fix(detector): detect function defaults replay

kernelguard.py

@@ -2459,6 +2459,96 @@ def _has_ver(expr: ast.AST | None) -> bool:
                 ),
             }]
 
+        # Fourth pass: replay through local storage loaded from the
+        # entrypoint's mutable __defaults__ tuple.
+        #
+        # Pattern: store = custom_kernel.__defaults__[0]
+        #          if store: return store[0]
+        #          store.append(data.clone())
+        params = {
+            arg.arg
+            for args in (node.args.posonlyargs, node.args.args, node.args.kwonlyargs)
+            for arg in args
+        }
+        if node.args.vararg is not None:
+            params.add(node.args.vararg.arg)
+        if node.args.kwarg is not None:
+            params.add(node.args.kwarg.arg)
+
+        defaults_mutated = False
+        for stmt in getattr(tree, "body", []):
+            if not isinstance(stmt, ast.Assign):
+                continue
+            if not any(
+                isinstance(target, ast.Attribute)
+                and target.attr == "__defaults__"
+                and isinstance(target.value, ast.Name)
+                and is_entrypoint_name(target.value.id)
+                for target in stmt.targets
+            ):
+                continue
+            if any(isinstance(part, (ast.List, ast.Dict, ast.Set)) for part in ast.walk(stmt.value)):
+                defaults_mutated = True
+                break
+
+        default_aliases: set[str] = set()
+        mutated_aliases: set[str] = set()
+        mutating_methods = {
+            "add", "append", "clear", "extend", "insert", "pop", "popitem",
+            "remove", "rotate", "setdefault", "update",
+        }
+        if defaults_mutated:
+            for child in ast.walk(node):
+                if isinstance(child, ast.Assign):
+                    if (len(child.targets) == 1
+                            and isinstance(child.targets[0], ast.Name)
+                            and isinstance(child.value, ast.Subscript)
+                            and isinstance(child.value.value, ast.Attribute)
+                            and child.value.value.attr == "__defaults__"
+                            and isinstance(child.value.value.value, ast.Name)
+                            and is_entrypoint_name(child.value.value.value.id)):
+                        default_aliases.add(child.targets[0].id)
+                    for target in child.targets:
+                        if _ast_root_name(target) in default_aliases:
+                            mutated_aliases.add(_ast_root_name(target))
+                elif isinstance(child, ast.AugAssign):
+                    target_root = _ast_root_name(child.target)
+                    if target_root in default_aliases:
+                        mutated_aliases.add(target_root)
+                elif (isinstance(child, ast.Call)
+                      and isinstance(child.func, ast.Attribute)
+                      and child.func.attr in mutating_methods
+                      and _ast_root_name(child.func.value) in default_aliases):
+                    mutated_aliases.add(_ast_root_name(child.func.value))
+
+        if mutated_aliases:
+            for child in ast.walk(node):
+                if not isinstance(child, ast.If):
+                    continue
+                if _body_has_calls(child.body):
+                    continue
+                if _expr_names(child.test) & params:
+                    continue
+                if not (_expr_names(child.test) & mutated_aliases):
+                    continue
+                if not any(
+                    isinstance(stmt, ast.Return)
+                    and stmt.value is not None
+                    and isinstance(stmt.value, ast.Subscript)
+                    and _ast_root_name(stmt.value) in mutated_aliases
+                    for stmt in child.body
+                ):
+                    continue
+
+                return [{
+                    "pattern": "LAST_CALL_REPLAY",
+                    "severity": "critical",
+                    "evidence": (
+                        f"{entrypoint_name} replays output through mutable "
+                        "__defaults__ storage"
+                    ),
+                }]
+
     return []