linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <20150723212042.GN25159@twins.programming.kicks-ass.net>
Date:	Thu, 23 Jul 2015 23:20:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
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>,
	Willy Tarreau <w@....eu>, Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	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:

> And the "take them and disable them" is really simple. No "am I in an
> NMI contect" thing (because that leads to the whole question about
> "what is NMI context"). That's not the real rule anyway.
> 
> No, make it very simple and straightforward. Make the test be "uhhuh,
> I got a #DB in kernel mode, and interrupts were disabled - I know I'm
> going to return with "ret", so I'm just going to have to disable this
> breakpoint".
> 
> Nothing clever. Nothing subtle. Nothing that needs "this range of
> instructions is magical". No.  Just a very simple rule: if the context
> we return to is kernel mode and interrupts are disabled, we're using
> 'ret', so we cannot suppress debug faults.
> 
> Did I miss something? There were a lot of emails flying around, but I
> *thought* I saw them all..

So the NMI could trigger userspace debug register faults, and simply
disabling them would make the whole debug register thing entirely
unreliable.

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