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Date:   Mon, 3 Oct 2022 10:25:28 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        joao.moreira@intel.com, John Allen <john.allen@amd.com>,
        kcc@google.com, eranian@google.com, rppt@kernel.org,
        jamorris@linux.microsoft.com, dethoma@microsoft.com,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v2 02/39] x86/cet/shstk: Add Kconfig option for Shadow
 Stack
Message-ID: <202210031020.0E93C75F9@keescook>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-3-rick.p.edgecombe@intel.com>
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On Thu, Sep 29, 2022 at 03:28:59PM -0700, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
> 
> Shadow Stack provides protection against function return address
> corruption. It is active when the processor supports it, the kernel has
> CONFIG_X86_SHADOW_STACK 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@intel.com>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
> 
> ---
> 
> v2:
>  - Remove already wrong kernel size increase info (tlgx)
>  - Change prompt to remove "Intel" (tglx)
>  - Update line about what CPUs are supported (Dave)
> 
> Yu-cheng v25:
>  - Remove X86_CET and use X86_SHADOW_STACK directly.
> 
> Yu-cheng v24:
>  - Update for the splitting X86_CET to X86_SHADOW_STACK and X86_IBT.
> 
>  arch/x86/Kconfig           | 18 ++++++++++++++++++
>  arch/x86/Kconfig.assembler |  5 +++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index f9920f1341c8..b68eb75887b8 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -26,6 +26,7 @@ config X86_64
>  	depends on 64BIT
>  	# Options that are inherently 64-bit kernel only:
>  	select ARCH_HAS_GIGANTIC_PAGE
> +	select ARCH_HAS_SHADOW_STACK
>  	select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
>  	select ARCH_USE_CMPXCHG_LOCKREF
>  	select HAVE_ARCH_SOFT_DIRTY
> @@ -1936,6 +1937,23 @@ config X86_SGX
>  
>  	  If unsure, say N.
>  
> +config ARCH_HAS_SHADOW_STACK
> +	def_bool n
> +
> +config X86_SHADOW_STACK
> +	prompt "X86 Shadow Stack"
> +	def_bool n

I hope we can switch this to "default y" soon, given it's a hardware
feature that is disabled at runtime when not available.

> +	depends on ARCH_HAS_SHADOW_STACK

Doesn't this depend on AS_WRUSS too?

> +	select ARCH_USES_HIGH_VMA_FLAGS
> +	help
> +	  Shadow Stack protection is a hardware feature that detects function
> +	  return address corruption. Today the kernel's support is limited to
> +	  virtualizing it in KVM guests.
> +
> +	  CPUs supporting shadow stacks were first released in 2020.
> +
> +	  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

Otherwise, I don't see anything else using OCNFIG_AS_WRUSS:

$ git grep AS_WRUSS
arch/x86/Kconfig.assembler:config AS_WRUSS

-Kees

-- 
Kees Cook