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1 //== ArrayBoundCheckerV2.cpp ------------------------------------*- C++ -*--==//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file defines ArrayBoundCheckerV2, which is a path-sensitive check
11 // which looks for an out-of-bound array element access.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "ClangSACheckers.h"
16 #include "clang/AST/CharUnits.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/CheckerContext.h"
21 #include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
22 #include "llvm/ADT/SmallString.h"
23 #include "llvm/Support/raw_ostream.h"
24 
25 using namespace clang;
26 using namespace ento;
27 
28 namespace {
29 class ArrayBoundCheckerV2 :
30     public Checker<check::Location> {
31   mutable std::unique_ptr<BuiltinBug> BT;
32 
33   enum OOB_Kind { OOB_Precedes, OOB_Excedes, OOB_Tainted };
34 
35   void reportOOB(CheckerContext &C, ProgramStateRef errorState,
36                  OOB_Kind kind) const;
37 
38 public:
39   void checkLocation(SVal l, bool isLoad, const Stmt*S,
40                      CheckerContext &C) const;
41 };
42 
43 // FIXME: Eventually replace RegionRawOffset with this class.
44 class RegionRawOffsetV2 {
45 private:
46   const SubRegion *baseRegion;
47   SVal byteOffset;
48 
RegionRawOffsetV2()49   RegionRawOffsetV2()
50     : baseRegion(nullptr), byteOffset(UnknownVal()) {}
51 
52 public:
RegionRawOffsetV2(const SubRegion * base,SVal offset)53   RegionRawOffsetV2(const SubRegion* base, SVal offset)
54     : baseRegion(base), byteOffset(offset) {}
55 
getByteOffset() const56   NonLoc getByteOffset() const { return byteOffset.castAs<NonLoc>(); }
getRegion() const57   const SubRegion *getRegion() const { return baseRegion; }
58 
59   static RegionRawOffsetV2 computeOffset(ProgramStateRef state,
60                                          SValBuilder &svalBuilder,
61                                          SVal location);
62 
63   void dump() const;
64   void dumpToStream(raw_ostream &os) const;
65 };
66 }
67 
computeExtentBegin(SValBuilder & svalBuilder,const MemRegion * region)68 static SVal computeExtentBegin(SValBuilder &svalBuilder,
69                                const MemRegion *region) {
70   while (true)
71     switch (region->getKind()) {
72       default:
73         return svalBuilder.makeZeroArrayIndex();
74       case MemRegion::SymbolicRegionKind:
75         // FIXME: improve this later by tracking symbolic lower bounds
76         // for symbolic regions.
77         return UnknownVal();
78       case MemRegion::ElementRegionKind:
79         region = cast<SubRegion>(region)->getSuperRegion();
80         continue;
81     }
82 }
83 
checkLocation(SVal location,bool isLoad,const Stmt * LoadS,CheckerContext & checkerContext) const84 void ArrayBoundCheckerV2::checkLocation(SVal location, bool isLoad,
85                                         const Stmt* LoadS,
86                                         CheckerContext &checkerContext) const {
87 
88   // NOTE: Instead of using ProgramState::assumeInBound(), we are prototyping
89   // some new logic here that reasons directly about memory region extents.
90   // Once that logic is more mature, we can bring it back to assumeInBound()
91   // for all clients to use.
92   //
93   // The algorithm we are using here for bounds checking is to see if the
94   // memory access is within the extent of the base region.  Since we
95   // have some flexibility in defining the base region, we can achieve
96   // various levels of conservatism in our buffer overflow checking.
97   ProgramStateRef state = checkerContext.getState();
98   ProgramStateRef originalState = state;
99 
100   SValBuilder &svalBuilder = checkerContext.getSValBuilder();
101   const RegionRawOffsetV2 &rawOffset =
102     RegionRawOffsetV2::computeOffset(state, svalBuilder, location);
103 
104   if (!rawOffset.getRegion())
105     return;
106 
107   // CHECK LOWER BOUND: Is byteOffset < extent begin?
108   //  If so, we are doing a load/store
109   //  before the first valid offset in the memory region.
110 
111   SVal extentBegin = computeExtentBegin(svalBuilder, rawOffset.getRegion());
112 
113   if (Optional<NonLoc> NV = extentBegin.getAs<NonLoc>()) {
114     SVal lowerBound =
115         svalBuilder.evalBinOpNN(state, BO_LT, rawOffset.getByteOffset(), *NV,
116                                 svalBuilder.getConditionType());
117 
118     Optional<NonLoc> lowerBoundToCheck = lowerBound.getAs<NonLoc>();
119     if (!lowerBoundToCheck)
120       return;
121 
122     ProgramStateRef state_precedesLowerBound, state_withinLowerBound;
123     std::tie(state_precedesLowerBound, state_withinLowerBound) =
124       state->assume(*lowerBoundToCheck);
125 
126     // Are we constrained enough to definitely precede the lower bound?
127     if (state_precedesLowerBound && !state_withinLowerBound) {
128       reportOOB(checkerContext, state_precedesLowerBound, OOB_Precedes);
129       return;
130     }
131 
132     // Otherwise, assume the constraint of the lower bound.
133     assert(state_withinLowerBound);
134     state = state_withinLowerBound;
135   }
136 
137   do {
138     // CHECK UPPER BOUND: Is byteOffset >= extent(baseRegion)?  If so,
139     // we are doing a load/store after the last valid offset.
140     DefinedOrUnknownSVal extentVal =
141       rawOffset.getRegion()->getExtent(svalBuilder);
142     if (!extentVal.getAs<NonLoc>())
143       break;
144 
145     SVal upperbound
146       = svalBuilder.evalBinOpNN(state, BO_GE, rawOffset.getByteOffset(),
147                                 extentVal.castAs<NonLoc>(),
148                                 svalBuilder.getConditionType());
149 
150     Optional<NonLoc> upperboundToCheck = upperbound.getAs<NonLoc>();
151     if (!upperboundToCheck)
152       break;
153 
154     ProgramStateRef state_exceedsUpperBound, state_withinUpperBound;
155     std::tie(state_exceedsUpperBound, state_withinUpperBound) =
156       state->assume(*upperboundToCheck);
157 
158     // If we are under constrained and the index variables are tainted, report.
159     if (state_exceedsUpperBound && state_withinUpperBound) {
160       if (state->isTainted(rawOffset.getByteOffset()))
161         reportOOB(checkerContext, state_exceedsUpperBound, OOB_Tainted);
162         return;
163     }
164 
165     // If we are constrained enough to definitely exceed the upper bound, report.
166     if (state_exceedsUpperBound) {
167       assert(!state_withinUpperBound);
168       reportOOB(checkerContext, state_exceedsUpperBound, OOB_Excedes);
169       return;
170     }
171 
172     assert(state_withinUpperBound);
173     state = state_withinUpperBound;
174   }
175   while (false);
176 
177   if (state != originalState)
178     checkerContext.addTransition(state);
179 }
180 
reportOOB(CheckerContext & checkerContext,ProgramStateRef errorState,OOB_Kind kind) const181 void ArrayBoundCheckerV2::reportOOB(CheckerContext &checkerContext,
182                                     ProgramStateRef errorState,
183                                     OOB_Kind kind) const {
184 
185   ExplodedNode *errorNode = checkerContext.generateErrorNode(errorState);
186   if (!errorNode)
187     return;
188 
189   if (!BT)
190     BT.reset(new BuiltinBug(this, "Out-of-bound access"));
191 
192   // FIXME: This diagnostics are preliminary.  We should get far better
193   // diagnostics for explaining buffer overruns.
194 
195   SmallString<256> buf;
196   llvm::raw_svector_ostream os(buf);
197   os << "Out of bound memory access ";
198   switch (kind) {
199   case OOB_Precedes:
200     os << "(accessed memory precedes memory block)";
201     break;
202   case OOB_Excedes:
203     os << "(access exceeds upper limit of memory block)";
204     break;
205   case OOB_Tainted:
206     os << "(index is tainted)";
207     break;
208   }
209 
210   checkerContext.emitReport(
211       llvm::make_unique<BugReport>(*BT, os.str(), errorNode));
212 }
213 
dump() const214 LLVM_DUMP_METHOD void RegionRawOffsetV2::dump() const {
215   dumpToStream(llvm::errs());
216 }
217 
dumpToStream(raw_ostream & os) const218 void RegionRawOffsetV2::dumpToStream(raw_ostream &os) const {
219   os << "raw_offset_v2{" << getRegion() << ',' << getByteOffset() << '}';
220 }
221 
222 
223 // Lazily computes a value to be used by 'computeOffset'.  If 'val'
224 // is unknown or undefined, we lazily substitute '0'.  Otherwise,
225 // return 'val'.
getValue(SVal val,SValBuilder & svalBuilder)226 static inline SVal getValue(SVal val, SValBuilder &svalBuilder) {
227   return val.getAs<UndefinedVal>() ? svalBuilder.makeArrayIndex(0) : val;
228 }
229 
230 // Scale a base value by a scaling factor, and return the scaled
231 // value as an SVal.  Used by 'computeOffset'.
scaleValue(ProgramStateRef state,NonLoc baseVal,CharUnits scaling,SValBuilder & sb)232 static inline SVal scaleValue(ProgramStateRef state,
233                               NonLoc baseVal, CharUnits scaling,
234                               SValBuilder &sb) {
235   return sb.evalBinOpNN(state, BO_Mul, baseVal,
236                         sb.makeArrayIndex(scaling.getQuantity()),
237                         sb.getArrayIndexType());
238 }
239 
240 // Add an SVal to another, treating unknown and undefined values as
241 // summing to UnknownVal.  Used by 'computeOffset'.
addValue(ProgramStateRef state,SVal x,SVal y,SValBuilder & svalBuilder)242 static SVal addValue(ProgramStateRef state, SVal x, SVal y,
243                      SValBuilder &svalBuilder) {
244   // We treat UnknownVals and UndefinedVals the same here because we
245   // only care about computing offsets.
246   if (x.isUnknownOrUndef() || y.isUnknownOrUndef())
247     return UnknownVal();
248 
249   return svalBuilder.evalBinOpNN(state, BO_Add, x.castAs<NonLoc>(),
250                                  y.castAs<NonLoc>(),
251                                  svalBuilder.getArrayIndexType());
252 }
253 
254 /// Compute a raw byte offset from a base region.  Used for array bounds
255 /// checking.
computeOffset(ProgramStateRef state,SValBuilder & svalBuilder,SVal location)256 RegionRawOffsetV2 RegionRawOffsetV2::computeOffset(ProgramStateRef state,
257                                                    SValBuilder &svalBuilder,
258                                                    SVal location)
259 {
260   const MemRegion *region = location.getAsRegion();
261   SVal offset = UndefinedVal();
262 
263   while (region) {
264     switch (region->getKind()) {
265       default: {
266         if (const SubRegion *subReg = dyn_cast<SubRegion>(region)) {
267           offset = getValue(offset, svalBuilder);
268           if (!offset.isUnknownOrUndef())
269             return RegionRawOffsetV2(subReg, offset);
270         }
271         return RegionRawOffsetV2();
272       }
273       case MemRegion::ElementRegionKind: {
274         const ElementRegion *elemReg = cast<ElementRegion>(region);
275         SVal index = elemReg->getIndex();
276         if (!index.getAs<NonLoc>())
277           return RegionRawOffsetV2();
278         QualType elemType = elemReg->getElementType();
279         // If the element is an incomplete type, go no further.
280         ASTContext &astContext = svalBuilder.getContext();
281         if (elemType->isIncompleteType())
282           return RegionRawOffsetV2();
283 
284         // Update the offset.
285         offset = addValue(state,
286                           getValue(offset, svalBuilder),
287                           scaleValue(state,
288                           index.castAs<NonLoc>(),
289                           astContext.getTypeSizeInChars(elemType),
290                           svalBuilder),
291                           svalBuilder);
292 
293         if (offset.isUnknownOrUndef())
294           return RegionRawOffsetV2();
295 
296         region = elemReg->getSuperRegion();
297         continue;
298       }
299     }
300   }
301   return RegionRawOffsetV2();
302 }
303 
registerArrayBoundCheckerV2(CheckerManager & mgr)304 void ento::registerArrayBoundCheckerV2(CheckerManager &mgr) {
305   mgr.registerChecker<ArrayBoundCheckerV2>();
306 }
307