Re: [PATCH v19 02/25] x86/cet/shstk: Add Kconfig option for user-mode control-flow protection

[Kees Cook]

On Wed, Feb 03, 2021 at 02:55:24PM -0800, Yu-cheng Yu wrote:
> Shadow Stack provides protection against function return address
> corruption.  It is active when the processor supports it, the kernel has
> CONFIG_X86_CET enabled, and the application is built for the feature.
> This is only implemented for the 64-bit kernel.  When it is enabled, legacy
> non-Shadow Stack applications continue to work, but without protection.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/Kconfig           | 22 ++++++++++++++++++++++
>  arch/x86/Kconfig.assembler |  5 +++++
>  2 files changed, 27 insertions(+)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 21f851179ff0..074b3c0e6bf6 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1951,6 +1951,28 @@ config X86_SGX
>  
>  	  If unsure, say N.
>  
> +config ARCH_HAS_SHADOW_STACK
> +	def_bool n
> +
> +config X86_CET
> +	prompt "Intel Control-flow protection for user-mode"
> +	def_bool n
> +	depends on X86_64
> +	depends on AS_WRUSS
> +	select ARCH_USES_HIGH_VMA_FLAGS
> +	select ARCH_HAS_SHADOW_STACK

This seems backwards to me? Shouldn't 'config X86_64' do the 'select
ARCH_HAS_SHADOW_STACK' and 'config X86_CET' do a 'depends on
ARCH_HAS_SHADOW_STACK' instead?

> +	help
> +	  Control-flow protection is a set of hardware features which place
> +	  additional restrictions on indirect branches.  These help
> +	  mitigate ROP attacks.  Applications must be enabled to use it,
> +	  and old userspace does not get protection "for free".
> +	  Support for this feature is present on Tiger Lake family of
> +	  processors released in 2020 or later.  Enabling this feature
> +	  increases kernel text size by 3.7 KB.
> +	  See Documentation/x86/intel_cet.rst for more information.
> +
> +	  If unsure, say N.
> +
>  config EFI
>  	bool "EFI runtime service support"
>  	depends on ACPI
> diff --git a/arch/x86/Kconfig.assembler b/arch/x86/Kconfig.assembler
> index 26b8c08e2fc4..00c79dd93651 100644
> --- a/arch/x86/Kconfig.assembler
> +++ b/arch/x86/Kconfig.assembler
> @@ -19,3 +19,8 @@ config AS_TPAUSE
>  	def_bool $(as-instr,tpause %ecx)
>  	help
>  	  Supported by binutils >= 2.31.1 and LLVM integrated assembler >= V7
> +
> +config AS_WRUSS
> +	def_bool $(as-instr,wrussq %rax$(comma)(%rbx))
> +	help
> +	  Supported by binutils >= 2.31 and LLVM integrated assembler
> -- 
> 2.21.0
> 

-- 
Kees Cook