linux: Randomize layout of sensitive kernel structures

To quote from the kernel documentation:

> If you say Y here, the layouts of structures that are entirely
> function pointers (and have not been manually annotated with
> __no_randomize_layout), or structures that have been explicitly
> marked with __randomize_layout, will be randomized at compile-time.
> This can introduce the requirement of an additional information
> exposure vulnerability for exploits targeting these structure
> types.
>
> Enabling this feature will introduce some performance impact,
> slightly increase memory usage, and prevent the use of forensic
> tools like Volatility against the system (unless the kernel
> source tree isn't cleaned after kernel installation).
>
> The seed used for compilation is located at
> scripts/gcc-plgins/randomize_layout_seed.h. It remains after
> a make clean to allow for external modules to be compiled with
> the existing seed and will be removed by a make mrproper or
> make distclean.
>
> Note that the implementation requires gcc 4.7 or newer.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Acked-by: Michael Tremer <michael.tremer@ipfire.org>

config/kernel/kernel.config.x86_64-ipfire

@@ -793,7 +793,8 @@ CONFIG_HAVE_GCC_PLUGINS=y
 CONFIG_GCC_PLUGINS=y
 # CONFIG_GCC_PLUGIN_CYC_COMPLEXITY is not set
 CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y
-# CONFIG_GCC_PLUGIN_RANDSTRUCT is not set
+CONFIG_GCC_PLUGIN_RANDSTRUCT=y
+CONFIG_GCC_PLUGIN_RANDSTRUCT_PERFORMANCE=y
 # end of General architecture-dependent options
 
 CONFIG_RT_MUTEXES=y