linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <20150723205207.GA3052@1wt.eu>
Date:	Thu, 23 Jul 2015 22:52:07 +0200
From:	Willy Tarreau <w@....eu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...capital.net>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Thu, Jul 23, 2015 at 01:38:33PM -0700, Linus Torvalds wrote:
> On Thu, Jul 23, 2015 at 1:21 PM, Andy Lutomirski <luto@...capital.net> wrote:
> >
> > 2. Forbid IRET inside NMIs.  Doable but maybe not that pretty.
> >
> > We haven't considered:
> >
> > 3. Forbid faults (other than MCE) inside NMI.
> 
> I'd really prefer #2. #3 depends on us getting many things right, and
> never introducing new cases in the future.
> 
> #2, in contrast, seems to be fairly localized. Yes, RF is an issue,
> but returning to user space with RF clear doesn't really seem to be
> all that problematic.

What's the worst case that can happen with RF cleared when returing
to user space ? My understanding is that it's just that we risk to
break again on an instruction that had a break point set and which
already triggered the breakpoint, right ?

If so the problem probably is whether there's a risk of looping again
without ever getting a chance to execute this instruction normally.
But if the NMIs don't bomb as fast as we can process them, at some
point the instruction should get a chance to be executed, so the
problem doesn't seem dramatic.

That makes me think that I have no idea what happens if we try to
step-trace "int 2", I don't even know if we pass through the NMI
handler.

Willy

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