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Date: Tue, 22 Apr 2014 19:08:48 -0400
Message-ID: <CAMzpN2im7kO1NvjyHsP8va7iQCXWRZO6HGgkkySW_AzpeufMWg@mail.gmail.com>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*
From: Brian Gerst <brgerst@gmail.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andrew Lutomirski <amluto@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Alexander van Heukelum <heukelum@fastmail.fm>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Arjan van de Ven <arjan.van.de.ven@intel.com>,
        Alexandre Julliard <julliard@winehq.com>,
        Andi Kleen <andi@firstfloor.org>, Thomas Gleixner <tglx@linutronix.de>
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On Tue, Apr 22, 2014 at 4:17 PM, H. Peter Anvin <hpa@zytor.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@zytor.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.