linux-kernel - Re: [PATCH][GIT PULL][v3.3] x86: Test saved %rip in NMI to determine nested NMI

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Message-ID: <1329748907.25686.29.camel@gandalf.stny.rr.com>
Date:	Mon, 20 Feb 2012 09:41:47 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH][GIT PULL][v3.3] x86: Test saved %rip in NMI to
 determine nested NMI

On Mon, 2012-02-20 at 09:10 +0100, Ingo Molnar wrote:
> > 
> > Note, it does not seem to cause any destruction, but screw up 
> > profiling.
> 
> Except if the source of the NMI was not profiling, right?

Well, what else are NMIs used for? All I can think of is profiling and
watchdogs. I'm not sure how much damage a watchdog NMI being missed in
userspace will hurt it. Unless it takes a watchdog to set off the next
watchdog. That is, if watchdogs are enabled in a one shot mode. Then a
missed NMI could cause the watchdog to shut off. This is all theory.

The test that %cs is __KERNEL_CS before considering the NMI nested seems
to fix this bug. And if userspace really can't change the %cs to
__KERNEL_CS than we are safe. The bug only exists in the 3.3-rc releases
and by fixing it now we don't need to worry about what problems the
original bug can cause.

Thanks,

-- Steve

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