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

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Message-ID: <1329756701.25686.31.camel@gandalf.stny.rr.com>
Date:	Mon, 20 Feb 2012 11:51:41 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	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,
	Greg KH <greg@...ah.com>
Cc:	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, 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.

-- Steve



> Link: http://lkml.kernel.org/r/1329687817.1561.27.camel@acer.local.home
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> ---
>  arch/x86/kernel/entry_64.S |    7 +++++++
>  1 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 3fe8239..debd851 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -1532,6 +1532,13 @@ ENTRY(nmi)
>  	pushq_cfi %rdx
>  
>  	/*
> +	 * If %cs was not the kernel segment, then the NMI triggered in user
> +	 * space, which means it is definitely not nested.
> +	 */
> +	cmp $__KERNEL_CS, 16(%rsp)
> +	jne first_nmi
> +
> +	/*
>  	 * Check the special variable on the stack to see if NMIs are
>  	 * executing.
>  	 */


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