linux-kernel - Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

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Date:	Tue, 22 Apr 2014 19:08:48 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andrew Lutomirski <amluto@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Arjan van de Ven <arjan.van.de.ven@...el.com>,
	Alexandre Julliard <julliard@...ehq.com>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

On Tue, Apr 22, 2014 at 4:17 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 04/22/2014 12:55 PM, Brian Gerst wrote:
>> On Tue, Apr 22, 2014 at 2:51 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>> On 04/22/2014 11:17 AM, Brian Gerst wrote:
>>>>>
>>>>> That is the entry condition that we have to deal with.  The fact that
>>>>> the switch to the IST is unconditional is what makes ISTs hard to deal with.
>>>>
>>>> Right, that is why you switch away from the IST as soon as possible,
>>>> copying the data that is already pushed there to another stack so it
>>>> won't be overwritten by a recursive fault.
>>>>
>>>
>>> That simply will not work if you can take a #GP due to the "safe" MSR
>>> functions from NMI and #MC context, which would be my main concern.
>>
>> In that case (#2 above), you would switch to the previous %rsp (in the
>> NMI/MC stack), copy the exception frame from the IST, and continue
>> with the #GP handler.  That effectively is the same as it is today,
>> where no stack switch occurs on the #GP fault.
>>
>
> 1. You take #GP.  This causes an IST stack switch.
> 2. You immediately thereafter take an NMI.  This switches stacks again.
> 3. Now you take another #GP.  This causes another IST stack, and now you
> have clobbered your return information, and cannot resume.

You are right.  That will not work.
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