linux-kernel - Re: [PATCH v15 05/26] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack

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Message-ID: <20201127171012.GD13163@zn.tnic>
Date:   Fri, 27 Nov 2020 18:10:12 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v15 05/26] x86/cet/shstk: Add Kconfig option for
 user-mode Shadow Stack

On Tue, Nov 10, 2020 at 08:21:50AM -0800, Yu-cheng Yu wrote:
> +config X86_CET
> +	def_bool n
> +
> +config ARCH_HAS_SHADOW_STACK
> +	def_bool n
> +
> +config X86_SHADOW_STACK_USER

Is X86_SHADOW_STACK_KERNEL coming too?

Regardless, you can add it when it comes and you can use only X86_CET
for now and drop this one and simplify this pile of Kconfig symbols.

> +	prompt "Intel Shadow Stacks for user-mode"
> +	def_bool n
> +	depends on CPU_SUP_INTEL && X86_64
> +	depends on AS_HAS_SHADOW_STACK
> +	select ARCH_USES_HIGH_VMA_FLAGS
> +	select X86_CET
> +	select ARCH_HAS_SHADOW_STACK
> +	help
> +	  Shadow Stacks provides protection against program stack
> +	  corruption.  It's a hardware feature.  This only matters
> +	  if you have the right hardware.  It's a security hardening
> +	  feature and apps must be enabled to use it.  You get no
> +	  protection "for free" on old userspace.  The hardware can
> +	  support user and kernel, but this option is for user space
> +	  only.
> +	  Support for this feature is only known to be present on
> +	  processors released in 2020 or later.  CET features are also
> +	  known to increase kernel text size by 3.7 KB.

This help text needs some rewriting. You can find an inspiration about
more adequate style in that same Kconfig file.

> +
> +	  If unsure, say N.
> +
>  config EFI
>  	bool "EFI runtime service support"
>  	depends on ACPI
> diff --git a/scripts/as-x86_64-has-shadow-stack.sh b/scripts/as-x86_64-has-shadow-stack.sh
> new file mode 100755
> index 000000000000..fac1d363a1b8
> --- /dev/null
> +++ b/scripts/as-x86_64-has-shadow-stack.sh
> @@ -0,0 +1,4 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0
> +
> +echo "wrussq %rax, (%rbx)" | $* -x assembler -c -

						      2> /dev/null

otherwise you get

{standard input}: Assembler messages:
{standard input}:1: Error: no such instruction: `wrussq %rax,(%rbx)

on non-enlightened toolchains during build.

Thx.

-- 
Regards/Gruss,
    Boris.

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