linux-kernel - Re: [PATCH v2 1/6] x86/cpu: Allow feature bit names from /proc/cpuinfo in clearcpuid=

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Date:   Mon, 7 Feb 2022 14:04:04 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/6] x86/cpu: Allow feature bit names from
 /proc/cpuinfo in clearcpuid=

On Thu, Jan 27, 2022 at 12:56:21PM +0100, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
> 
> Having to give the X86_FEATURE array indices in order to disable a
> feature bit for testing is not really user-friendly. So accept the
> feature bit names too.
> 
> Some feature bits don't have names so there the array indices are still
> accepted, of course.
> 
> Clearing CPUID flags is not something which should be done in production
> so taint the kernel too.
> 
> An exemplary cmdline would then be something like:
> 
>   clearcpuid=de,440,smca,succory,bmi1,3dnow
> 
> ("succory" is wrong on purpose). And it says:
> 
> [    0.000000] Clearing CPUID bits: de 13:24 smca bmi1 3dnow
> 
> Signed-off-by: Borislav Petkov <bp@...e.de>

I like the taint flag addition!

Even though it reports what it does actually clear, do you think it
might be more "friendly" to yell about unknown stuff too? i.e.:

[    0.000000] Clearing CPUID bits: unknown bit 'succory'
[    0.000000] Clearing CPUID bits: de 13:24 smca bmi1 3dnow

Either way:

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

-- 
Kees Cook