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

["H. Peter Anvin" <hpa@zytor.com>]

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.

	-hpa