ksmbd: fix slab-use-after-free in ksmbd_smb2_session_create

There is a race condition between ksmbd_smb2_session_create and
ksmbd_expire_session. This patch add missing sessions_table_lock
while adding/deleting session from global session table.

Cc: stable@vger.kernel.org # v5.15+
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

@@ -178,6 +178,7 @@ static void ksmbd_expire_session(struct ksmbd_conn *conn)
 	unsigned long id;
 	struct ksmbd_session *sess;
 
+	down_write(&sessions_table_lock);
 	down_write(&conn->session_lock);
 	xa_for_each(&conn->sessions, id, sess) {
 		if (atomic_read(&sess->refcnt) == 0 &&
@@ -191,6 +192,7 @@ static void ksmbd_expire_session(struct ksmbd_conn *conn)
 		}
 	}
 	up_write(&conn->session_lock);
+	up_write(&sessions_table_lock);
 }
 
 int ksmbd_session_register(struct ksmbd_conn *conn,
@@ -232,7 +234,6 @@ void ksmbd_sessions_deregister(struct ksmbd_conn *conn)
 			}
 		}
 	}
-	up_write(&sessions_table_lock);
 
 	down_write(&conn->session_lock);
 	xa_for_each(&conn->sessions, id, sess) {
@@ -252,6 +253,7 @@ void ksmbd_sessions_deregister(struct ksmbd_conn *conn)
 		}
 	}
 	up_write(&conn->session_lock);
+	up_write(&sessions_table_lock);
 }
 
 struct ksmbd_session *ksmbd_session_lookup(struct ksmbd_conn *conn,