linux-kernel - Re: STI architectural question (and lretq -- I'm not even kidding)

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Message-ID: <CALCETrVzKJZiEmxuVNv8_7dybh-4AQYRA5Yb5ms+vK9CKYEorA@mail.gmail.com>
Date:	Wed, 23 Jul 2014 08:12:32 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>
Subject: Re: STI architectural question (and lretq -- I'm not even kidding)

On Jul 23, 2014 3:49 AM, "Borislav Petkov" <bp@...en8.de> wrote:
>
> On Tue, Jul 22, 2014 at 06:33:02PM -0700, Andy Lutomirski wrote:
> > Of course, this does nothing at all to protect us from #MC after sti
> > on return from #MC to userspace, but I think we're screwed regardless
> > -- we could just as easily get a second #MC before the sti. Machine
> > check broadcast was the worst idea ever.
>
> Please do not think that a raised #MC means the machine is gone. There
> are MC errors which are reported with the exception mechanism and from
> which we can and do recover, regardless of broadcasting or not.
>

How are we supposed to survive two machine checks in rapid succession?
 The second will fire as soon as the first one is acked, I imagine.
Unless we switch stacks before acking the MCE, the return address of
the first one will be lost.

In any event, I'll do a manual fixup for this in my patch.


--Andy
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