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

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Message-ID: <20140925144025.GA14030@khazad-dum.debian.net>
Date:	Thu, 25 Sep 2014 11:40:25 -0300
From:	Henrique de Moraes Holschuh <hmh@....eng.br>
To:	Borislav Petkov <bp@...en8.de>
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 Thu, 25 Sep 2014, Borislav Petkov wrote:
> On Thu, Sep 25, 2014 at 08:36:45AM -0300, Henrique de Moraes Holschuh wrote:
> > This isn't an useless fix, it will allow systems without early initramfs
> > support to operate correctly after a microcode update.
> 
> So what do we do if we update the microcode late and some userspace task
> is using HLE and all of a sudden it segfaults and gets killed due to
> #UD. I'll forward all those complaint emails to you then, no?
> 
> :-)
> 
> What's saying is, a reboot in this case is maybe the lesser of two evils.

In that case we should blacklist to refuse to apply the update, and reboot
only if the blacklist wasn't good enough and we detect that something really
important in the cpu feature cpuid bits changed.

However, a reboot is even worse than everything linked to libpthread
segfaulting, as it will also cause data loss for the stuff that didn't get
SIGILL'd to death.  Meh.

Backporting early initramfs support to 3.0/3.2/3.4 doesn't seem doable, or
wise.

At this point, what alternatives are left?

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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