCallExplicitelyAllowed(const IdentifierInfo *II,
54                                 CheckerContext &C) const;
55 
56   void reportBug(const char *What, CheckerContext &C,
57                  const char *Details = nullptr) const;
58 
59 public:
60   VforkChecker() : II_vfork(nullptr) {}
61 
62   void checkPreCall(const CallEvent &Call, CheckerContext &C) const;
63   void checkPostCall(const CallEvent &Call, CheckerContext &C) const;
64   void checkBind(SVal L, SVal V, const Stmt *S, CheckerContext &C) const;
65   void checkPreStmt(const ReturnStmt *RS, CheckerContext &C) const;
66 };
67 
68 } // end anonymous namespace
69 
70 // This trait holds region of variable that is assigned with vfork's
71 // return value (this is the only region child is allowed to write).
72 // VFORK_RESULT_INVALID means that we are in parent process.
73 // VFORK_RESULT_NONE means that vfork's return value hasn't been assigned.
74 // Other values point to valid regions.
75 REGISTER_TRAIT_WITH_PROGRAMSTATE(VforkResultRegion, const void *)
76 #define VFORK_RESULT_INVALID 0
77 #define VFORK_RESULT_NONE ((void *)(uintptr_t)1)
78 
79 bool VforkChecker::isChildProcess(const ProgramStateRef State) {
80   return State->get<VforkResultRegion>() != VFORK_RESULT_INVALID;
81 }
82 
83 bool VforkChecker::isVforkCall(const Decl *D, CheckerContext &C) const {
84   auto FD = dyn_cast_or_null<FunctionDecl>(D);
85   if (!FD || !C.isCLibraryFunction(FD))
86     return false;
87 
88   if (!II_vfork) {
89     ASTContext &AC = C.getASTContext();
90     II_vfork = &AC.Idents.get("vfork");
91   }
92 
93   return FD->getIdentifier() == II_vfork;
94 }
95 
96 // Returns true iff ok to call function after successful vfork.
97 bool VforkChecker::isCallExplicitelyAllowed(const IdentifierInfo *II,
98                                             CheckerContext &C) const {
99   if (VforkAllowlist.empty()) {
100     // According to manpage.
101     const char *ids[] = {
102       "_Exit",
103       "_exit",
104       "execl",
105       "execle",
106       "execlp",
107       "execv",
108       "execve",
109       "execvp",
110       "execvpe",
111       nullptr
112     };
113 
114     ASTContext &AC = C.getASTContext();
115     for (const char **id = ids; *id; ++id)
116       VforkAllowlist.insert(&AC.Idents.get(*id));
117   }
118 
119   return VforkAllowlist.count(II);
120 }
121 
122 void VforkChecker::reportBug(const char *What, CheckerContext &C,
123                              const char *Details) const {
124   if (ExplodedNode *N = C.generateErrorNode(C.getState())) {
125     if (!BT)
126       BT.reset(new BuiltinBug(this,
127                               "Dangerous construct in a vforked process"));
128 
129     SmallString<256> buf;
130     llvm::raw_svector_ostream os(buf);
131 
132     os << What << " is prohibited after a successful vfork";
133 
134     if (Details)
135       os << "; " << Details;
136 
137     auto Report = std::make_unique<PathSensitiveBugReport>(*BT, os.str(), N);
138     // TODO: mark vfork call in BugReportVisitor
139     C.emitReport(std::move(Report));
140   }
141 }
142 
143 // Detect calls to vfork and split execution appropriately.
144 void VforkChecker::checkPostCall(const CallEvent &Call,
145                                  CheckerContext &C) const {
146   // We can't call vfork in child so don't bother
147   // (corresponding warning has already been emitted in checkPreCall).
148   ProgramStateRef State = C.getState();
149   if (isChildProcess(State))
150     return;
151 
152   if (!isVforkCall(Call.getDecl(), C))
153     return;
154 
155   // Get return value of vfork.
156   SVal VforkRetVal = Call.getReturnValue();
157   Optional<DefinedOrUnknownSVal> DVal =
158     VforkRetVal.getAs<DefinedOrUnknownSVal>();
159   if (!DVal)
160     return;
161 
162   // Get assigned variable.
163   const ParentMap &PM = C.getLocationContext()->getParentMap();
164   const Stmt *P = PM.getParentIgnoreParenCasts(Call.getOriginExpr());
165   const VarDecl *LhsDecl;
166   std::tie(LhsDecl, std::ignore) = parseAssignment(P);
167 
168   // Get assigned memory region.
169   MemRegionManager &M = C.getStoreManager().getRegionManager();
170   const MemRegion *LhsDeclReg =
171     LhsDecl
172       ? M.getVarRegion(LhsDecl, C.getLocationContext())
173       : (const MemRegion *)VFORK_RESULT_NONE;
174 
175   // Parent branch gets nonzero return value (according to manpage).
176   ProgramStateRef ParentState, ChildState;
177   std::tie(ParentState, ChildState) = C.getState()->assume(*DVal);
178   C.addTransition(ParentState);
179   ChildState = ChildState->set<VforkResultRegion>(LhsDeclReg);
180   C.addTransition(ChildState);
181 }
182 
183 // Prohibit calls to functions in child process which are not explicitly
184 // allowed.
185 void VforkChecker::checkPreCall(const CallEvent &Call,
186                                 CheckerContext &C) const {
187   ProgramStateRef State = C.getState();
188   if (isChildProcess(State) &&
189       !isCallExplicitelyAllowed(Call.getCalleeIdentifier(), C))
190     reportBug("This function call", C);
191 }
192 
193 // Prohibit writes in child process (except for vfork's lhs).
194 void VforkChecker::checkBind(SVal L, SVal V, const Stmt *S,
195                              CheckerContext &C) const {
196   ProgramStateRef State = C.getState();
197   if (!isChildProcess(State))
198     return;
199 
200   const MemRegion *VforkLhs =
201     static_cast<const MemRegion *>(State->get<VforkResultRegion>());
202   const MemRegion *MR = L.getAsRegion();
203 
204   // Child is allowed to modify only vfork's lhs.
205   if (!MR || MR == VforkLhs)
206     return;
207 
208   reportBug("This assignment", C);
209 }
210 
211 // Prohibit return from function in child process.
212 void VforkChecker::checkPreStmt(const ReturnStmt *RS, CheckerContext &C) const {
213   ProgramStateRef State = C.getState();
214   if (isChildProcess(State))
215     reportBug("Return", C, "call _exit() instead");
216 }
217 
218 void ento::registerVforkChecker(CheckerManager &mgr) {
219   mgr.registerChecker<VforkChecker>();
220 }
221 
222 bool ento::shouldRegisterVforkChecker(const CheckerManager &mgr) {
223   return true;
224 }
225