linux-kernel - Re: [tip:x86/urgent] x86/nmi: Test saved %cs in NMI to determine nested NMI case

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Date:	Mon, 27 Feb 2012 13:29:47 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
	stable@...nel.org, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/nmi: Test saved %cs in NMI to determine
 nested NMI case

On Mon, Feb 20, 2012 at 11:51:41AM -0500, Steven Rostedt wrote:
> On Mon, 2012-02-20 at 03:47 -0800, tip-bot for Steven Rostedt wrote:
> > Commit-ID:  45d5a1683c04be28abdf5c04c27b1417e0374486
> > Gitweb:     http://git.kernel.org/tip/45d5a1683c04be28abdf5c04c27b1417e0374486
> > Author:     Steven Rostedt <rostedt@...dmis.org>
> > AuthorDate: Sun, 19 Feb 2012 16:43:37 -0500
> > Committer:  Ingo Molnar <mingo@...e.hu>
> > CommitDate: Mon, 20 Feb 2012 09:09:57 +0100
> > 
> > x86/nmi: Test saved %cs in NMI to determine nested NMI case
> > 
> > Currently, the NMI handler tests if it is nested by checking the
> > special variable saved on the stack (set during NMI handling)
> > and whether the saved stack is the NMI stack as well (to prevent
> > the race when the variable is set to zero).
> > 
> > But userspace may set their %rsp to any value as long as they do
> > not derefence it, and it may make it point to the NMI stack,
> > which will prevent NMIs from triggering while the userspace app
> > is running. (I tested this, and it is indeed the case)
> > 
> > Add another check to determine nested NMIs by looking at the
> > saved %cs (code segment register) and making sure that it is the
> > kernel code segment.
> > 
> > Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> > Cc: H. Peter Anvin <hpa@...or.com>
> > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: <stable@...nel.org>
> 
> Just so that we do not confuse Greg, the bug was introduced in the 3.3
> merge window. It does not exist in 3.2 or earlier.

Thanks for letting me know, I would have been confused :)

greg k-h
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