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 #include <optional>
39 
40 using namespace clang;
41 using namespace ento;
42 
43 namespace {
44 
45 class VforkChecker : public Checker<check::PreCall, check::PostCall,
46                                     check::Bind, check::PreStmt<ReturnStmt>> {
47   mutable std::unique_ptr<BuiltinBug> BT;
48   mutable llvm::SmallSet<const IdentifierInfo *, 10> VforkAllowlist;
49   mutable const IdentifierInfo *II_vfork;
50 
51   static bool isChildProcess(const ProgramStateRef State);
52 
53   bool isVforkCall(const Decl *D, CheckerContext &C) const;
54   bool isCallExplicitelyAllowed(const IdentifierInfo *II,
55                                 CheckerContext &C) const;
56 
57   void reportBug(const char *What, CheckerContext &C,
58                  const char *Details = nullptr) const;
59 
60 public:
61   VforkChecker() : II_vfork(nullptr) {}
62 
63   void checkPreCall(const CallEvent &Call, CheckerContext &C) const;
64   void checkPostCall(const CallEvent &Call, CheckerContext &C) const;
65   void checkBind(SVal L, SVal V, const Stmt *S, CheckerContext &C) const;
66   void checkPreStmt(const ReturnStmt *RS, CheckerContext &C) const;
67 };
68 
69 } // end anonymous namespace
70 
71 // This trait holds region of variable that is assigned with vfork's
72 // return value (this is the only region child is allowed to write).
73 // VFORK_RESULT_INVALID means that we are in parent process.
74 // VFORK_RESULT_NONE means that vfork's return value hasn't been assigned.
75 // Other values point to valid regions.
76 REGISTER_TRAIT_WITH_PROGRAMSTATE(VforkResultRegion, const void *)
77 #define VFORK_RESULT_INVALID 0
78 #define VFORK_RESULT_NONE ((void *)(uintptr_t)1)
79 
80 bool VforkChecker::isChildProcess(const ProgramStateRef State) {
81   return State->get<VforkResultRegion>() != VFORK_RESULT_INVALID;
82 }
83 
84 bool VforkChecker::isVforkCall(const Decl *D, CheckerContext &C) const {
85   auto FD = dyn_cast_or_null<FunctionDecl>(D);
86   if (!FD || !C.isCLibraryFunction(FD))
87     return false;
88 
89   if (!II_vfork) {
90     ASTContext &AC = C.getASTContext();
91     II_vfork = &AC.Idents.get("vfork");
92   }
93 
94   return FD->getIdentifier() == II_vfork;
95 }
96 
97 // Returns true iff ok to call function after successful vfork.
98 bool VforkChecker::isCallExplicitelyAllowed(const IdentifierInfo *II,
99                                             CheckerContext &C) const {
100   if (VforkAllowlist.empty()) {
101     // According to manpage.
102     const char *ids[] = {
103       "_Exit",
104       "_exit",
105       "execl",
106       "execle",
107       "execlp",
108       "execv",
109       "execve",
110       "execvp",
111       "execvpe",
112       nullptr
113     };
114 
115     ASTContext &AC = C.getASTContext();
116     for (const char **id = ids; *id; ++id)
117       VforkAllowlist.insert(&AC.Idents.get(*id));
118   }
119 
120   return VforkAllowlist.count(II);
121 }
122 
123 void VforkChecker::reportBug(const char *What, CheckerContext &C,
124                              const char *Details) const {
125   if (ExplodedNode *N = C.generateErrorNode(C.getState())) {
126     if (!BT)
127       BT.reset(new BuiltinBug(this,
128                               "Dangerous construct in a vforked process"));
129 
130     SmallString<256> buf;
131     llvm::raw_svector_ostream os(buf);
132 
133     os << What << " is prohibited after a successful vfork";
134 
135     if (Details)
136       os << "; " << Details;
137 
138     auto Report = std::make_unique<PathSensitiveBugReport>(*BT, os.str(), N);
139     // TODO: mark vfork call in BugReportVisitor
140     C.emitReport(std::move(Report));
141   }
142 }
143 
144 // Detect calls to vfork and split execution appropriately.
145 void VforkChecker::checkPostCall(const CallEvent &Call,
146                                  CheckerContext &C) const {
147   // We can't call vfork in child so don't bother
148   // (corresponding warning has already been emitted in checkPreCall).
149   ProgramStateRef State = C.getState();
150   if (isChildProcess(State))
151     return;
152 
153   if (!isVforkCall(Call.getDecl(), C))
154     return;
155 
156   // Get return value of vfork.
157   SVal VforkRetVal = Call.getReturnValue();
158   std::optional<DefinedOrUnknownSVal> DVal =
159       VforkRetVal.getAs<DefinedOrUnknownSVal>();
160   if (!DVal)
161     return;
162 
163   // Get assigned variable.
164   const ParentMap &PM = C.getLocationContext()->getParentMap();
165   const Stmt *P = PM.getParentIgnoreParenCasts(Call.getOriginExpr());
166   const VarDecl *LhsDecl;
167   std::tie(LhsDecl, std::ignore) = parseAssignment(P);
168 
169   // Get assigned memory region.
170   MemRegionManager &M = C.getStoreManager().getRegionManager();
171   const MemRegion *LhsDeclReg =
172     LhsDecl
173       ? M.getVarRegion(LhsDecl, C.getLocationContext())
174       : (const MemRegion *)VFORK_RESULT_NONE;
175 
176   // Parent branch gets nonzero return value (according to manpage).
177   ProgramStateRef ParentState, ChildState;
178   std::tie(ParentState, ChildState) = C.getState()->assume(*DVal);
179   C.addTransition(ParentState);
180   ChildState = ChildState->set<VforkResultRegion>(LhsDeclReg);
181   C.addTransition(ChildState);
182 }
183 
184 // Prohibit calls to functions in child process which are not explicitly
185 // allowed.
186 void VforkChecker::checkPreCall(const CallEvent &Call,
187                                 CheckerContext &C) const {
188   ProgramStateRef State = C.getState();
189   if (isChildProcess(State) &&
190       !isCallExplicitelyAllowed(Call.getCalleeIdentifier(), C))
191     reportBug("This function call", C);
192 }
193 
194 // Prohibit writes in child process (except for vfork's lhs).
195 void VforkChecker::checkBind(SVal L, SVal V, const Stmt *S,
196                              CheckerContext &C) const {
197   ProgramStateRef State = C.getState();
198   if (!isChildProcess(State))
199     return;
200 
201   const MemRegion *VforkLhs =
202     static_cast<const MemRegion *>(State->get<VforkResultRegion>());
203   const MemRegion *MR = L.getAsRegion();
204 
205   // Child is allowed to modify only vfork's lhs.
206   if (!MR || MR == VforkLhs)
207     return;
208 
209   reportBug("This assignment", C);
210 }
211 
212 // Prohibit return from function in child process.
213 void VforkChecker::checkPreStmt(const ReturnStmt *RS, CheckerContext &C) const {
214   ProgramStateRef State = C.getState();
215   if (isChildProcess(State))
216     reportBug("Return", C, "call _exit() instead");
217 }
218 
219 void ento::registerVforkChecker(CheckerManager &mgr) {
220   mgr.registerChecker<VforkChecker>();
221 }
222 
223 bool ento::shouldRegisterVforkChecker(const CheckerManager &mgr) {
224   return true;
225 }
226