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

Brian Gerst
2014-04-22T19:55:17Z

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.