linux-kernel - Re: x86, microcode: BUG: microcode update that changes x86

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Message-ID: <20140925085158.GF22317@nazgul.tnic>
Date:	Thu, 25 Sep 2014 10:51:58 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	Chuck Ebbert <cebbert.lkml@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: x86, microcode: BUG: microcode update that changes x86_capability

On Wed, Sep 24, 2014 at 11:56:58AM -0300, Henrique de Moraes Holschuh wrote:
> I don't know about AMD, but on Intel, the time it takes to update the
> microcode on a core is anything but negligible[1], so the microcode
> version skew window still exists, and it is not small.  It is much smaller
> than it once was, but it is still there.

I think that window is unsafe but yeah, we probably should take your
empirical observation as good enough for now.

> But IMHO we still need to detect and do something smart when
> x86_capability changes due to a microcode update.
> 
> And I'd really prefer it to be "update x86_capability, warn the user and
> carry on" for anything that is not going to crash the kernel.

The problem is with hiding CPUID bits and userspace using HLE after
having detected it previously. I think we'll be on the safe side if we
reboot thus the suggestion to the user that rebooting should be done
ASAP.

-- 
Regards/Gruss,
    Boris.
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