On recent versions of Ubuntu, ordinary users do not by default have access to either System.map or any decompressed kernel images (I did however notice that on my RHEL system they have read access to System.map), and ptmx_fops does not appear in kallsyms. Is ensuring correct permissions on System.map and kernel images enough to render the exploit harmless?

Edit: On second thought, given that the kernel isn't custom, using files from another system with the same kernel is a trivial workaround.