Re: Dealing with the NMI mess

[Willy Tarreau <w@1wt.eu>]

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@amacapital.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