1 //== ArrayBoundCheckerV2.cpp ------------------------------------*- C++ -*--==//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file defines ArrayBoundCheckerV2, which is a path-sensitive check
10 // which looks for an out-of-bound array element access.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "Taint.h"
15 #include "clang/AST/CharUnits.h"
16 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
17 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
18 #include "clang/StaticAnalyzer/Core/Checker.h"
19 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
20 #include "clang/StaticAnalyzer/Core/PathSensitive/APSIntType.h"
21 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
22 #include "clang/StaticAnalyzer/Core/PathSensitive/DynamicSize.h"
23 #include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
24 #include "llvm/ADT/SmallString.h"
25 #include "llvm/Support/raw_ostream.h"
26 
27 using namespace clang;
28 using namespace ento;
29 using namespace taint;
30 
31 namespace {
32 class ArrayBoundCheckerV2 :
33     public Checker<check::Location> {
34   mutable std::unique_ptr<BuiltinBug> BT;
35 
36   enum OOB_Kind { OOB_Precedes, OOB_Excedes, OOB_Tainted };
37 
38   void reportOOB(CheckerContext &C, ProgramStateRef errorState, OOB_Kind kind,
39                  std::unique_ptr<BugReporterVisitor> Visitor = nullptr) const;
40 
41 public:
42   void checkLocation(SVal l, bool isLoad, const Stmt*S,
43                      CheckerContext &C) const;
44 };
45 
46 // FIXME: Eventually replace RegionRawOffset with this class.
47 class RegionRawOffsetV2 {
48 private:
49   const SubRegion *baseRegion;
50   SVal byteOffset;
51 
52   RegionRawOffsetV2()
53     : baseRegion(nullptr), byteOffset(UnknownVal()) {}
54 
55 public:
56   RegionRawOffsetV2(const SubRegion* base, SVal offset)
57     : baseRegion(base), byteOffset(offset) {}
58 
59   NonLoc getByteOffset() const { return byteOffset.castAs<NonLoc>(); }
60   const SubRegion *getRegion() const { return baseRegion; }
61 
62   static RegionRawOffsetV2 computeOffset(ProgramStateRef state,
63                                          SValBuilder &svalBuilder,
64                                          SVal location);
65 
66   void dump() const;
67   void dumpToStream(raw_ostream &os) const;
68 };
69 }
70 
71 static SVal computeExtentBegin(SValBuilder &svalBuilder,
72                                const MemRegion *region) {
73   const MemSpaceRegion *SR = region->getMemorySpace();
74   if (SR->getKind() == MemRegion::UnknownSpaceRegionKind)
75     return UnknownVal();
76   else
77     return svalBuilder.makeZeroArrayIndex();
78 }
79 
80 // TODO: once the constraint manager is smart enough to handle non simplified
81 // symbolic expressions remove this function. Note that this can not be used in
82 // the constraint manager as is, since this does not handle overflows. It is
83 // safe to assume, however, that memory offsets will not overflow.
84 static std::pair<NonLoc, nonloc::ConcreteInt>
85 getSimplifiedOffsets(NonLoc offset, nonloc::ConcreteInt extent,
86                      SValBuilder &svalBuilder) {
87   Optional<nonloc::SymbolVal> SymVal = offset.getAs<nonloc::SymbolVal>();
88   if (SymVal && SymVal->isExpression()) {
89     if (const SymIntExpr *SIE = dyn_cast<SymIntExpr>(SymVal->getSymbol())) {
90       llvm::APSInt constant =
91           APSIntType(extent.getValue()).convert(SIE->getRHS());
92       switch (SIE->getOpcode()) {
93       case BO_Mul:
94         // The constant should never be 0 here, since it the result of scaling
95         // based on the size of a type which is never 0.
96         if ((extent.getValue() % constant) != 0)
97           return std::pair<NonLoc, nonloc::ConcreteInt>(offset, extent);
98         else
99           return getSimplifiedOffsets(
100               nonloc::SymbolVal(SIE->getLHS()),
101               svalBuilder.makeIntVal(extent.getValue() / constant),
102               svalBuilder);
103       case BO_Add:
104         return getSimplifiedOffsets(
105             nonloc::SymbolVal(SIE->getLHS()),
106             svalBuilder.makeIntVal(extent.getValue() - constant), svalBuilder);
107       default:
108         break;
109       }
110     }
111   }
112 
113   return std::pair<NonLoc, nonloc::ConcreteInt>(offset, extent);
114 }
115 
116 void ArrayBoundCheckerV2::checkLocation(SVal location, bool isLoad,
117                                         const Stmt* LoadS,
118                                         CheckerContext &checkerContext) const {
119 
120   // NOTE: Instead of using ProgramState::assumeInBound(), we are prototyping
121   // some new logic here that reasons directly about memory region extents.
122   // Once that logic is more mature, we can bring it back to assumeInBound()
123   // for all clients to use.
124   //
125   // The algorithm we are using here for bounds checking is to see if the
126   // memory access is within the extent of the base region.  Since we
127   // have some flexibility in defining the base region, we can achieve
128   // various levels of conservatism in our buffer overflow checking.
129   ProgramStateRef state = checkerContext.getState();
130 
131   SValBuilder &svalBuilder = checkerContext.getSValBuilder();
132   const RegionRawOffsetV2 &rawOffset =
133     RegionRawOffsetV2::computeOffset(state, svalBuilder, location);
134 
135   if (!rawOffset.getRegion())
136     return;
137 
138   NonLoc rawOffsetVal = rawOffset.getByteOffset();
139 
140   // CHECK LOWER BOUND: Is byteOffset < extent begin?
141   //  If so, we are doing a load/store
142   //  before the first valid offset in the memory region.
143 
144   SVal extentBegin = computeExtentBegin(svalBuilder, rawOffset.getRegion());
145 
146   if (Optional<NonLoc> NV = extentBegin.getAs<NonLoc>()) {
147     if (NV->getAs<nonloc::ConcreteInt>()) {
148       std::pair<NonLoc, nonloc::ConcreteInt> simplifiedOffsets =
149           getSimplifiedOffsets(rawOffset.getByteOffset(),
150                                NV->castAs<nonloc::ConcreteInt>(),
151                                svalBuilder);
152       rawOffsetVal = simplifiedOffsets.first;
153       *NV = simplifiedOffsets.second;
154     }
155 
156     SVal lowerBound = svalBuilder.evalBinOpNN(state, BO_LT, rawOffsetVal, *NV,
157                                               svalBuilder.getConditionType());
158 
159     Optional<NonLoc> lowerBoundToCheck = lowerBound.getAs<NonLoc>();
160     if (!lowerBoundToCheck)
161       return;
162 
163     ProgramStateRef state_precedesLowerBound, state_withinLowerBound;
164     std::tie(state_precedesLowerBound, state_withinLowerBound) =
165       state->assume(*lowerBoundToCheck);
166 
167     // Are we constrained enough to definitely precede the lower bound?
168     if (state_precedesLowerBound && !state_withinLowerBound) {
169       reportOOB(checkerContext, state_precedesLowerBound, OOB_Precedes);
170       return;
171     }
172 
173     // Otherwise, assume the constraint of the lower bound.
174     assert(state_withinLowerBound);
175     state = state_withinLowerBound;
176   }
177 
178   do {
179     // CHECK UPPER BOUND: Is byteOffset >= size(baseRegion)?  If so,
180     // we are doing a load/store after the last valid offset.
181     const MemRegion *MR = rawOffset.getRegion();
182     DefinedOrUnknownSVal Size = getDynamicSize(state, MR, svalBuilder);
183     if (!Size.getAs<NonLoc>())
184       break;
185 
186     if (Size.getAs<nonloc::ConcreteInt>()) {
187       std::pair<NonLoc, nonloc::ConcreteInt> simplifiedOffsets =
188           getSimplifiedOffsets(rawOffset.getByteOffset(),
189                                Size.castAs<nonloc::ConcreteInt>(), svalBuilder);
190       rawOffsetVal = simplifiedOffsets.first;
191       Size = simplifiedOffsets.second;
192     }
193 
194     SVal upperbound = svalBuilder.evalBinOpNN(state, BO_GE, rawOffsetVal,
195                                               Size.castAs<NonLoc>(),
196                                               svalBuilder.getConditionType());
197 
198     Optional<NonLoc> upperboundToCheck = upperbound.getAs<NonLoc>();
199     if (!upperboundToCheck)
200       break;
201 
202     ProgramStateRef state_exceedsUpperBound, state_withinUpperBound;
203     std::tie(state_exceedsUpperBound, state_withinUpperBound) =
204       state->assume(*upperboundToCheck);
205 
206     // If we are under constrained and the index variables are tainted, report.
207     if (state_exceedsUpperBound && state_withinUpperBound) {
208       SVal ByteOffset = rawOffset.getByteOffset();
209       if (isTainted(state, ByteOffset)) {
210         reportOOB(checkerContext, state_exceedsUpperBound, OOB_Tainted,
211                   std::make_unique<TaintBugVisitor>(ByteOffset));
212         return;
213       }
214     } else if (state_exceedsUpperBound) {
215       // If we are constrained enough to definitely exceed the upper bound,
216       // report.
217       assert(!state_withinUpperBound);
218       reportOOB(checkerContext, state_exceedsUpperBound, OOB_Excedes);
219       return;
220     }
221 
222     assert(state_withinUpperBound);
223     state = state_withinUpperBound;
224   }
225   while (false);
226 
227   checkerContext.addTransition(state);
228 }
229 
230 void ArrayBoundCheckerV2::reportOOB(
231     CheckerContext &checkerContext, ProgramStateRef errorState, OOB_Kind kind,
232     std::unique_ptr<BugReporterVisitor> Visitor) const {
233 
234   ExplodedNode *errorNode = checkerContext.generateErrorNode(errorState);
235   if (!errorNode)
236     return;
237 
238   if (!BT)
239     BT.reset(new BuiltinBug(this, "Out-of-bound access"));
240 
241   // FIXME: This diagnostics are preliminary.  We should get far better
242   // diagnostics for explaining buffer overruns.
243 
244   SmallString<256> buf;
245   llvm::raw_svector_ostream os(buf);
246   os << "Out of bound memory access ";
247   switch (kind) {
248   case OOB_Precedes:
249     os << "(accessed memory precedes memory block)";
250     break;
251   case OOB_Excedes:
252     os << "(access exceeds upper limit of memory block)";
253     break;
254   case OOB_Tainted:
255     os << "(index is tainted)";
256     break;
257   }
258 
259   auto BR = std::make_unique<PathSensitiveBugReport>(*BT, os.str(), errorNode);
260   BR->addVisitor(std::move(Visitor));
261   checkerContext.emitReport(std::move(BR));
262 }
263 
264 #ifndef NDEBUG
265 LLVM_DUMP_METHOD void RegionRawOffsetV2::dump() const {
266   dumpToStream(llvm::errs());
267 }
268 
269 void RegionRawOffsetV2::dumpToStream(raw_ostream &os) const {
270   os << "raw_offset_v2{" << getRegion() << ',' << getByteOffset() << '}';
271 }
272 #endif
273 
274 // Lazily computes a value to be used by 'computeOffset'.  If 'val'
275 // is unknown or undefined, we lazily substitute '0'.  Otherwise,
276 // return 'val'.
277 static inline SVal getValue(SVal val, SValBuilder &svalBuilder) {
278   return val.getAs<UndefinedVal>() ? svalBuilder.makeArrayIndex(0) : val;
279 }
280 
281 // Scale a base value by a scaling factor, and return the scaled
282 // value as an SVal.  Used by 'computeOffset'.
283 static inline SVal scaleValue(ProgramStateRef state,
284                               NonLoc baseVal, CharUnits scaling,
285                               SValBuilder &sb) {
286   return sb.evalBinOpNN(state, BO_Mul, baseVal,
287                         sb.makeArrayIndex(scaling.getQuantity()),
288                         sb.getArrayIndexType());
289 }
290 
291 // Add an SVal to another, treating unknown and undefined values as
292 // summing to UnknownVal.  Used by 'computeOffset'.
293 static SVal addValue(ProgramStateRef state, SVal x, SVal y,
294                      SValBuilder &svalBuilder) {
295   // We treat UnknownVals and UndefinedVals the same here because we
296   // only care about computing offsets.
297   if (x.isUnknownOrUndef() || y.isUnknownOrUndef())
298     return UnknownVal();
299 
300   return svalBuilder.evalBinOpNN(state, BO_Add, x.castAs<NonLoc>(),
301                                  y.castAs<NonLoc>(),
302                                  svalBuilder.getArrayIndexType());
303 }
304 
305 /// Compute a raw byte offset from a base region.  Used for array bounds
306 /// checking.
307 RegionRawOffsetV2 RegionRawOffsetV2::computeOffset(ProgramStateRef state,
308                                                    SValBuilder &svalBuilder,
309                                                    SVal location)
310 {
311   const MemRegion *region = location.getAsRegion();
312   SVal offset = UndefinedVal();
313 
314   while (region) {
315     switch (region->getKind()) {
316       default: {
317         if (const SubRegion *subReg = dyn_cast<SubRegion>(region)) {
318           offset = getValue(offset, svalBuilder);
319           if (!offset.isUnknownOrUndef())
320             return RegionRawOffsetV2(subReg, offset);
321         }
322         return RegionRawOffsetV2();
323       }
324       case MemRegion::ElementRegionKind: {
325         const ElementRegion *elemReg = cast<ElementRegion>(region);
326         SVal index = elemReg->getIndex();
327         if (!index.getAs<NonLoc>())
328           return RegionRawOffsetV2();
329         QualType elemType = elemReg->getElementType();
330         // If the element is an incomplete type, go no further.
331         ASTContext &astContext = svalBuilder.getContext();
332         if (elemType->isIncompleteType())
333           return RegionRawOffsetV2();
334 
335         // Update the offset.
336         offset = addValue(state,
337                           getValue(offset, svalBuilder),
338                           scaleValue(state,
339                           index.castAs<NonLoc>(),
340                           astContext.getTypeSizeInChars(elemType),
341                           svalBuilder),
342                           svalBuilder);
343 
344         if (offset.isUnknownOrUndef())
345           return RegionRawOffsetV2();
346 
347         region = elemReg->getSuperRegion();
348         continue;
349       }
350     }
351   }
352   return RegionRawOffsetV2();
353 }
354 
355 void ento::registerArrayBoundCheckerV2(CheckerManager &mgr) {
356   mgr.registerChecker<ArrayBoundCheckerV2>();
357 }
358 
359 bool ento::shouldRegisterArrayBoundCheckerV2(const CheckerManager &mgr) {
360   return true;
361 }
362