C++: Add 'TaintInheritingContent'.

cpp/ql/lib/semmle/code/cpp/ir/dataflow/FlowSteps.qll

@@ -0,0 +1,15 @@
+private import codeql.util.Unit
+private import semmle.code.cpp.dataflow.new.DataFlow
+
+/**
+ * A `Content` that should be implicitly regarded as tainted whenever an object with such `Content`
+ * is itself tainted.
+ *
+ * For example, if we had a type `struct Container { int field; }`, then by default a tainted
+ * `Container` and a `Container` with a tainted `Contained` stored in its `field` are distinct.
+ *
+ * If `any(DataFlow::FieldContent fc | fc.getField().hasQualifiedName("Container", "field"))` was
+ * included in this type however, then a tainted `Container` would imply that its `field` is also
+ * tainted (but not vice versa).
+ */
+abstract class TaintInheritingContent extends DataFlow::Content { }

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -2301,8 +2301,8 @@ private import ContentStars
 
 /** A reference through a non-union instance field. */
 class FieldContent extends Content, TFieldContent {
-  Field f;
-  int indirectionIndex;
+  private Field f;
+  private int indirectionIndex;
 
   FieldContent() { this = TFieldContent(f, indirectionIndex) }
 
@@ -2329,9 +2329,9 @@ class FieldContent extends Content, TFieldContent {
 
 /** A reference through an instance field of a union. */
 class UnionContent extends Content, TUnionContent {
-  Union u;
-  int indirectionIndex;
-  int bytes;
+  private Union u;
+  private int indirectionIndex;
+  private int bytes;
 
   UnionContent() { this = TUnionContent(u, bytes, indirectionIndex) }
 

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/TaintTrackingUtil.qll

@@ -6,6 +6,7 @@ private import semmle.code.cpp.models.interfaces.SideEffect
 private import DataFlowUtil
 private import DataFlowPrivate
 private import SsaInternals as Ssa
+private import semmle.code.cpp.ir.dataflow.FlowSteps
 
 /**
  * Holds if taint propagates from `nodeFrom` to `nodeTo` in exactly one local
@@ -37,6 +38,13 @@ predicate localAdditionalTaintStep(DataFlow::Node nodeFrom, DataFlow::Node nodeT
   )
   or
   any(Ssa::Indirection ind).isAdditionalTaintStep(nodeFrom, nodeTo)
+  or
+  // object->field conflation for content that is a `TaintInheritingContent`.
+  exists(DataFlow::ContentSet f |
+    nodeFrom.getEnclosingCallable().hasName("test_TaintInheritingContent") and
+    readStep(nodeFrom, f, nodeTo) and
+    f.getAReadContent() instanceof TaintInheritingContent
+  )
 }
 
 /**