[analyzer] Introduce MallocOverflowSecurityChecker, a simple flow-sensitive checker that may be useful for security auditing. This checker is currently too noisy to be on by default.

llvm-svn: 136804

clang/lib/StaticAnalyzer/Checkers/CMakeLists.txt

@@ -34,6 +34,7 @@ add_clang_library(clangStaticAnalyzerCheckers
   MacOSKeychainAPIChecker.cpp
   MacOSXAPIChecker.cpp
   MallocChecker.cpp
+  MallocOverflowSecurityChecker.cpp
   NSAutoreleasePoolChecker.cpp
   NSErrorChecker.cpp
   NoReturnFunctionChecker.cpp

clang/lib/StaticAnalyzer/Checkers/Checkers.td

@@ -222,6 +222,10 @@ def ReturnPointerRangeChecker : Checker<"ReturnPtrRange">,
   HelpText<"Check for an out-of-bound pointer being returned to callers">,
   DescFile<"ReturnPointerRangeChecker.cpp">;
 
+def MallocOverflowSecurityChecker : Checker<"MallocOverflow">,
+  HelpText<"Check for overflows in the arguments to malloc()">,
+  DescFile<"MallocOverflowSecurityChecker.cpp">;
+
 } // end "security.experimental"
 
 //===----------------------------------------------------------------------===//

clang/lib/StaticAnalyzer/Checkers/MallocOverflowSecurityChecker.cpp

@@ -0,0 +1,262 @@
+// MallocOverflowSecurityChecker.cpp - Check for malloc overflows -*- C++ -*-=//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This checker detects a common memory allocation security flaw.
+// Suppose 'unsigned int n' comes from an untrusted source. If the
+// code looks like 'malloc (n * 4)', and an attacker can make 'n' be
+// say MAX_UINT/4+2, then instead of allocating the correct 'n' 4-byte
+// elements, this will actually allocate only two because of overflow.
+// Then when the rest of the program attempts to store values past the
+// second element, these values will actually overwrite other items in
+// the heap, probably allowing the attacker to execute arbitrary code.
+//
+//===----------------------------------------------------------------------===//
+
+#include "ClangSACheckers.h"
+#include "clang/AST/EvaluatedExprVisitor.h"
+#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
+#include "clang/StaticAnalyzer/Core/Checker.h"
+#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
+#include "llvm/ADT/SmallVector.h"
+
+using namespace clang;
+using namespace ento;
+
+namespace {
+struct MallocOverflowCheck {
+  const BinaryOperator *mulop;
+  const Expr *variable;
+
+  MallocOverflowCheck (const BinaryOperator *m, const Expr *v) 
+    : mulop(m), variable (v)
+  {}
+};
+
+class MallocOverflowSecurityChecker : public Checker<check::ASTCodeBody> {
+public:
+  void checkASTCodeBody(const Decl *D, AnalysisManager &mgr,
+                        BugReporter &BR) const;
+
+  void CheckMallocArgument(
+    llvm::SmallVectorImpl<MallocOverflowCheck> &PossibleMallocOverflows,
+    const Expr *TheArgument, ASTContext &Context) const;
+
+  void OutputPossibleOverflows(
+    llvm::SmallVectorImpl<MallocOverflowCheck> &PossibleMallocOverflows,
+    const Decl *D, BugReporter &BR, AnalysisManager &mgr) const;
+
+};
+} // end anonymous namespace
+
+void MallocOverflowSecurityChecker::CheckMallocArgument(
+  llvm::SmallVectorImpl<MallocOverflowCheck> &PossibleMallocOverflows,
+  const Expr *TheArgument,
+  ASTContext &Context) const {
+
+  /* Look for a linear combination with a single variable, and at least
+   one multiplication.
+   Reject anything that applies to the variable: an explicit cast,
+   conditional expression, an operation that could reduce the range
+   of the result, or anything too complicated :-).  */
+  const Expr * e = TheArgument;
+  const BinaryOperator * mulop = NULL;
+
+  for (;;) {
+    e = e->IgnoreParenImpCasts();
+    if (isa<BinaryOperator>(e)) {
+      const BinaryOperator * binop = dyn_cast<BinaryOperator>(e);
+      BinaryOperatorKind opc = binop->getOpcode();
+      // TODO: ignore multiplications by 1, reject if multiplied by 0.
+      if (mulop == NULL && opc == BO_Mul)
+        mulop = binop;
+      if (opc != BO_Mul && opc != BO_Add && opc != BO_Sub && opc != BO_Shl)
+        return;
+
+      const Expr *lhs = binop->getLHS();
+      const Expr *rhs = binop->getRHS();
+      if (rhs->isEvaluatable(Context))
+        e = lhs;
+      else if ((opc == BO_Add || opc == BO_Mul)
+               && lhs->isEvaluatable(Context))
+        e = rhs;
+      else
+        return;
+    }
+    else if (isa<DeclRefExpr>(e) || isa<MemberExpr>(e))
+      break;
+    else
+      return;
+  }
+
+  if (mulop == NULL)
+    return;
+
+  //  We've found the right structure of malloc argument, now save
+  // the data so when the body of the function is completely available
+  // we can check for comparisons.
+
+  // TODO: Could push this into the innermost scope where 'e' is
+  // defined, rather than the whole function.
+  PossibleMallocOverflows.push_back(MallocOverflowCheck(mulop, e));
+}
+
+namespace {
+// A worker class for OutputPossibleOverflows.
+class CheckOverflowOps :
+  public EvaluatedExprVisitor<CheckOverflowOps> {
+public:
+  typedef llvm::SmallVectorImpl<MallocOverflowCheck> theVecType;
+
+private:
+    theVecType &toScanFor;
+    ASTContext &Context;
+
+    bool isIntZeroExpr(const Expr *E) const {
+      return (E->getType()->isIntegralOrEnumerationType()
+              && E->isEvaluatable(Context)
+              && E->EvaluateAsInt(Context) == 0);
+    }
+
+    void CheckExpr(const Expr *E_p) {
+      const Expr *E = E_p->IgnoreParenImpCasts();
+
+      theVecType::iterator i = toScanFor.end();
+      theVecType::iterator e = toScanFor.begin();
+
+      if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E)) {
+        const Decl * EdreD = DR->getDecl();
+        while (i != e) {
+          --i;
+          if (const DeclRefExpr *DR_i = dyn_cast<DeclRefExpr>(i->variable)) {
+            if (DR_i->getDecl() == EdreD)
+              i = toScanFor.erase(i);
+          }
+        }
+      }
+      else if (isa<MemberExpr>(E)) {
+        // No points-to analysis, just look at the member
+        const Decl * EmeMD = dyn_cast<MemberExpr>(E)->getMemberDecl();
+        while (i != e) {
+          --i;
+          if (isa<MemberExpr>(i->variable)) {
+            if (dyn_cast<MemberExpr>(i->variable)->getMemberDecl() == EmeMD)
+              i = toScanFor.erase (i);
+          }
+        }
+      }
+    }
+
+  public:
+    void VisitBinaryOperator(BinaryOperator *E) {
+      if (E->isComparisonOp()) {
+        const Expr * lhs = E->getLHS();
+        const Expr * rhs = E->getRHS();
+        // Ignore comparisons against zero, since they generally don't
+        // protect against an overflow.
+        if (!isIntZeroExpr(lhs) && ! isIntZeroExpr(rhs)) {
+          CheckExpr(lhs);
+          CheckExpr(rhs);
+        }
+      }
+      EvaluatedExprVisitor<CheckOverflowOps>::VisitBinaryOperator(E);
+    }
+
+    /* We specifically ignore loop conditions, because they're typically
+     not error checks.  */
+    void VisitWhileStmt(WhileStmt *S) {
+      return this->Visit(S->getBody());
+    }
+    void VisitForStmt(ForStmt *S) {
+      return this->Visit(S->getBody());
+    }
+    void VisitDoStmt(DoStmt *S) {
+      return this->Visit(S->getBody());
+    }
+
+    CheckOverflowOps(theVecType &v, ASTContext &ctx)
+    : EvaluatedExprVisitor<CheckOverflowOps>(ctx),
+      toScanFor(v), Context(ctx)
+    { }
+  };
+}
+
+// OutputPossibleOverflows - We've found a possible overflow earlier,
+// now check whether Body might contain a comparison which might be
+// preventing the overflow.
+// This doesn't do flow analysis, range analysis, or points-to analysis; it's
+// just a dumb "is there a comparison" scan.  The aim here is to
+// detect the most blatent cases of overflow and educate the
+// programmer.
+void MallocOverflowSecurityChecker::OutputPossibleOverflows(
+  llvm::SmallVectorImpl<MallocOverflowCheck> &PossibleMallocOverflows,
+  const Decl *D, BugReporter &BR, AnalysisManager &mgr) const {
+  // By far the most common case: nothing to check.
+  if (PossibleMallocOverflows.empty())
+    return;
+
+  // Delete any possible overflows which have a comparison.
+  CheckOverflowOps c(PossibleMallocOverflows, BR.getContext());
+  c.Visit(mgr.getAnalysisContext(D)->getBody());
+
+  // Output warnings for all overflows that are left.
+  for (CheckOverflowOps::theVecType::iterator
+       i = PossibleMallocOverflows.begin(),
+       e = PossibleMallocOverflows.end();
+       i != e;
+       ++i) {
+    SourceRange R = i->mulop->getSourceRange();
+    BR.EmitBasicReport("MallocOverflowSecurityChecker",
+        "the computation of the size of the memory allocation may overflow",
+        i->mulop->getOperatorLoc(), &R, 1);
+  }
+}
+
+void MallocOverflowSecurityChecker::checkASTCodeBody(const Decl *D,
+                                             AnalysisManager &mgr,
+                                             BugReporter &BR) const {
+
+  CFG *cfg = mgr.getCFG(D);
+  if (!cfg)
+    return;
+
+  // A list of variables referenced in possibly overflowing malloc operands.
+  llvm::SmallVector<MallocOverflowCheck, 2> PossibleMallocOverflows;
+
+  for (CFG::iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it) {
+    CFGBlock *block = *it;
+    for (CFGBlock::iterator bi = block->begin(), be = block->end();
+         bi != be; ++bi) {
+      if (const CFGStmt *CS = bi->getAs<CFGStmt>()) {
+        if (const CallExpr *TheCall = dyn_cast<CallExpr>(CS->getStmt())) {
+          // Get the callee.
+          const FunctionDecl *FD = TheCall->getDirectCallee();
+
+          if (!FD)
+            return;
+
+          // Get the name of the callee. If it's a builtin, strip off the prefix.
+          IdentifierInfo *FnInfo = FD->getIdentifier();
+
+          if (FnInfo->isStr ("malloc") || FnInfo->isStr ("_MALLOC")) {
+            if (TheCall->getNumArgs() == 1)
+              CheckMallocArgument(PossibleMallocOverflows, TheCall->getArg(0),
+                                  mgr.getASTContext());
+          }
+        }
+      }
+    }
+  }
+
+  OutputPossibleOverflows(PossibleMallocOverflows, D, BR, mgr);
+}
+
+void ento::registerMallocOverflowSecurityChecker(CheckerManager &mgr) {
+  mgr.registerChecker<MallocOverflowSecurityChecker>();
+}
+

clang/test/Analysis/malloc-overflow.c

@@ -0,0 +1,114 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.7.0 -analyze -analyzer-checker=security.experimental.MallocOverflow -verify %s
+
+typedef __typeof__(sizeof(int)) size_t;
+extern void * malloc(size_t);
+
+void * f1(int n)
+{
+  return malloc(n * sizeof(int));  // expected-warning {{the computation of the size of the memory allocation may overflow}}
+}
+
+void * f2(int n)
+{
+  return malloc(sizeof(int) * n); // // expected-warning {{the computation of the size of the memory allocation may overflow}}
+}
+
+void * f3()
+{
+  return malloc(4 * sizeof(int));  // no-warning
+}
+
+struct s4
+{
+  int n;
+};
+
+void * f4(struct s4 *s)
+{
+  return malloc(s->n * sizeof(int)); // expected-warning {{the computation of the size of the memory allocation may overflow}}
+}
+
+void * f5(struct s4 *s)
+{
+  struct s4 s2 = *s;
+  return malloc(s2.n * sizeof(int)); // expected-warning {{the computation of the size of the memory allocation may overflow}}
+}
+
+void * f6(int n)
+{
+  return malloc((n + 1) * sizeof(int)); // expected-warning {{the computation of the size of the memory allocation may overflow}}
+}
+
+#include <stddef.h>
+extern void * malloc (size_t);
+
+void * f7(int n)
+{
+  if (n > 10)
+    return NULL;
+  return malloc(n * sizeof(int));  // no-warning
+}
+
+void * f8(int n)
+{
+  if (n < 10)
+    return malloc(n * sizeof(int));  // no-warning
+  else
+    return NULL;
+}
+
+void * f9(int n)
+{
+  int * x = malloc(n * sizeof(int));  // expected-warning {{the computation of the size of the memory allocation may overflow}}
+  for (int i = 0; i < n; i++)
+    x[i] = i;
+  return x;
+}
+
+void * f10(int n)
+{
+  int * x = malloc(n * sizeof(int));  // expected-warning {{the computation of the size of the memory allocation may overflow}}
+  int i = 0;
+  while (i < n)
+    x[i++] = 0;
+  return x;
+}
+
+void * f11(int n)
+{
+  int * x = malloc(n * sizeof(int));  // expected-warning {{the computation of the size of the memory allocation may overflow}}
+  int i = 0;
+  do {
+    x[i++] = 0;
+  } while (i < n);
+  return x;
+}
+
+void * f12(int n)
+{
+  n = (n > 10 ? 10 : n);
+  int * x = malloc(n * sizeof(int));  // no-warning
+  for (int i = 0; i < n; i++)
+    x[i] = i;
+  return x;
+}
+
+struct s13
+{
+  int n;
+};
+
+void * f13(struct s13 *s)
+{
+  if (s->n > 10)
+    return NULL;
+  return malloc(s->n * sizeof(int));  // no warning
+}
+
+void * f14(int n)
+{
+  if (n < 0)
+    return NULL;
+  return malloc(n * sizeof(int));  // expected-warning {{the computation of the size of the memory allocation may overflow}}
+}
+