Subject: Re: [PATCH 5/5] arm64: bti: Provide Kconfig for kernel mode BTI

From: Kees Cook <[email-masked]>
Date: Sat, 28 Mar 2020 14:19:49 -0700

On Fri, Mar 27, 2020 at 07:21:07PM +0000, Mark Brown wrote:
> Now that all the code is in place provide a Kconfig option allowing users
> to enable BTI for the kernel if their toolchain supports it, defaulting it
> on since this has security benefits. This is a separate configuration
> option since we currently don't support secondary CPUs that lack BTI if
> the boot CPU supports it.
> 
> Current testing appears to show an issue with GCC which causes it to
> emit unsuitable landing pads for function entries in some cases, until
> this issue is understood and either our usage fixed or a suitable
> version of GCC is identified the feature is marked as incompatible with
> GCC.  No issues have been identified with clang.
> 
> Signed-off-by: Mark Brown <[email-masked]>
> ---
>  arch/arm64/Kconfig | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 00cb4db4beab..25bb0931b2ba 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1607,6 +1607,24 @@ config ARM64_BTI
>  	  BTI, such binaries can still run, but you get no additional
>  	  enforcement of branch destinations.
>  
> +config ARM64_BTI_KERNEL
> +	bool "Use Branch Target Identification for kernel"
> +	default y
> +	depends on ARM64_BTI
> +	depends on ARM64_PTR_AUTH
> +	depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI

(this depend can be dropped with my crazy Makefile suggestion)

> +	depends on !CC_IS_GCC
> +	depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS)
> +	help
> +	  Build the kernel with Branch Target Identification annotations
> +	  and enable enforcement of this for kernel code. When this option
> +	  is enabled and the system supports BTI all kernel code including
> +	  modular code must have BTI enabled.
> +
> +config CC_HAS_BRANCH_PROT_PAC_RET_BTI
> +	# GCC 9 or later, clang 8 or later
> +	def_bool $(cc-option,-mbranch-protection=pac-ret+leaf+bti)

Exciting! :)

> +
>  config ARM64_E0PD
>  	bool "Enable support for E0PD"
>  	default y
> -- 
> 2.20.1
> 

-- 
Kees Cook

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