[analyzer] Fix core.VLASize checker false positive taint reports (#68140)

The checker reported a false positive on this code 

void testTaintedSanitizedVLASize(void) {
  int x;
  scanf("%d", &x);
  if (x<1)
    return;
  int vla[x]; // no-warning
}

After the fix, the checker only emits tainted warning if the vla size is
coming from a tainted source and it cannot prove that it is positive.

Author: dkrupp

clang/docs/analyzer/checkers.rst

@@ -213,9 +213,8 @@ Check for undefined results of binary operators.
 
 core.VLASize (C)
 """"""""""""""""
-Check for declarations of Variable Length Arrays of undefined or zero size.
-
- Check for declarations of VLA of undefined or zero size.
+Check for declarations of Variable Length Arrays (VLA) of undefined, zero or negative
+size.
 
 .. code-block:: c
 
@@ -229,6 +228,28 @@ Check for declarations of Variable Length Arrays of undefined or zero size.
    int vla2[x]; // warn: zero size
  }
 
+
+The checker also gives warning if the `TaintPropagation` checker is switched on
+and an unbound, attacker controlled (tainted) value is used to define
+the size of the VLA.
+
+.. code-block:: c
+
+ void taintedVLA(void) {
+   int x;
+   scanf("%d", &x);
+   int vla[x]; // Declared variable-length array (VLA) has tainted (attacker controlled) size, that can be 0 or negative
+ }
+
+ void taintedVerfieidVLA(void) {
+   int x;
+   scanf("%d", &x);
+   if (x<1)
+     return;
+   int vla[x]; // no-warning. The analyzer can prove that x must be positive.
+ }
+
+
 .. _core-uninitialized-ArraySubscript:
 
 core.uninitialized.ArraySubscript (C)

clang/lib/StaticAnalyzer/Checkers/VLASizeChecker.cpp

@@ -164,12 +164,6 @@ ProgramStateRef VLASizeChecker::checkVLAIndexSize(CheckerContext &C,
   if (SizeV.isUnknown())
     return nullptr;
 
-  // Check if the size is tainted.
-  if (isTainted(State, SizeV)) {
-    reportTaintBug(SizeE, State, C, SizeV);
-    return nullptr;
-  }
-
   // Check if the size is zero.
   DefinedSVal SizeD = SizeV.castAs<DefinedSVal>();
 
@@ -192,10 +186,10 @@ ProgramStateRef VLASizeChecker::checkVLAIndexSize(CheckerContext &C,
 
   SVal LessThanZeroVal =
       SVB.evalBinOp(State, BO_LT, SizeD, Zero, SVB.getConditionType());
+  ProgramStateRef StatePos, StateNeg;
   if (std::optional<DefinedSVal> LessThanZeroDVal =
           LessThanZeroVal.getAs<DefinedSVal>()) {
     ConstraintManager &CM = C.getConstraintManager();
-    ProgramStateRef StatePos, StateNeg;
 
     std::tie(StateNeg, StatePos) = CM.assumeDual(State, *LessThanZeroDVal);
     if (StateNeg && !StatePos) {
@@ -205,6 +199,12 @@ ProgramStateRef VLASizeChecker::checkVLAIndexSize(CheckerContext &C,
     State = StatePos;
   }
 
+  // Check if the size is tainted.
+  if ((StateNeg || StateZero) && isTainted(State, SizeV)) {
+    reportTaintBug(SizeE, State, C, SizeV);
+    return nullptr;
+  }
+
   return State;
 }
 
@@ -218,7 +218,7 @@ void VLASizeChecker::reportTaintBug(const Expr *SizeE, ProgramStateRef State,
   SmallString<256> buf;
   llvm::raw_svector_ostream os(buf);
   os << "Declared variable-length array (VLA) ";
-  os << "has tainted size";
+  os << "has tainted (attacker controlled) size that can be 0 or negative";
 
   auto report = std::make_unique<PathSensitiveBugReport>(TaintBT, os.str(), N);
   report->addRange(SizeE->getSourceRange());

clang/test/Analysis/taint-diagnostic-visitor.c

@@ -46,8 +46,8 @@ void taintDiagnosticVLA(void) {
   scanf("%d", &x); // expected-note {{Value assigned to 'x'}}
                    // expected-note@-1 {{Taint originated here}}
                    // expected-note@-2 {{Taint propagated to the 2nd argument}}
-  int vla[x]; // expected-warning {{Declared variable-length array (VLA) has tainted size}}
-              // expected-note@-1 {{Declared variable-length array (VLA) has tainted size}}
+  int vla[x]; // expected-warning {{Declared variable-length array (VLA) has tainted}}
+              // expected-note@-1 {{Declared variable-length array (VLA) has tainted}}
 }
 
 

clang/test/Analysis/taint-generic.c

@@ -405,7 +405,16 @@ int testDivByZero(void) {
 void testTaintedVLASize(void) {
   int x;
   scanf("%d", &x);
-  int vla[x]; // expected-warning{{Declared variable-length array (VLA) has tainted size}}
+  int vla[x]; // expected-warning{{Declared variable-length array (VLA) has tainted (attacker controlled) size that can be 0 or negative}}
+}
+
+// Tainted-sanitized VLAs.
+void testTaintedSanitizedVLASize(void) {
+  int x;
+  scanf("%d", &x);
+  if (x<1)
+    return;
+  int vla[x]; // no-warning
 }
 
 int testTaintedAllocaMem() {