linux-kernel - Re: recent x86-64 nested NMI adjustments

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Message-Id: <4F5E08C30200007800077ACC@nat28.tlf.novell.com>
Date:	Mon, 12 Mar 2012 13:31:31 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	<mingo@...e.hu>, "Peter Zijlstra" <peterz@...radead.org>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: recent x86-64 nested NMI adjustments

>>> On 12.03.12 at 14:16, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Mon, 2012-03-12 at 12:10 +0000, Jan Beulich wrote:
>> Hi Steven,
>> 
>> the explanation of 45d5a1683c04be28abdf5c04c27b1417e0374486
>> seems bogus to me: When arriving from user mode, %rsp won't point
>> to the user stack anymore, as it gets switched away from during the
>> processing of the exception (the more that the IDT entry specifies a
>> separate stack anyway, which even guarantees this for kernel mode
>> entries).
> 
> No it is real, and I had a test program that exploited it. I'm not
> worried about the current %rsp, I'm worried about what %rsp is saved on
> the stack. Two things are used to check if the incoming NMI is nested or
> not.
> 
> 1) if the on-stack "in-nmi" variable is set
> 
> 2) if the saved %rsp is pointing to the NMI stack.

Ah, right - the way the new check got placed I (wrongly) implied
it to guard the immediately succeeding check of the "special
variable", whereas it really guards (only) test_in_nmi.

Thanks for the explanation, and sorry for the noise then.

Jan

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