Enable return-thunks

configname: CONFIG_RETHUNK

Linux Kernel Configuration
└─>Mitigations for speculative execution vulnerabilities
└─>Enable return-thunks
In linux kernel since version 5.2 (release Date: 2019-07-07)  
Compile the kernel with the return-thunks compiler option to guard
against kernel-to-user data leaks by avoiding return speculation.
Requires a compiler with -mfunction-return=thunk-extern
support for full protection. The kernel may run slower.