Re: [PATCH] arm64: Always force a branch protection mode when the compiler has one

From: Kees Cook <keescook@chromium.org>
To: Mark Brown <broonie@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Szabolcs Nagy <szabolcs.nagy@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	stable@vger.kernel.org,
	Amit Daniel Kachhap <amit.kachhap@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: Always force a branch protection mode when the compiler has one
Date: Tue, 31 Mar 2020 12:46:33 -0700	[thread overview]
Message-ID: <202003311246.3BAE76AF1@keescook> (raw)
In-Reply-To: <20200331194459.54740-1-broonie@kernel.org>

On Tue, Mar 31, 2020 at 08:44:59PM +0100, Mark Brown wrote:
> Compilers with branch protection support can be configured to enable it by
> default, it is likely that distributions will do this as part of deploying
> branch protection system wide. As well as the slight overhead from having
> some extra NOPs for unused branch protection features this can cause more
> serious problems when the kernel is providing pointer authentication to
> userspace but not built for pointer authentication itself. In that case our
> switching of keys for userspace can affect the kernel unexpectedly, causing
> pointer authentication instructions in the kernel to corrupt addresses.
> 
> To ensure that we get consistent and reliable behaviour always explicitly
> initialise the branch protection mode, ensuring that the kernel is built
> the same way regardless of the compiler defaults.
> 
> Fixes: 7503197562567 (arm64: add basic pointer authentication support)
> Reported-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
> Signed-off-by: Mark Brown <broonie@kernel.org>

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

-Kees

> Cc: stable@vger.kernel.org
> ---
>  arch/arm64/Kconfig  | 4 ++++
>  arch/arm64/Makefile | 7 ++++++-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index d3efdc095a17..1e46746e8392 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1537,6 +1537,10 @@ config ARM64_PTR_AUTH
>  	  This feature works with FUNCTION_GRAPH_TRACER option only if
>  	  DYNAMIC_FTRACE_WITH_REGS is enabled.
>  
> +config CC_HAS_BRANCH_PROT_NONE
> +	# GCC 9 or later, clang 8 or later
> +	def_bool $(cc-option,-mbranch-protection=none)
> +
>  config CC_HAS_BRANCH_PROT_PAC_RET
>  	# GCC 9 or later, clang 8 or later
>  	def_bool $(cc-option,-mbranch-protection=pac-ret+leaf)
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index f15f92ba53e6..370fca6663c8 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -65,6 +65,10 @@ stack_protector_prepare: prepare0
>  					include/generated/asm-offsets.h))
>  endif
>  
> +# Ensure that if the compiler supports branch protection we default it
> +# off, this will be overridden if we are using branch protection.
> +branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_NONE) := -mbranch-protection=none
> +
>  ifeq ($(CONFIG_ARM64_PTR_AUTH),y)
>  branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all
>  branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pac-ret+leaf
> @@ -73,9 +77,10 @@ branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pa
>  # we pass it only to the assembler. This option is utilized only in case of non
>  # integrated assemblers.
>  branch-prot-flags-$(CONFIG_AS_HAS_PAC) += -Wa,-march=armv8.3-a
> -KBUILD_CFLAGS += $(branch-prot-flags-y)
>  endif
>  
> +KBUILD_CFLAGS += $(branch-prot-flags-y)
> +
>  ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
>  KBUILD_CPPFLAGS	+= -mbig-endian
>  CHECKFLAGS	+= -D__AARCH64EB__
> -- 
> 2.20.1
> 

-- 
Kees Cook

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