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

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Date:	Sun, 19 Feb 2012 12:45:09 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 02/19/2012 12:34 PM, Steven Rostedt wrote:
> On Sun, 2012-02-19 at 05:46 -0800, hpanvin@...il.com wrote:
>> Vsyscall page, not vdso...
>
> Peter,
>
> My original patch was to check the %cs register against __KERNEL_CS, but
> IIRC, you said that userspace can change that register to anything it
> wanted before doing a long jump or something. Is this true for x86_64 as
> well? I guess it would be because x86_64 can support 32bit apps.
>
> Anyway, I'll add a check that makes sure that the RIP is less than the
> FIXADDR sections as well.
>

User space can change %cs, but it can never set it to __KERNEL_CS; 
specifically user space can never set the bottom two bits in CS to zero.

So this should be a better test.

(Now, doing the test that way plays havoc with the braindamage knows as 
Xen paravirt, but I don't think it's subject to this particular NMI code.)

	-hpa

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