linux-kernel - Re: [PATCH] x86: Let AS_WRUSS depend on X86

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Message-ID: <2b345b1c-73ce-4719-812f-ce638f665f77@intel.com>
Date:   Tue, 31 Oct 2023 06:53:09 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Jiri Slaby (SUSE)" <jirislaby@...nel.org>, bp@...en8.de
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Mike Rapoport <rppt@...nel.org>,
        Pengfei Xu <pengfei.xu@...el.com>,
        John Allen <john.allen@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH] x86: Let AS_WRUSS depend on X86_64

On 10/31/23 03:21, Jiri Slaby (SUSE) wrote:
...
> Provided the wruss instruction is 64-bit only (and used in pure 64-bit
> X86_USER_SHADOW_STACK), it has little sense to have AS_WRUSS=y set on
> 32-bit.
> 
> Therefore, make the whole test dependent on X86_64 to ensure it's set
> only on 64-bit.
...
>  config AS_WRUSS
>  	def_bool $(as-instr,wrussq %rax$(comma)(%rbx))
> +	depends on X86_64
>  	help
>  	  Supported by binutils >= 2.31 and LLVM integrated assembler

What's the downside to just leaving this alone?

This patch just seems wrong logically.  Suppose some deranged person
wanted 32-bit shadow stack support.  They'd have to go hunt this down
via trial and error instead of just enabling X86_USER_SHADOW_STACK.

Granted, that would take one crazy person five minutes to figure out why
their .config is broken, but it still seems wrong.  It's especially
wrong without a comment because it logically reads something along the
lines of "WRUSS is only available on x86_64 configs".

A better way to do this would be:

config HAS_SHADOW_STACKS
	depends on X86_64

config AS_WRUSS
	...
	# Avoid setting AS_WRUSS on configs that don't need it:
	depends on HAS_SHADOW_STACKS

config X86_USER_SHADOW_STACK
        bool "X86 userspace shadow stack"
        depends on AS_WRUSS
        depends on HAS_SHADOW_STACKS

But that honestly doesn't seem worth it because (circling back to the
first thing I wrote...) I don't really know what the benefit is to doing
this in the first place.