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

From: Kees Cook <keescook@chromium.org>
To: Mark Brown <broonie@kernel.org>
Cc: "Catalin Marinas" <catalin.marinas@arm.com>,
	"Will Deacon" <will@kernel.org>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Paul Elliott" <paul.elliott@arm.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Yu-cheng Yu" <yu-cheng.yu@intel.com>,
	"Amit Kachhap" <amit.kachhap@arm.com>,
	"Vincenzo Frascino" <vincenzo.frascino@arm.com>,
	"Marc Zyngier" <maz@kernel.org>,
	"Eugene Syromiatnikov" <esyr@redhat.com>,
	"Szabolcs Nagy" <szabolcs.nagy@arm.com>,
	"H . J . Lu " <hjl.tools@gmail.com>,
	"Andrew Jones" <drjones@redhat.com>,
	"Arnd Bergmann" <arnd@arndb.de>, "Jann Horn" <jannh@google.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Kristina Martšenko" <kristina.martsenko@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Florian Weimer" <fweimer@redhat.com>,
	"Sudakshina Das" <sudi.das@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	"Dave Martin" <Dave.Martin@arm.com>
Subject: Re: [PATCH v9 13/13] arm64: BTI: Add Kconfig entry for userspace BTI
Date: Thu, 12 Mar 2020 17:01:47 -0700	[thread overview]
Message-ID: <202003121700.9260E027@keescook> (raw)
In-Reply-To: <20200311192608.40095-14-broonie@kernel.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