/usr/lib/mono/2.0/System.Runtime.Serialization.dll is in libmono-wcf3.0a-cil 3.2.8+dfsg-4ubuntu1.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/2.0/../gac/System.Runtime.Serialization/3.0.0.0__b77a5c561934e089/System.Runtime.Serialization.dll