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

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Date:	Mon, 20 Feb 2012 09:10:45 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
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


* Steven Rostedt <rostedt@...dmis.org> wrote:

> On Sun, 2012-02-19 at 13:56 +0100, Ingo Molnar wrote:
> > * Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > Ingo,
> > > 
> > > I found that it is possible for userspace to prevent an NMI 
> > > from triggering while it is running by setting its stack 
> > > pointer to that of the NMI stack. This tricks the NMI nested 
> > > algorithm in thinking that the NMI is nested. The easy 
> > > solution to this is to test the %rip to make sure that the NMI 
> > > happened in kernel mode before testing for nesting.
> > 
> > Ouch...
> 
> Note, it does not seem to cause any destruction, but screw up 
> profiling.

Except if the source of the NMI was not profiling, right?

Thanks,

	Ingo
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