StaticAnalyzer/Checkers/CheckSecuritySyntaxOnly.cpp

//==- CheckSecuritySyntaxOnly.cpp - Basic security checks --------*- C++ -*-==//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//  This file defines a set of flow-insensitive security checks.
//
//===----------------------------------------------------------------------===//

#include "ClangSACheckers.h"
#include "clang/Analysis/AnalysisContext.h"
#include "clang/AST/StmtVisitor.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/raw_ostream.h"

using namespace clang;
using namespace ento;

static bool isArc4RandomAvailable(const ASTContext &Ctx) {
  const llvm::Triple &T = Ctx.getTargetInfo().getTriple();
  return T.getVendor() == llvm::Triple::Apple ||
         T.getOS() == llvm::Triple::FreeBSD ||
         T.getOS() == llvm::Triple::NetBSD ||
         T.getOS() == llvm::Triple::OpenBSD ||
         T.getOS() == llvm::Triple::DragonFly;
}

namespace {
class WalkAST : public StmtVisitor<WalkAST> {
  BugReporter &BR;
  AnalysisContext* AC;
  enum { num_setids = 6 };
  IdentifierInfo *II_setid[num_setids];

  const bool CheckRand;

public:
  WalkAST(BugReporter &br, AnalysisContext* ac)
  : BR(br), AC(ac), II_setid(),
    CheckRand(isArc4RandomAvailable(BR.getContext())) {}

  // Statement visitor methods.
  void VisitCallExpr(CallExpr *CE);
  void VisitForStmt(ForStmt *S);
  void VisitCompoundStmt (CompoundStmt *S);
  void VisitStmt(Stmt *S) { VisitChildren(S); }

  void VisitChildren(Stmt *S);

  // Helpers.
  bool checkCall_strCommon(const CallExpr *CE, const FunctionDecl *FD);

  typedef void (WalkAST::*FnCheck)(const CallExpr *,
				   const FunctionDecl *);

  // Checker-specific methods.
  void checkLoopConditionForFloat(const ForStmt *FS);
  void checkCall_gets(const CallExpr *CE, const FunctionDecl *FD);
  void checkCall_getpw(const CallExpr *CE, const FunctionDecl *FD);
  void checkCall_mktemp(const CallExpr *CE, const FunctionDecl *FD);
  void checkCall_strcpy(const CallExpr *CE, const FunctionDecl *FD);
  void checkCall_strcat(const CallExpr *CE, const FunctionDecl *FD);
  void checkCall_rand(const CallExpr *CE, const FunctionDecl *FD);
  void checkCall_random(const CallExpr *CE, const FunctionDecl *FD);
  void checkUncheckedReturnValue(CallExpr *CE);
};
} // end anonymous namespace

//===----------------------------------------------------------------------===//
// AST walking.
//===----------------------------------------------------------------------===//

void WalkAST::VisitChildren(Stmt *S) {
  for (Stmt::child_iterator I = S->child_begin(), E = S->child_end(); I!=E; ++I)
    if (Stmt *child = *I)
      Visit(child);
}

void WalkAST::VisitCallExpr(CallExpr *CE) {
  // Get the callee.
  const FunctionDecl *FD = CE->getDirectCallee();

  if (!FD)
    return;

  // Get the name of the callee. If it's a builtin, strip off the prefix.
  IdentifierInfo *II = FD->getIdentifier();
  if (!II)   // if no identifier, not a simple C function
    return;
  StringRef Name = II->getName();
  if (Name.startswith("__builtin_"))
    Name = Name.substr(10);

  // Set the evaluation function by switching on the callee name.
  FnCheck evalFunction = llvm::StringSwitch<FnCheck>(Name)
    .Case("gets", &WalkAST::checkCall_gets)
    .Case("getpw", &WalkAST::checkCall_getpw)
    .Case("mktemp", &WalkAST::checkCall_mktemp)
    .Cases("strcpy", "__strcpy_chk", &WalkAST::checkCall_strcpy)
    .Cases("strcat", "__strcat_chk", &WalkAST::checkCall_strcat)
    .Case("drand48", &WalkAST::checkCall_rand)
    .Case("erand48", &WalkAST::checkCall_rand)
    .Case("jrand48", &WalkAST::checkCall_rand)
    .Case("lrand48", &WalkAST::checkCall_rand)
    .Case("mrand48", &WalkAST::checkCall_rand)
    .Case("nrand48", &WalkAST::checkCall_rand)
    .Case("lcong48", &WalkAST::checkCall_rand)
    .Case("rand", &WalkAST::checkCall_rand)
    .Case("rand_r", &WalkAST::checkCall_rand)
    .Case("random", &WalkAST::checkCall_random)
    .Default(NULL);

  // If the callee isn't defined, it is not of security concern.
  // Check and evaluate the call.
  if (evalFunction)
    (this->*evalFunction)(CE, FD);

  // Recurse and check children.
  VisitChildren(CE);
}

void WalkAST::VisitCompoundStmt(CompoundStmt *S) {
  for (Stmt::child_iterator I = S->child_begin(), E = S->child_end(); I!=E; ++I)
    if (Stmt *child = *I) {
      if (CallExpr *CE = dyn_cast<CallExpr>(child))
        checkUncheckedReturnValue(CE);
      Visit(child);
    }
}

void WalkAST::VisitForStmt(ForStmt *FS) {
  checkLoopConditionForFloat(FS);

  // Recurse and check children.
  VisitChildren(FS);
}

//===----------------------------------------------------------------------===//
// Check: floating poing variable used as loop counter.
// Originally: <rdar://problem/6336718>
// Implements: CERT security coding advisory FLP-30.
//===----------------------------------------------------------------------===//

static const DeclRefExpr*
getIncrementedVar(const Expr *expr, const VarDecl *x, const VarDecl *y) {
  expr = expr->IgnoreParenCasts();

  if (const BinaryOperator *B = dyn_cast<BinaryOperator>(expr)) {
    if (!(B->isAssignmentOp() || B->isCompoundAssignmentOp() ||
          B->getOpcode() == BO_Comma))
      return NULL;

    if (const DeclRefExpr *lhs = getIncrementedVar(B->getLHS(), x, y))
      return lhs;

    if (const DeclRefExpr *rhs = getIncrementedVar(B->getRHS(), x, y))
      return rhs;

    return NULL;
  }

  if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(expr)) {
    const NamedDecl *ND = DR->getDecl();
    return ND == x || ND == y ? DR : NULL;
  }

  if (const UnaryOperator *U = dyn_cast<UnaryOperator>(expr))
    return U->isIncrementDecrementOp()
      ? getIncrementedVar(U->getSubExpr(), x, y) : NULL;

  return NULL;
}

/// CheckLoopConditionForFloat - This check looks for 'for' statements that
///  use a floating point variable as a loop counter.
///  CERT: FLP30-C, FLP30-CPP.
///
void WalkAST::checkLoopConditionForFloat(const ForStmt *FS) {
  // Does the loop have a condition?
  const Expr *condition = FS->getCond();

  if (!condition)
    return;

  // Does the loop have an increment?
  const Expr *increment = FS->getInc();

  if (!increment)
    return;

  // Strip away '()' and casts.
  condition = condition->IgnoreParenCasts();
  increment = increment->IgnoreParenCasts();

  // Is the loop condition a comparison?
  const BinaryOperator *B = dyn_cast<BinaryOperator>(condition);

  if (!B)
    return;

  // Is this a comparison?
  if (!(B->isRelationalOp() || B->isEqualityOp()))
    return;

  // Are we comparing variables?
  const DeclRefExpr *drLHS =
    dyn_cast<DeclRefExpr>(B->getLHS()->IgnoreParenLValueCasts());
  const DeclRefExpr *drRHS =
    dyn_cast<DeclRefExpr>(B->getRHS()->IgnoreParenLValueCasts());

  // Does at least one of the variables have a floating point type?
  drLHS = drLHS && drLHS->getType()->isRealFloatingType() ? drLHS : NULL;
  drRHS = drRHS && drRHS->getType()->isRealFloatingType() ? drRHS : NULL;

  if (!drLHS && !drRHS)
    return;

  const VarDecl *vdLHS = drLHS ? dyn_cast<VarDecl>(drLHS->getDecl()) : NULL;
  const VarDecl *vdRHS = drRHS ? dyn_cast<VarDecl>(drRHS->getDecl()) : NULL;

  if (!vdLHS && !vdRHS)
    return;

  // Does either variable appear in increment?
  const DeclRefExpr *drInc = getIncrementedVar(increment, vdLHS, vdRHS);

  if (!drInc)
    return;

  // Emit the error.  First figure out which DeclRefExpr in the condition
  // referenced the compared variable.
  const DeclRefExpr *drCond = vdLHS == drInc->getDecl() ? drLHS : drRHS;

  SmallVector<SourceRange, 2> ranges;
  llvm::SmallString<256> sbuf;
  llvm::raw_svector_ostream os(sbuf);

  os << "Variable '" << drCond->getDecl()->getName()
     << "' with floating point type '" << drCond->getType().getAsString()
     << "' should not be used as a loop counter";

  ranges.push_back(drCond->getSourceRange());
  ranges.push_back(drInc->getSourceRange());

  const char *bugType = "Floating point variable used as loop counter";

  PathDiagnosticLocation FSLoc =
    PathDiagnosticLocation::createBegin(FS, BR.getSourceManager(), AC);
  BR.EmitBasicReport(bugType, "Security", os.str(),
                     FSLoc, ranges.data(), ranges.size());
}

//===----------------------------------------------------------------------===//
// Check: Any use of 'gets' is insecure.
// Originally: <rdar://problem/6335715>
// Implements (part of): 300-BSI (buildsecurityin.us-cert.gov)
// CWE-242: Use of Inherently Dangerous Function
//===----------------------------------------------------------------------===//

void WalkAST::checkCall_gets(const CallExpr *CE, const FunctionDecl *FD) {
  const FunctionProtoType *FPT
    = dyn_cast<FunctionProtoType>(FD->getType().IgnoreParens());
  if (!FPT)
    return;

  // Verify that the function takes a single argument.
  if (FPT->getNumArgs() != 1)
    return;

  // Is the argument a 'char*'?
  const PointerType *PT = dyn_cast<PointerType>(FPT->getArgType(0));
  if (!PT)
    return;

  if (PT->getPointeeType().getUnqualifiedType() != BR.getContext().CharTy)
    return;

  // Issue a warning.
  SourceRange R = CE->getCallee()->getSourceRange();
  PathDiagnosticLocation CELoc =
    PathDiagnosticLocation::createBegin(CE, BR.getSourceManager(), AC);
  BR.EmitBasicReport("Potential buffer overflow in call to 'gets'",
                     "Security",
                     "Call to function 'gets' is extremely insecure as it can "
                     "always result in a buffer overflow",
                     CELoc, &R, 1);
}

//===----------------------------------------------------------------------===//
// Check: Any use of 'getpwd' is insecure.
// CWE-477: Use of Obsolete Functions
//===----------------------------------------------------------------------===//

void WalkAST::checkCall_getpw(const CallExpr *CE, const FunctionDecl *FD) {
  const FunctionProtoType *FPT
    = dyn_cast<FunctionProtoType>(FD->getType().IgnoreParens());
  if (!FPT)
    return;

  // Verify that the function takes two arguments.
  if (FPT->getNumArgs() != 2)
    return;

  // Verify the first argument type is integer.
  if (!FPT->getArgType(0)->isIntegerType())
    return;

  // Verify the second argument type is char*.
  const PointerType *PT = dyn_cast<PointerType>(FPT->getArgType(1));
  if (!PT)
    return;

  if (PT->getPointeeType().getUnqualifiedType() != BR.getContext().CharTy)
    return;

  // Issue a warning.
  SourceRange R = CE->getCallee()->getSourceRange();
  PathDiagnosticLocation CELoc =
    PathDiagnosticLocation::createBegin(CE, BR.getSourceManager(), AC);
  BR.EmitBasicReport("Potential buffer overflow in call to 'getpw'",
                     "Security",
                     "The getpw() function is dangerous as it may overflow the "
                     "provided buffer. It is obsoleted by getpwuid().",
                     CELoc, &R, 1);
}

//===----------------------------------------------------------------------===//
// Check: Any use of 'mktemp' is insecure.It is obsoleted by mkstemp().
// CWE-377: Insecure Temporary File
//===----------------------------------------------------------------------===//

void WalkAST::checkCall_mktemp(const CallExpr *CE, const FunctionDecl *FD) {
  const FunctionProtoType *FPT
    = dyn_cast<FunctionProtoType>(FD->getType().IgnoreParens());
  if(!FPT)
    return;

  // Verify that the function takes a single argument.
  if (FPT->getNumArgs() != 1)
    return;

  // Verify that the argument is Pointer Type.
  const PointerType *PT = dyn_cast<PointerType>(FPT->getArgType(0));
  if (!PT)
    return;

  // Verify that the argument is a 'char*'.
  if (PT->getPointeeType().getUnqualifiedType() != BR.getContext().CharTy)
    return;

  // Issue a waring.
  SourceRange R = CE->getCallee()->getSourceRange();
  PathDiagnosticLocation CELoc =
    PathDiagnosticLocation::createBegin(CE, BR.getSourceManager(), AC);
  BR.EmitBasicReport("Potential insecure temporary file in call 'mktemp'",
                     "Security",
                     "Call to function 'mktemp' is insecure as it always "
                     "creates or uses insecure temporary file.  Use 'mkstemp' instead",
                     CELoc, &R, 1);
}

//===----------------------------------------------------------------------===//
// Check: Any use of 'strcpy' is insecure.
//
// CWE-119: Improper Restriction of Operations within
// the Bounds of a Memory Buffer
//===----------------------------------------------------------------------===//
void WalkAST::checkCall_strcpy(const CallExpr *CE, const FunctionDecl *FD) {
  if (!checkCall_strCommon(CE, FD))
    return;

  // Issue a warning.
  SourceRange R = CE->getCallee()->getSourceRange();
  PathDiagnosticLocation CELoc =
    PathDiagnosticLocation::createBegin(CE, BR.getSourceManager(), AC);
  BR.EmitBasicReport("Potential insecure memory buffer bounds restriction in "
                     "call 'strcpy'",
                     "Security",
                     "Call to function 'strcpy' is insecure as it does not "
		     "provide bounding of the memory buffer. Replace "
		     "unbounded copy functions with analogous functions that "
		     "support length arguments such as 'strncpy'. CWE-119.",
                     CELoc, &R, 1);
}

//===----------------------------------------------------------------------===//
// Check: Any use of 'strcat' is insecure.
//
// CWE-119: Improper Restriction of Operations within
// the Bounds of a Memory Buffer
//===----------------------------------------------------------------------===//
void WalkAST::checkCall_strcat(const CallExpr *CE, const FunctionDecl *FD) {
  if (!checkCall_strCommon(CE, FD))
    return;

  // Issue a warning.
  SourceRange R = CE->getCallee()->getSourceRange();
  PathDiagnosticLocation CELoc =
    PathDiagnosticLocation::createBegin(CE, BR.getSourceManager(), AC);
  BR.EmitBasicReport("Potential insecure memory buffer bounds restriction in "
		     "call 'strcat'",
		     "Security",
		     "Call to function 'strcat' is insecure as it does not "
		     "provide bounding of the memory buffer. Replace "
		     "unbounded copy functions with analogous functions that "
		     "support length arguments such as 'strncat'. CWE-119.",
                     CELoc, &R, 1);
}

//===----------------------------------------------------------------------===//
// Common check for str* functions with no bounds parameters.
//===----------------------------------------------------------------------===//
bool WalkAST::checkCall_strCommon(const CallExpr *CE, const FunctionDecl *FD) {
  const FunctionProtoType *FPT
    = dyn_cast<FunctionProtoType>(FD->getType().IgnoreParens());
  if (!FPT)
    return false;

  // Verify the function takes two arguments, three in the _chk version.
  int numArgs = FPT->getNumArgs();
  if (numArgs != 2 && numArgs != 3)
    return false;

  // Verify the type for both arguments.
  for (int i = 0; i < 2; i++) {
    // Verify that the arguments are pointers.
    const PointerType *PT = dyn_cast<PointerType>(FPT->getArgType(i));
    if (!PT)
      return false;

    // Verify that the argument is a 'char*'.
    if (PT->getPointeeType().getUnqualifiedType() != BR.getContext().CharTy)
      return false;
  }

  return true;
}

//===----------------------------------------------------------------------===//
// Check: Linear congruent random number generators should not be used
// Originally: <rdar://problem/63371000>
// CWE-338: Use of cryptographically weak prng
//===----------------------------------------------------------------------===//

void WalkAST::checkCall_rand(const CallExpr *CE, const FunctionDecl *FD) {
  if (!CheckRand)
    return;

  const FunctionProtoType *FTP
    = dyn_cast<FunctionProtoType>(FD->getType().IgnoreParens());
  if (!FTP)
    return;

  if (FTP->getNumArgs() == 1) {
    // Is the argument an 'unsigned short *'?
    // (Actually any integer type is allowed.)
    const PointerType *PT = dyn_cast<PointerType>(FTP->getArgType(0));
    if (!PT)
      return;

    if (! PT->getPointeeType()->isIntegerType())
      return;
  }
  else if (FTP->getNumArgs() != 0)
    return;

  // Issue a warning.
  llvm::SmallString<256> buf1;
  llvm::raw_svector_ostream os1(buf1);
  os1 << '\'' << FD << "' is a poor random number generator";

  llvm::SmallString<256> buf2;
  llvm::raw_svector_ostream os2(buf2);
  os2 << "Function '" << FD
      << "' is obsolete because it implements a poor random number generator."
      << "  Use 'arc4random' instead";

  SourceRange R = CE->getCallee()->getSourceRange();
  PathDiagnosticLocation CELoc =
    PathDiagnosticLocation::createBegin(CE, BR.getSourceManager(), AC);
  BR.EmitBasicReport(os1.str(), "Security", os2.str(), CELoc, &R, 1);
}

//===----------------------------------------------------------------------===//
// Check: 'random' should not be used
// Originally: <rdar://problem/63371000>
//===----------------------------------------------------------------------===//

void WalkAST::checkCall_random(const CallExpr *CE, const FunctionDecl *FD) {
  if (!CheckRand)
    return;

  const FunctionProtoType *FTP
    = dyn_cast<FunctionProtoType>(FD->getType().IgnoreParens());
  if (!FTP)
    return;

  // Verify that the function takes no argument.
  if (FTP->getNumArgs() != 0)
    return;

  // Issue a warning.
  SourceRange R = CE->getCallee()->getSourceRange();
  PathDiagnosticLocation CELoc =
    PathDiagnosticLocation::createBegin(CE, BR.getSourceManager(), AC);
  BR.EmitBasicReport("'random' is not a secure random number generator",
                     "Security",
                     "The 'random' function produces a sequence of values that "
                     "an adversary may be able to predict.  Use 'arc4random' "
                     "instead", CELoc, &R, 1);
}

//===----------------------------------------------------------------------===//
// Check: Should check whether privileges are dropped successfully.
// Originally: <rdar://problem/6337132>
//===----------------------------------------------------------------------===//

void WalkAST::checkUncheckedReturnValue(CallExpr *CE) {
  const FunctionDecl *FD = CE->getDirectCallee();
  if (!FD)
    return;

  if (II_setid[0] == NULL) {
    static const char * const identifiers[num_setids] = {
      "setuid", "setgid", "seteuid", "setegid",
      "setreuid", "setregid"
    };

    for (size_t i = 0; i < num_setids; i++)
      II_setid[i] = &BR.getContext().Idents.get(identifiers[i]);
  }

  const IdentifierInfo *id = FD->getIdentifier();
  size_t identifierid;

  for (identifierid = 0; identifierid < num_setids; identifierid++)
    if (id == II_setid[identifierid])
      break;

  if (identifierid >= num_setids)
    return;

  const FunctionProtoType *FTP
    = dyn_cast<FunctionProtoType>(FD->getType().IgnoreParens());
  if (!FTP)
    return;

  // Verify that the function takes one or two arguments (depending on
  //   the function).
  if (FTP->getNumArgs() != (identifierid < 4 ? 1 : 2))
    return;

  // The arguments must be integers.
  for (unsigned i = 0; i < FTP->getNumArgs(); i++)
    if (! FTP->getArgType(i)->isIntegerType())
      return;

  // Issue a warning.
  llvm::SmallString<256> buf1;
  llvm::raw_svector_ostream os1(buf1);
  os1 << "Return value is not checked in call to '" << FD << '\'';

  llvm::SmallString<256> buf2;
  llvm::raw_svector_ostream os2(buf2);
  os2 << "The return value from the call to '" << FD
      << "' is not checked.  If an error occurs in '" << FD
      << "', the following code may execute with unexpected privileges";

  SourceRange R = CE->getCallee()->getSourceRange();
  PathDiagnosticLocation CELoc =
    PathDiagnosticLocation::createBegin(CE, BR.getSourceManager(), AC);
  BR.EmitBasicReport(os1.str(), "Security", os2.str(), CELoc, &R, 1);
}

//===----------------------------------------------------------------------===//
// SecuritySyntaxChecker
//===----------------------------------------------------------------------===//

namespace {
class SecuritySyntaxChecker : public Checker<check::ASTCodeBody> {
public:
  void checkASTCodeBody(const Decl *D, AnalysisManager& mgr,
                        BugReporter &BR) const {
    WalkAST walker(BR, mgr.getAnalysisContext(D));
    walker.Visit(D->getBody());
  }
};
}

void ento::registerSecuritySyntaxChecker(CheckerManager &mgr) {
  mgr.registerChecker<SecuritySyntaxChecker>();
}