Re: 64bit x86: NMI nesting still buggy?

[Vojtech Pavlik]

On Wed, May 21, 2014 at 04:20:55PM +0200, Borislav Petkov wrote:
> On Wed, May 21, 2014 at 03:42:24PM +0200, Jiri Kosina wrote:
> > Alright, Andy's iret optimization efforts do immediately bring a
> > followup question -- why is this not a problem with iret-based return
> > from #MC possibly interrupting NMI?
> 
> Yeah, and frankly, I don't see this nesting fun at all protected against
> a #MC interrupting it at any point actually. Because once the #MC
> handler returns, it goes into paranoid_exit and that place doesn't
> account for NMIs at all, AFAICS.
> 
> Which would mean:
> 
> * NMI goes off
> * MCE happens, we switch to machine_check which is paranoidzeroentry
> * #MC handler is done -> paranoid_exit -> IRET
> -> boom! Or if not "boom", at least, the NMI gets forgotten.
> 
> Am I missing something?

I think to get a full BOOM you need a bit more complex process, namely:

	* NMI triggered
	* NMI handler starts
	* MCE happens
	* Second NMI triggered and queued
	* handler done, IRET
	* Second NMI handler starts and overwrites NMI return address on stack
	* Second NMI handler ends
	* First NMI handler ends and goes into an infinite IRET loop, always
	  returning to the beginning of itself

But you do have all the ingredients.

And I don't see any other way out than not calling IRET for MCEs.

-- 
Vojtech Pavlik
Director SUSE Labs
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