[PATCH] arm64: fix the doc of RANDOMIZE_MODULE_REGION_FULL

[Will Deacon]

On Fri, Jul 30, 2021 at 10:50:56PM +1200, Barry Song wrote:
> Obviously kaslr is setting the module region to 2GB rather than 4GB since
> commit b2eed9b588112 ("arm64/kernel: kaslr: reduce module randomization
> range to 2 GB").
> On the other hand, module_alloc() can fall back to a 2GB window even though
> RANDOMIZE_MODULE_REGION_FULL is not set. In this case, PLT is still needed.
> So fix the doc to be more precise to avoid any confusion to the readers of
> the code.
> 
> Cc: Masami Hiramatsu <mhiramat at kernel.org>
> Cc: Ard Biesheuvel <ard.biesheuvel at arm.com>
> Cc: Qi Liu <liuqi115 at huawei.com>
> Signed-off-by: Barry Song <song.bao.hua at hisilicon.com>
> ---
>  arch/arm64/Kconfig        | 7 ++++---
>  arch/arm64/kernel/kaslr.c | 4 +++-
>  2 files changed, 7 insertions(+), 4 deletions(-)

Ack for the general idea, but I think this still isn't quite right...

> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b5b13a932561..525c1f0b2430 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1800,11 +1800,11 @@ config RANDOMIZE_BASE
>  	  If unsure, say N.
>  
>  config RANDOMIZE_MODULE_REGION_FULL
> -	bool "Randomize the module region over a 4 GB range"
> +	bool "Randomize the module region over a 2 GB range"
>  	depends on RANDOMIZE_BASE
>  	default y
>  	help
> -	  Randomizes the location of the module region inside a 4 GB window
> +	  Randomizes the location of the module region inside a 2 GB window
>  	  covering the core kernel. This way, it is less likely for modules
>  	  to leak information about the location of core kernel data structures
>  	  but it does imply that function calls between modules and the core
> @@ -1812,7 +1812,8 @@ config RANDOMIZE_MODULE_REGION_FULL
>  
>  	  When this option is not set, the module region will be randomized over
>  	  a limited range that contains the [_stext, _etext] interval of the
> -	  core kernel, so branch relocations are always in range.
> +	  core kernel, so branch relocations are almost always in range unless
> +	  the region is exhausted and we fall back to a larger 2GB area.

Hmm, that's only true if ARM64_MODULE_PLTS=y.

Will