Re: [PATCH v2 4/9] x86: Makefile: Add build and config option for CONFIG_FG_KASLR

[Kees Cook]

On Thu, May 21, 2020 at 09:56:35AM -0700, Kristen Carlson Accardi wrote:
> Allow user to select CONFIG_FG_KASLR if dependencies are met. Change
> the make file to build with -ffunction-sections if CONFIG_FG_KASLR
> 
> Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
> Tested-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---
>  Makefile         |  4 ++++
>  arch/x86/Kconfig | 13 +++++++++++++
>  2 files changed, 17 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index 04f5662ae61a..28e515baa824 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -862,6 +862,10 @@ ifdef CONFIG_LIVEPATCH
>  KBUILD_CFLAGS += $(call cc-option, -flive-patching=inline-clone)
>  endif
>  
> +ifdef CONFIG_FG_KASLR
> +KBUILD_CFLAGS += -ffunction-sections
> +endif
> +
>  # arch Makefile may override CC so keep this after arch Makefile is included
>  NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
>  
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 2d3f963fd6f1..50e83ea57d70 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2183,6 +2183,19 @@ config RANDOMIZE_BASE
>  
>  	  If unsure, say Y.
>  
> +config FG_KASLR
> +	bool "Function Granular Kernel Address Space Layout Randomization"
> +	depends on $(cc-option, -ffunction-sections)
> +	depends on RANDOMIZE_BASE && X86_64
> +	help
> +	  This option improves the randomness of the kernel text
> +	  over basic Kernel Address Space Layout Randomization (KASLR)
> +	  by reordering the kernel text at boot time. This feature
> +	  uses information generated at compile time to re-layout the
> +	  kernel text section at boot time at function level granularity.
> +
> +	  If unsure, say N.
> +
>  # Relocation on x86 needs some additional build support
>  config X86_NEED_RELOCS
>  	def_bool y

Kconfig bikeshedding: how about putting FG_KASLR in arch/Kconfig, add
a "depends on ARCH_HAS_FG_KASLR", and remove the arch-specific depends.

Then in arch/x86 have ARCH_HAS_FG_KASLR as a def_bool y with the
RANDOMIZE_BASE && X86_64 depends.

This will more cleanly split the build elements (compiler flags) from
the arch elements (64-bit x86, arch-specific flags, etc).

With that split out:

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook