clang  10.0.0git
VforkChecker.cpp
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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 whitelist, 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 
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> VforkWhitelist;
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 isCallWhitelisted(const IdentifierInfo *II, CheckerContext &C) const;
54 
55  void reportBug(const char *What, CheckerContext &C,
56  const char *Details = nullptr) const;
57 
58 public:
59  VforkChecker() : II_vfork(nullptr) {}
60 
61  void checkPreCall(const CallEvent &Call, CheckerContext &C) const;
62  void checkPostCall(const CallEvent &Call, CheckerContext &C) const;
63  void checkBind(SVal L, SVal V, const Stmt *S, CheckerContext &C) const;
64  void checkPreStmt(const ReturnStmt *RS, CheckerContext &C) const;
65 };
66 
67 } // end anonymous namespace
68 
69 // This trait holds region of variable that is assigned with vfork's
70 // return value (this is the only region child is allowed to write).
71 // VFORK_RESULT_INVALID means that we are in parent process.
72 // VFORK_RESULT_NONE means that vfork's return value hasn't been assigned.
73 // Other values point to valid regions.
74 REGISTER_TRAIT_WITH_PROGRAMSTATE(VforkResultRegion, const void *)
75 #define VFORK_RESULT_INVALID 0
76 #define VFORK_RESULT_NONE ((void *)(uintptr_t)1)
77 
78 bool VforkChecker::isChildProcess(const ProgramStateRef State) {
79  return State->get<VforkResultRegion>() != VFORK_RESULT_INVALID;
80 }
81 
82 bool VforkChecker::isVforkCall(const Decl *D, CheckerContext &C) const {
83  auto FD = dyn_cast_or_null<FunctionDecl>(D);
84  if (!FD || !C.isCLibraryFunction(FD))
85  return false;
86 
87  if (!II_vfork) {
88  ASTContext &AC = C.getASTContext();
89  II_vfork = &AC.Idents.get("vfork");
90  }
91 
92  return FD->getIdentifier() == II_vfork;
93 }
94 
95 // Returns true iff ok to call function after successful vfork.
96 bool VforkChecker::isCallWhitelisted(const IdentifierInfo *II,
97  CheckerContext &C) const {
98  if (VforkWhitelist.empty()) {
99  // According to manpage.
100  const char *ids[] = {
101  "_exit",
102  "_Exit",
103  "execl",
104  "execlp",
105  "execle",
106  "execv",
107  "execvp",
108  "execvpe",
109  nullptr
110  };
111 
112  ASTContext &AC = C.getASTContext();
113  for (const char **id = ids; *id; ++id)
114  VforkWhitelist.insert(&AC.Idents.get(*id));
115  }
116 
117  return VforkWhitelist.count(II);
118 }
119 
120 void VforkChecker::reportBug(const char *What, CheckerContext &C,
121  const char *Details) const {
122  if (ExplodedNode *N = C.generateErrorNode(C.getState())) {
123  if (!BT)
124  BT.reset(new BuiltinBug(this,
125  "Dangerous construct in a vforked process"));
126 
127  SmallString<256> buf;
128  llvm::raw_svector_ostream os(buf);
129 
130  os << What << " is prohibited after a successful vfork";
131 
132  if (Details)
133  os << "; " << Details;
134 
135  auto Report = std::make_unique<PathSensitiveBugReport>(*BT, os.str(), N);
136  // TODO: mark vfork call in BugReportVisitor
137  C.emitReport(std::move(Report));
138  }
139 }
140 
141 // Detect calls to vfork and split execution appropriately.
142 void VforkChecker::checkPostCall(const CallEvent &Call,
143  CheckerContext &C) const {
144  // We can't call vfork in child so don't bother
145  // (corresponding warning has already been emitted in checkPreCall).
146  ProgramStateRef State = C.getState();
147  if (isChildProcess(State))
148  return;
149 
150  if (!isVforkCall(Call.getDecl(), C))
151  return;
152 
153  // Get return value of vfork.
154  SVal VforkRetVal = Call.getReturnValue();
156  VforkRetVal.getAs<DefinedOrUnknownSVal>();
157  if (!DVal)
158  return;
159 
160  // Get assigned variable.
161  const ParentMap &PM = C.getLocationContext()->getParentMap();
162  const Stmt *P = PM.getParentIgnoreParenCasts(Call.getOriginExpr());
163  const VarDecl *LhsDecl;
164  std::tie(LhsDecl, std::ignore) = parseAssignment(P);
165 
166  // Get assigned memory region.
167  MemRegionManager &M = C.getStoreManager().getRegionManager();
168  const MemRegion *LhsDeclReg =
169  LhsDecl
170  ? M.getVarRegion(LhsDecl, C.getLocationContext())
171  : (const MemRegion *)VFORK_RESULT_NONE;
172 
173  // Parent branch gets nonzero return value (according to manpage).
174  ProgramStateRef ParentState, ChildState;
175  std::tie(ParentState, ChildState) = C.getState()->assume(*DVal);
176  C.addTransition(ParentState);
177  ChildState = ChildState->set<VforkResultRegion>(LhsDeclReg);
178  C.addTransition(ChildState);
179 }
180 
181 // Prohibit calls to non-whitelist functions in child process.
182 void VforkChecker::checkPreCall(const CallEvent &Call,
183  CheckerContext &C) const {
184  ProgramStateRef State = C.getState();
185  if (isChildProcess(State)
186  && !isCallWhitelisted(Call.getCalleeIdentifier(), C))
187  reportBug("This function call", C);
188 }
189 
190 // Prohibit writes in child process (except for vfork's lhs).
191 void VforkChecker::checkBind(SVal L, SVal V, const Stmt *S,
192  CheckerContext &C) const {
193  ProgramStateRef State = C.getState();
194  if (!isChildProcess(State))
195  return;
196 
197  const MemRegion *VforkLhs =
198  static_cast<const MemRegion *>(State->get<VforkResultRegion>());
199  const MemRegion *MR = L.getAsRegion();
200 
201  // Child is allowed to modify only vfork's lhs.
202  if (!MR || MR == VforkLhs)
203  return;
204 
205  reportBug("This assignment", C);
206 }
207 
208 // Prohibit return from function in child process.
209 void VforkChecker::checkPreStmt(const ReturnStmt *RS, CheckerContext &C) const {
210  ProgramStateRef State = C.getState();
211  if (isChildProcess(State))
212  reportBug("Return", C, "call _exit() instead");
213 }
214 
215 void ento::registerVforkChecker(CheckerManager &mgr) {
216  mgr.registerChecker<VforkChecker>();
217 }
218 
219 bool ento::shouldRegisterVforkChecker(const LangOptions &LO) {
220  return true;
221 }
Stmt - This represents one statement.
Definition: Stmt.h:66
Decl - This represents one declaration (or definition), e.g.
Definition: DeclBase.h:88
IntrusiveRefCntPtr< const ProgramState > ProgramStateRef
StringRef P
Represents a variable declaration or definition.
Definition: Decl.h:820
One of these records is kept for each identifier that is lexed.
Holds long-lived AST nodes (such as types and decls) that can be referred to throughout the semantic ...
Definition: ASTContext.h:168
LineState State
Keeps track of the various options that can be enabled, which controls the dialect of C or C++ that i...
Definition: LangOptions.h:53
IdentifierTable & Idents
Definition: ASTContext.h:580
std::pair< const clang::VarDecl *, const clang::Expr * > parseAssignment(const Stmt *S)
#define V(N, I)
Definition: ASTContext.h:2941
ReturnStmt - This represents a return, optionally of an expression: return; return 4;...
Definition: Stmt.h:2636
#define VFORK_RESULT_INVALID
#define REGISTER_TRAIT_WITH_PROGRAMSTATE(Name, Type)
Declares a program state trait for type Type called Name, and introduce a type named NameTy...
IdentifierInfo & get(StringRef Name)
Return the identifier token info for the specified named identifier.
#define VFORK_RESULT_NONE
Dataflow Directional Tag Classes.
Stmt * getParentIgnoreParenCasts(Stmt *) const
Definition: ParentMap.cpp:145