linux-kernel - Re: 64bit x86: NMI nesting still buggy?

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Message-ID: <20140521142055.GH21205@pd.tnic>
Date:	Wed, 21 May 2014 16:20:55 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Salman Qazi <sqazi@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Michal Hocko <mhocko@...e.cz>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Petr Tesarik <ptesarik@...e.cz>, Petr Mladek <pmladek@...e.cz>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: 64bit x86: NMI nesting still buggy?

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?

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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