Subject: Re: [PATCH v8 03/11] arm64: Basic Branch Target Identification support

From: Catalin Marinas <[email-masked]>
Date: Wed, 11 Mar 2020 16:31:03 +0000

On Thu, Feb 27, 2020 at 05:44:09PM +0000, Mark Brown wrote:
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 0b30e884e088..e37f4f07b990 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1519,6 +1519,28 @@ 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.

To keep the series bisectable, I'd move the Kconfig into a separate
patch towards the end. It looks like the feature is only partially
supported after patch 3, so let's not advertise it here.

-- 
Catalin

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