75cb0dd5ed
Assuming strlcat is used with strlcpy we check as we can if the last argument does not equal os not larger than the buffer. Advising the proper usual pattern. Reviewers: george.karpenkov, NoQ, MaskRay Reviewed By: MaskRay Differential Revision: https://reviews.llvm.org/D49722 llvm-svn: 342832
293 lines
9.7 KiB
C++
293 lines
9.7 KiB
C++
//== CStringSyntaxChecker.cpp - CoreFoundation containers API *- C++ -*-==//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// An AST checker that looks for common pitfalls when using C string APIs.
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// - Identifies erroneous patterns in the last argument to strncat - the number
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// of bytes to copy.
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//
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//===----------------------------------------------------------------------===//
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#include "ClangSACheckers.h"
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#include "clang/AST/Expr.h"
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#include "clang/AST/OperationKinds.h"
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#include "clang/AST/StmtVisitor.h"
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#include "clang/Analysis/AnalysisDeclContext.h"
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#include "clang/Basic/TargetInfo.h"
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#include "clang/Basic/TypeTraits.h"
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#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
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#include "clang/StaticAnalyzer/Core/Checker.h"
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#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
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#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
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#include "llvm/ADT/SmallString.h"
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#include "llvm/Support/raw_ostream.h"
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using namespace clang;
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using namespace ento;
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namespace {
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class WalkAST: public StmtVisitor<WalkAST> {
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const CheckerBase *Checker;
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BugReporter &BR;
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AnalysisDeclContext* AC;
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/// Check if two expressions refer to the same declaration.
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bool sameDecl(const Expr *A1, const Expr *A2) {
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if (const auto *D1 = dyn_cast<DeclRefExpr>(A1->IgnoreParenCasts()))
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if (const auto *D2 = dyn_cast<DeclRefExpr>(A2->IgnoreParenCasts()))
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return D1->getDecl() == D2->getDecl();
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return false;
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}
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/// Check if the expression E is a sizeof(WithArg).
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bool isSizeof(const Expr *E, const Expr *WithArg) {
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if (const auto *UE = dyn_cast<UnaryExprOrTypeTraitExpr>(E))
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if (UE->getKind() == UETT_SizeOf && !UE->isArgumentType())
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return sameDecl(UE->getArgumentExpr(), WithArg);
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return false;
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}
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/// Check if the expression E is a strlen(WithArg).
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bool isStrlen(const Expr *E, const Expr *WithArg) {
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if (const auto *CE = dyn_cast<CallExpr>(E)) {
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const FunctionDecl *FD = CE->getDirectCallee();
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if (!FD)
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return false;
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return (CheckerContext::isCLibraryFunction(FD, "strlen") &&
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sameDecl(CE->getArg(0), WithArg));
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}
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return false;
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}
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/// Check if the expression is an integer literal with value 1.
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bool isOne(const Expr *E) {
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if (const auto *IL = dyn_cast<IntegerLiteral>(E))
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return (IL->getValue().isIntN(1));
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return false;
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}
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StringRef getPrintableName(const Expr *E) {
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if (const auto *D = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
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return D->getDecl()->getName();
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return StringRef();
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}
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/// Identify erroneous patterns in the last argument to strncat - the number
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/// of bytes to copy.
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bool containsBadStrncatPattern(const CallExpr *CE);
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/// Identify erroneous patterns in the last argument to strlcpy - the number
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/// of bytes to copy.
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/// The bad pattern checked is when the size is known
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/// to be larger than the destination can handle.
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/// char dst[2];
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/// size_t cpy = 4;
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/// strlcpy(dst, "abcd", sizeof("abcd") - 1);
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/// strlcpy(dst, "abcd", 4);
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/// strlcpy(dst + 3, "abcd", 2);
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/// strlcpy(dst, "abcd", cpy);
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/// Identify erroneous patterns in the last argument to strlcat - the number
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/// of bytes to copy.
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/// The bad pattern checked is when the last argument is basically
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/// pointing to the destination buffer size or argument larger or
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/// equal to.
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/// char dst[2];
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/// strlcat(dst, src2, sizeof(dst));
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/// strlcat(dst, src2, 2);
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/// strlcat(dst, src2, 10);
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bool containsBadStrlcpyStrlcatPattern(const CallExpr *CE);
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public:
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WalkAST(const CheckerBase *Checker, BugReporter &BR, AnalysisDeclContext *AC)
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: Checker(Checker), BR(BR), AC(AC) {}
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// Statement visitor methods.
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void VisitChildren(Stmt *S);
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void VisitStmt(Stmt *S) {
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VisitChildren(S);
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}
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void VisitCallExpr(CallExpr *CE);
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};
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} // end anonymous namespace
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// The correct size argument should look like following:
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// strncat(dst, src, sizeof(dst) - strlen(dest) - 1);
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// We look for the following anti-patterns:
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// - strncat(dst, src, sizeof(dst) - strlen(dst));
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// - strncat(dst, src, sizeof(dst) - 1);
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// - strncat(dst, src, sizeof(dst));
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bool WalkAST::containsBadStrncatPattern(const CallExpr *CE) {
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if (CE->getNumArgs() != 3)
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return false;
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const Expr *DstArg = CE->getArg(0);
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const Expr *SrcArg = CE->getArg(1);
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const Expr *LenArg = CE->getArg(2);
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// Identify wrong size expressions, which are commonly used instead.
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if (const auto *BE = dyn_cast<BinaryOperator>(LenArg->IgnoreParenCasts())) {
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// - sizeof(dst) - strlen(dst)
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if (BE->getOpcode() == BO_Sub) {
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const Expr *L = BE->getLHS();
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const Expr *R = BE->getRHS();
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if (isSizeof(L, DstArg) && isStrlen(R, DstArg))
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return true;
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// - sizeof(dst) - 1
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if (isSizeof(L, DstArg) && isOne(R->IgnoreParenCasts()))
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return true;
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}
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}
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// - sizeof(dst)
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if (isSizeof(LenArg, DstArg))
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return true;
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// - sizeof(src)
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if (isSizeof(LenArg, SrcArg))
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return true;
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return false;
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}
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bool WalkAST::containsBadStrlcpyStrlcatPattern(const CallExpr *CE) {
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if (CE->getNumArgs() != 3)
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return false;
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const FunctionDecl *FD = CE->getDirectCallee();
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bool Append = CheckerContext::isCLibraryFunction(FD, "strlcat");
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const Expr *DstArg = CE->getArg(0);
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const Expr *LenArg = CE->getArg(2);
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const auto *DstArgDecl = dyn_cast<DeclRefExpr>(DstArg->IgnoreParenImpCasts());
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const auto *LenArgDecl = dyn_cast<DeclRefExpr>(LenArg->IgnoreParenLValueCasts());
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uint64_t DstOff = 0;
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if (isSizeof(LenArg, DstArg))
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return false;
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// - size_t dstlen = sizeof(dst)
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if (LenArgDecl) {
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const auto *LenArgVal = dyn_cast<VarDecl>(LenArgDecl->getDecl());
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if (LenArgVal->getInit())
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LenArg = LenArgVal->getInit();
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}
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// - integral value
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// We try to figure out if the last argument is possibly longer
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// than the destination can possibly handle if its size can be defined.
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if (const auto *IL = dyn_cast<IntegerLiteral>(LenArg->IgnoreParenImpCasts())) {
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uint64_t ILRawVal = IL->getValue().getZExtValue();
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// Case when there is pointer arithmetic on the destination buffer
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// especially when we offset from the base decreasing the
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// buffer length accordingly.
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if (!DstArgDecl) {
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if (const auto *BE = dyn_cast<BinaryOperator>(DstArg->IgnoreParenImpCasts())) {
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DstArgDecl = dyn_cast<DeclRefExpr>(BE->getLHS()->IgnoreParenImpCasts());
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if (BE->getOpcode() == BO_Add) {
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if ((IL = dyn_cast<IntegerLiteral>(BE->getRHS()->IgnoreParenImpCasts()))) {
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DstOff = IL->getValue().getZExtValue();
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}
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}
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}
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}
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if (DstArgDecl) {
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if (const auto *Buffer = dyn_cast<ConstantArrayType>(DstArgDecl->getType())) {
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ASTContext &C = BR.getContext();
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uint64_t BufferLen = C.getTypeSize(Buffer) / 8;
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auto RemainingBufferLen = BufferLen - DstOff;
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if (Append) {
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if (RemainingBufferLen <= ILRawVal)
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return true;
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} else {
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if (RemainingBufferLen < ILRawVal)
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return true;
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}
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}
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}
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}
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return false;
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}
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void WalkAST::VisitCallExpr(CallExpr *CE) {
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const FunctionDecl *FD = CE->getDirectCallee();
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if (!FD)
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return;
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if (CheckerContext::isCLibraryFunction(FD, "strncat")) {
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if (containsBadStrncatPattern(CE)) {
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const Expr *DstArg = CE->getArg(0);
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const Expr *LenArg = CE->getArg(2);
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PathDiagnosticLocation Loc =
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PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
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StringRef DstName = getPrintableName(DstArg);
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SmallString<256> S;
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llvm::raw_svector_ostream os(S);
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os << "Potential buffer overflow. ";
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if (!DstName.empty()) {
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os << "Replace with 'sizeof(" << DstName << ") "
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"- strlen(" << DstName <<") - 1'";
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os << " or u";
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} else
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os << "U";
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os << "se a safer 'strlcat' API";
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BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
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"C String API", os.str(), Loc,
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LenArg->getSourceRange());
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}
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} else if (CheckerContext::isCLibraryFunction(FD, "strlcpy") ||
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CheckerContext::isCLibraryFunction(FD, "strlcat")) {
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if (containsBadStrlcpyStrlcatPattern(CE)) {
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const Expr *DstArg = CE->getArg(0);
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const Expr *LenArg = CE->getArg(2);
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PathDiagnosticLocation Loc =
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PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
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StringRef DstName = getPrintableName(DstArg);
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SmallString<256> S;
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llvm::raw_svector_ostream os(S);
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os << "The third argument allows to potentially copy more bytes than it should. ";
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os << "Replace with the value ";
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if (!DstName.empty())
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os << "sizeof(" << DstName << ")";
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else
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os << "sizeof(<destination buffer>)";
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os << " or lower";
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BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
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"C String API", os.str(), Loc,
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LenArg->getSourceRange());
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}
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}
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// Recurse and check children.
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VisitChildren(CE);
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}
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void WalkAST::VisitChildren(Stmt *S) {
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for (Stmt *Child : S->children())
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if (Child)
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Visit(Child);
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}
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namespace {
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class CStringSyntaxChecker: public Checker<check::ASTCodeBody> {
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public:
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void checkASTCodeBody(const Decl *D, AnalysisManager& Mgr,
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BugReporter &BR) const {
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WalkAST walker(this, BR, Mgr.getAnalysisDeclContext(D));
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walker.Visit(D->getBody());
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}
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};
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}
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void ento::registerCStringSyntaxChecker(CheckerManager &mgr) {
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mgr.registerChecker<CStringSyntaxChecker>();
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}
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