fix(detector): detect try/except replay

kernelguard.py

@@ -2459,6 +2459,88 @@ def _has_ver(expr: ast.AST | None) -> bool:
                 ),
             }]
 
+        # Fourth pass: try-fast-path replay where the exception path fills
+        # the captured state.
+        #
+        # Pattern: try: return _state[0]
+        #          except IndexError: _state.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)
+
+        global_names = {
+            name
+            for stmt in node.body
+            if isinstance(stmt, ast.Global)
+            for name in stmt.names
+        }
+        local_names = {
+            target.id
+            for stmt in node.body
+            if isinstance(stmt, (ast.Assign, ast.AnnAssign, ast.AugAssign))
+            for target in (
+                stmt.targets if isinstance(stmt, ast.Assign)
+                else [stmt.target]
+            )
+            if isinstance(target, ast.Name) and target.id not in global_names
+        }
+        captured = _expr_names(node) - params - local_names
+
+        mutating_methods = {
+            "add", "append", "clear", "extend", "insert", "pop", "popitem",
+            "remove", "rotate", "setdefault", "update",
+        }
+        for child in ast.walk(node):
+            if not isinstance(child, ast.Try):
+                continue
+            if _body_has_calls(child.body):
+                continue
+            returned = {
+                _ast_root_name(stmt.value)
+                for stmt in child.body
+                if isinstance(stmt, ast.Return)
+                and isinstance(stmt.value, ast.Subscript)
+            } - {None}
+            replay_roots = returned & captured
+            if not replay_roots:
+                continue
+
+            mutated = set()
+            for handler in child.handlers:
+                for stmt in ast.walk(handler):
+                    if isinstance(stmt, ast.Assign):
+                        for target in stmt.targets:
+                            target_root = _ast_root_name(target)
+                            if target_root in replay_roots:
+                                mutated.add(target_root)
+                    elif isinstance(stmt, ast.AugAssign):
+                        target_root = _ast_root_name(stmt.target)
+                        if target_root in replay_roots:
+                            mutated.add(target_root)
+                    elif (isinstance(stmt, ast.Call)
+                          and isinstance(stmt.func, ast.Attribute)
+                          and stmt.func.attr in mutating_methods):
+                        target_root = _ast_root_name(stmt.func.value)
+                        if target_root in replay_roots:
+                            mutated.add(target_root)
+            if not mutated:
+                continue
+
+            return [{
+                "pattern": "LAST_CALL_REPLAY",
+                "severity": "critical",
+                "evidence": (
+                    f"{entrypoint_name} tries to return captured state and "
+                    "fills it in the exception path"
+                ),
+            }]
+
     return []