VLASizeChecker.cpp

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//=== VLASizeChecker.cpp - Undefined dereference checker --------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This defines VLASizeChecker, a builtin check in ExprEngine that
// performs checks for declaration of VLA of undefined or zero size.
// In addition, VLASizeChecker is responsible for defining the extent
// of the MemRegion that represents a VLA.
//
//===----------------------------------------------------------------------===//
 
#include "clang/AST/CharUnits.h"
#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
#include "clang/StaticAnalyzer/Checkers/Taint.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/CheckerManager.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/DynamicExtent.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/Support/raw_ostream.h"
#include <optional>
 
using namespace clang;
using namespace ento;
using namespace taint;
 
namespace {
class VLASizeChecker
    : public Checker<check::PreStmt<DeclStmt>,
                     check::PreStmt<UnaryExprOrTypeTraitExpr>> {
  const BugType BT{this, "Dangerous variable-length array (VLA) declaration"};
  const BugType TaintBT{this,
                        "Dangerous variable-length array (VLA) declaration",
                        categories::TaintedData};
  enum VLASize_Kind { VLA_Garbage, VLA_Zero, VLA_Negative, VLA_Overflow };
 
  /// Check a VLA for validity.
  /// Every dimension of the array and the total size is checked for validity.
  /// Returns null or a new state where the size is validated.
  /// 'ArraySize' will contain SVal that refers to the total size (in char)
  /// of the array.
  ProgramStateRef checkVLA(CheckerContext &C, ProgramStateRef State,
                           const VariableArrayType *VLA, SVal &ArraySize) const;
  /// Check a single VLA index size expression for validity.
  ProgramStateRef checkVLAIndexSize(CheckerContext &C, ProgramStateRef State,
                                    const Expr *SizeE) const;
 
  void reportBug(VLASize_Kind Kind, const Expr *SizeE, ProgramStateRef State,
                 CheckerContext &C) const;
 
  void reportTaintBug(const Expr *SizeE, ProgramStateRef State,
                      CheckerContext &C, SVal TaintedSVal) const;
 
public:
  void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const;
  void checkPreStmt(const UnaryExprOrTypeTraitExpr *UETTE,
                    CheckerContext &C) const;
};
} // end anonymous namespace
 
ProgramStateRef VLASizeChecker::checkVLA(CheckerContext &C,
                                         ProgramStateRef State,
                                         const VariableArrayType *VLA,
                                         SVal &ArraySize) const {
  assert(VLA && "Function should be called with non-null VLA argument.");
 
  const VariableArrayType *VLALast = nullptr;
  llvm::SmallVector<const Expr *, 2> VLASizes;
 
  // Walk over the VLAs for every dimension until a non-VLA is found.
  // There is a VariableArrayType for every dimension (fixed or variable) until
  // the most inner array that is variably modified.
  // Dimension sizes are collected into 'VLASizes'. 'VLALast' is set to the
  // innermost VLA that was encountered.
  // In "int vla[x][2][y][3]" this will be the array for index "y" (with type
  // int[3]). 'VLASizes' contains 'x', '2', and 'y'.
  while (VLA) {
    const Expr *SizeE = VLA->getSizeExpr();
    State = checkVLAIndexSize(C, State, SizeE);
    if (!State)
      return nullptr;
    VLASizes.push_back(SizeE);
    VLALast = VLA;
    VLA = C.getASTContext().getAsVariableArrayType(VLA->getElementType());
  };
  assert(VLALast &&
         "Array should have at least one variably-modified dimension.");
 
  ASTContext &Ctx = C.getASTContext();
  SValBuilder &SVB = C.getSValBuilder();
  CanQualType SizeTy = Ctx.getSizeType();
  uint64_t SizeMax =
      SVB.getBasicValueFactory().getMaxValue(SizeTy)->getZExtValue();
 
  // Get the element size.
  CharUnits EleSize = Ctx.getTypeSizeInChars(VLALast->getElementType());
  NonLoc ArrSize =
      SVB.makeIntVal(EleSize.getQuantity(), SizeTy).castAs<NonLoc>();
 
  // Try to calculate the known real size of the array in KnownSize.
  uint64_t KnownSize = 0;
  if (const llvm::APSInt *KV = SVB.getKnownValue(State, ArrSize))
    KnownSize = KV->getZExtValue();
 
  for (const Expr *SizeE : VLASizes) {
    auto SizeD = C.getSVal(SizeE).castAs<DefinedSVal>();
    // Convert the array length to size_t.
    NonLoc IndexLength =
        SVB.evalCast(SizeD, SizeTy, SizeE->getType()).castAs<NonLoc>();
    // Multiply the array length by the element size.
    SVal Mul = SVB.evalBinOpNN(State, BO_Mul, ArrSize, IndexLength, SizeTy);
    if (auto MulNonLoc = Mul.getAs<NonLoc>())
      ArrSize = *MulNonLoc;
    else
      // Extent could not be determined.
      return State;
 
    if (const llvm::APSInt *IndexLVal = SVB.getKnownValue(State, IndexLength)) {
      // Check if the array size will overflow.
      // Size overflow check does not work with symbolic expressions because a
      // overflow situation can not be detected easily.
      uint64_t IndexL = IndexLVal->getZExtValue();
      // FIXME: See https://reviews.llvm.org/D80903 for discussion of
      // some difference in assume and getKnownValue that leads to
      // unexpected behavior. Just bail on IndexL == 0 at this point.
      if (IndexL == 0)
        return nullptr;
 
      if (KnownSize <= SizeMax / IndexL) {
        KnownSize *= IndexL;
      } else {
        // Array size does not fit into size_t.
        reportBug(VLA_Overflow, SizeE, State, C);
        return nullptr;
      }
    } else {
      KnownSize = 0;
    }
  }
 
  ArraySize = ArrSize;
 
  return State;
}
 
ProgramStateRef VLASizeChecker::checkVLAIndexSize(CheckerContext &C,
                                                  ProgramStateRef State,
                                                  const Expr *SizeE) const {
  SVal SizeV = C.getSVal(SizeE);
 
  if (SizeV.isUndef()) {
    reportBug(VLA_Garbage, SizeE, State, C);
    return nullptr;
  }
 
  // See if the size value is known. It can't be undefined because we would have
  // warned about that already.
  if (SizeV.isUnknown())
    return nullptr;
 
  // Check if the size is zero.
  DefinedSVal SizeD = SizeV.castAs<DefinedSVal>();
 
  ProgramStateRef StateNotZero, StateZero;
  std::tie(StateNotZero, StateZero) = State->assume(SizeD);
 
  if (StateZero && !StateNotZero) {
    reportBug(VLA_Zero, SizeE, StateZero, C);
    return nullptr;
  }
 
  // From this point on, assume that the size is not zero.
  State = StateNotZero;
 
  // Check if the size is negative.
  SValBuilder &SVB = C.getSValBuilder();
 
  QualType SizeTy = SizeE->getType();
  DefinedOrUnknownSVal Zero = SVB.makeZeroVal(SizeTy);
 
  SVal LessThanZeroVal =
      SVB.evalBinOp(State, BO_LT, SizeD, Zero, SVB.getConditionType());
  ProgramStateRef StatePos, StateNeg;
  if (std::optional<DefinedSVal> LessThanZeroDVal =
          LessThanZeroVal.getAs<DefinedSVal>()) {
    ConstraintManager &CM = C.getConstraintManager();
 
    std::tie(StateNeg, StatePos) = CM.assumeDual(State, *LessThanZeroDVal);
    if (StateNeg && !StatePos) {
      reportBug(VLA_Negative, SizeE, State, C);
      return nullptr;
    }
    State = StatePos;
  }
 
  // Check if the size is tainted.
  if ((StateNeg || StateZero) && isTainted(State, SizeV)) {
    reportTaintBug(SizeE, State, C, SizeV);
    return nullptr;
  }
 
  return State;
}
 
void VLASizeChecker::reportTaintBug(const Expr *SizeE, ProgramStateRef State,
                                    CheckerContext &C, SVal TaintedSVal) const {
  // Generate an error node.
  ExplodedNode *N = C.generateErrorNode(State);
  if (!N)
    return;
 
  SmallString<256> buf;
  llvm::raw_svector_ostream os(buf);
  os << "Declared variable-length array (VLA) ";
  os << "has tainted (attacker controlled) size that can be 0 or negative";
 
  auto report = std::make_unique<PathSensitiveBugReport>(TaintBT, os.str(), N);
  report->addRange(SizeE->getSourceRange());
  bugreporter::trackExpressionValue(N, SizeE, *report);
  // The vla size may be a complex expression where multiple memory locations
  // are tainted.
  for (auto Sym : getTaintedSymbols(State, TaintedSVal))
    report->markInteresting(Sym);
  C.emitReport(std::move(report));
}
 
void VLASizeChecker::reportBug(VLASize_Kind Kind, const Expr *SizeE,
                               ProgramStateRef State, CheckerContext &C) const {
  // Generate an error node.
  ExplodedNode *N = C.generateErrorNode(State);
  if (!N)
    return;
 
  SmallString<256> buf;
  llvm::raw_svector_ostream os(buf);
  os << "Declared variable-length array (VLA) ";
  switch (Kind) {
  case VLA_Garbage:
    os << "uses a garbage value as its size";
    break;
  case VLA_Zero:
    os << "has zero size";
    break;
  case VLA_Negative:
    os << "has negative size";
    break;
  case VLA_Overflow:
    os << "has too large size";
    break;
  }
 
  auto report = std::make_unique<PathSensitiveBugReport>(BT, os.str(), N);
  report->addRange(SizeE->getSourceRange());
  bugreporter::trackExpressionValue(N, SizeE, *report);
  C.emitReport(std::move(report));
}
 
void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const {
  if (!DS->isSingleDecl())
    return;
 
  ASTContext &Ctx = C.getASTContext();
  ProgramStateRef State = C.getState();
  QualType TypeToCheck;
 
  const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl());
 
  if (VD)
    TypeToCheck = VD->getType().getCanonicalType();
  else if (const auto *TND = dyn_cast<TypedefNameDecl>(DS->getSingleDecl()))
    TypeToCheck = TND->getUnderlyingType().getCanonicalType();
  else
    return;
 
  const VariableArrayType *VLA = Ctx.getAsVariableArrayType(TypeToCheck);
  if (!VLA)
    return;
 
  // Check the VLA sizes for validity.
 
  SVal ArraySize;
 
  State = checkVLA(C, State, VLA, ArraySize);
  if (!State)
    return;
 
  if (!isa<NonLoc>(ArraySize)) {
    // Array size could not be determined but state may contain new assumptions.
    C.addTransition(State);
    return;
  }
 
  // VLASizeChecker is responsible for defining the extent of the array.
  if (VD) {
    State =
        setDynamicExtent(State, State->getRegion(VD, C.getLocationContext()),
                         ArraySize.castAs<NonLoc>());
  }
 
  // Remember our assumptions!
  C.addTransition(State);
}
 
void VLASizeChecker::checkPreStmt(const UnaryExprOrTypeTraitExpr *UETTE,
                                  CheckerContext &C) const {
  // Want to check for sizeof.
  if (UETTE->getKind() != UETT_SizeOf)
    return;
 
  // Ensure a type argument.
  if (!UETTE->isArgumentType())
    return;
 
  const VariableArrayType *VLA = C.getASTContext().getAsVariableArrayType(
      UETTE->getTypeOfArgument().getCanonicalType());
  // Ensure that the type is a VLA.
  if (!VLA)
    return;
 
  ProgramStateRef State = C.getState();
  SVal ArraySize;
  State = checkVLA(C, State, VLA, ArraySize);
  if (!State)
    return;
 
  C.addTransition(State);
}
 
void ento::registerVLASizeChecker(CheckerManager &mgr) {
  mgr.registerChecker<VLASizeChecker>();
}
 
bool ento::shouldRegisterVLASizeChecker(const CheckerManager &mgr) {
  return true;
}