linux-kernel - Re: [PATCH] x86/cpuid: Fix up "virtual" IBRS/IBPB/STIBP feature bits on Intel

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Date:   Tue, 30 Jan 2018 06:01:43 -0800
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     David Woodhouse <dwmw2@...radead.org>, karahmed@...zon.de,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org
Subject: Re: [PATCH] x86/cpuid: Fix up "virtual" IBRS/IBPB/STIBP feature bits
 on Intel

On 1/30/2018 5:11 AM, Borislav Petkov wrote:
> On Tue, Jan 30, 2018 at 01:57:21PM +0100, Thomas Gleixner wrote:
>> So much for the theory. That's not going to work. If the boot cpu has the
>> feature then the alternatives will have been applied. So even if the flag
>> mismatch can be observed when a secondary CPU comes up the outcome will be
>> access to a non existing MSR and #GP.
> 
> Yes, with mismatched microcode we're f*cked.

I think in the super early days of SMP there was an occasional broken BIOS.
(and when Linux then did the ucode update it was sane again)

Not since a long time though (I think the various certification suites check for it now)

> 
> So my question is: is there such microcode out there or is this
> something theoretical which we want to address?

at this point it's insane theoretical; no OS can actually cope with this, so
if you're an OEM selling this, your customer can run zero OSes ;-)


> 
> (.. and adressing this will be ugly, no matter what.)
> 
> And if I were able to wish, I'd like to blacklist that microcode in
> dracut so that it doesn't come anywhere near my system.

I'm not sure what you'd want dracut to do... panic() the system
on such a bios?