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