ksmbd: fix use-after-free in ksmbd_sessions_deregister()

In multichannel mode, UAF issue can occur in session_deregister
when the second channel sets up a session through the connection of
the first channel. session that is freed through the global session
table can be accessed again through ->sessions of connection.

Cc: stable@vger.kernel.org
Reported-by: Norbert Szetei <norbert@doyensec.com>
Tested-by: Norbert Szetei <norbert@doyensec.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: namjaejeon

fs/smb/server/mgmt/user_session.c

@@ -230,6 +230,9 @@ void ksmbd_sessions_deregister(struct ksmbd_conn *conn)
 			if (!ksmbd_chann_del(conn, sess) &&
 			    xa_empty(&sess->ksmbd_chann_list)) {
 				hash_del(&sess->hlist);
+				down_write(&conn->session_lock);
+				xa_erase(&conn->sessions, sess->id);
+				up_write(&conn->session_lock);
 				ksmbd_session_destroy(sess);
 			}
 		}