Re: [PATCH v9 13/13] arm64: BTI: Add Kconfig entry for userspace BTI

[Kees Cook <keescook@chromium.org>]

On Wed, Mar 11, 2020 at 07:26:08PM +0000, Mark Brown wrote:
> From: Dave Martin <Dave.Martin@arm.com>
> 
> Now that the code for userspace BTI support is in the kernel add the
> Kconfig entry so that it can be built and used.
> 
> [Split out of "arm64: Basic Branch Target Identification support" --
> broonie]
> 
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  arch/arm64/Kconfig | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 8a15bc68dadd..d65d226a77ec 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1522,6 +1522,28 @@ endmenu
>  
>  menu "ARMv8.5 architectural features"
>  
> +config ARM64_BTI
> +	bool "Branch Target Identification support"
> +	default y
> +	help
> +	  Branch Target Identification (part of the ARMv8.5 Extensions)
> +	  provides a mechanism to limit the set of locations to which computed
> +	  branch instructions such as BR or BLR can jump.
> +
> +	  To make use of BTI on CPUs that support it, say Y.
> +
> +	  BTI is intended to provide complementary protection to other control
> +	  flow integrity protection mechanisms, such as the Pointer
> +	  authentication mechanism provided as part of the ARMv8.3 Extensions.
> +	  For this reason, it does not make sense to enable this option without
> +	  also enabling support for pointer authentication.  Thus, when
> +	  enabling this option you should also select ARM64_PTR_AUTH=y.
> +
> +	  Userspace binaries must also be specifically compiled to make use of
> +	  this mechanism.  If you say N here or the hardware does not support
> +	  BTI, such binaries can still run, but you get no additional
> +	  enforcement of branch destinations.
> +
>  config ARM64_E0PD
>  	bool "Enable support for E0PD"
>  	default y
> -- 
> 2.20.1
> 

-- 
Kees Cook