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

H. Peter Anvin
2014-04-22T20:18:40Z

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