vfs: Check that open() resulted in a file-like object.

That is, an object whose type defines the protocol slot.

Note that due to protocol confusion, a variant of the original
crasher that returned e.g., a machine.Pin instance could still
lead to a crash. (#17852)

Closes: #17841

Signed-off-by: Jeff Epler <jepler@gmail.com>

extmod/vfs.c

@@ -338,7 +338,12 @@ mp_obj_t mp_vfs_open(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args)
     #endif
 
     mp_vfs_mount_t *vfs = lookup_path(args[ARG_file].u_obj, &args[ARG_file].u_obj);
-    return mp_vfs_proxy_call(vfs, MP_QSTR_open, 2, (mp_obj_t *)&args);
+    mp_obj_t result = mp_vfs_proxy_call(vfs, MP_QSTR_open, 2, (mp_obj_t *)&args);
+    const mp_obj_type_t *type = mp_obj_get_type(vfs->obj);
+    if (!MP_OBJ_TYPE_HAS_SLOT(type, protocol)) {
+        mp_raise_TypeError(NULL);
+    }
+    return result;
 }
 MP_DEFINE_CONST_FUN_OBJ_KW(mp_vfs_open_obj, 0, mp_vfs_open);
 

tests/extmod/vfs_open_error.py

@@ -0,0 +1,19 @@
+import os
+import vfs
+
+
+class Filesystem:
+    def mount(self, readonly, mkfs):
+        pass
+
+    def open(self, file, mode):
+        return None  # violates vfs contract
+
+
+fs = Filesystem()
+vfs.mount(fs, "/test_mnt")
+try:
+    execfile("/test_mnt/test.py")
+    print("ExecFile succeeded")
+except TypeError:
+    print("TypeError")

tests/extmod/vfs_open_error.py.exp

@@ -0,0 +1 @@
+TypeError