extmod/vfs_reader: 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).

Fixes issue #17841.

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

tests/micropython/builtin_execfile.py

@@ -75,3 +75,24 @@ except TypeError:
 
 # Unmount the VFS object.
 vfs.umount(fs)
+
+
+class EvilFilesystem:
+    def mount(self, readonly, mkfs):
+        print("mount", readonly, mkfs)
+
+    def umount(self):
+        print("umount")
+
+    def open(self, file, mode):
+        return None
+
+
+fs = EvilFilesystem()
+vfs.mount(fs, "/test_mnt")
+try:
+    execfile("/test_mnt/test.py")
+    print("ExecFile succeeded")
+except OSError:
+    print("OSError")
+vfs.umount(fs)

tests/micropython/builtin_execfile.py.exp

@@ -5,3 +5,6 @@ open /test.py rb
 123
 TypeError
 umount
+mount False False
+OSError
+umount