// 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/BugReporter/BugReporter.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.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(
SmallVectorImpl<MallocOverflowCheck> &PossibleMallocOverflows,
const Expr *TheArgument, ASTContext &Context) const;
void OutputPossibleOverflows(
SmallVectorImpl<MallocOverflowCheck> &PossibleMallocOverflows,
const Decl *D, BugReporter &BR, AnalysisManager &mgr) const;
};
} // end anonymous namespace
void MallocOverflowSecurityChecker::CheckMallocArgument(
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 = nullptr;
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 == nullptr && 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 == nullptr)
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 SmallVectorImpl<MallocOverflowCheck> theVecType;
private:
theVecType &toScanFor;
ASTContext &Context;
bool isIntZeroExpr(const Expr *E) const {
if (!E->getType()->isIntegralOrEnumerationType())
return false;
llvm::APSInt Result;
if (E->EvaluateAsInt(Result, Context))
return Result == 0;
return false;
}
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(
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.getAnalysisDeclContext(D)->getBody());
// Output warnings for all overflows that are left.
for (CheckOverflowOps::theVecType::iterator
i = PossibleMallocOverflows.begin(),
e = PossibleMallocOverflows.end();
i != e;
++i) {
BR.EmitBasicReport(
D, this, "malloc() size overflow", categories::UnixAPI,
"the computation of the size of the memory allocation may overflow",
PathDiagnosticLocation::createOperatorLoc(i->mulop,
BR.getSourceManager()),
i->mulop->getSourceRange());
}
}
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.
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 (Optional<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)
return;
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>();
}