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

H. Peter Anvin
2014-04-24T22:39:12Z

On 04/24/2014 03:31 PM, Andrew Lutomirski wrote:
>
> I was imagining just randomizing a couple of high bits so the whole
> espfix area moves as a unit.
>
>> We could XOR with a random constant with no penalty at all. Only
>> problem is that this happens early, so the entropy system is not yet
>> available. Fine if we have RDRAND, but...
>
> How many people have SMAP and not RDRAND? I think this is a complete
> nonissue for non-SMAP systems.
>

Most likely none, unless some "clever" virtualizer turns off RDRAND out
of spite.

>>> Peter, is this idea completely nuts? The only exceptions that can
>>> happen there are NMI, MCE, #DB, #SS, and #GP. The first four use IST,
>>> so they won't double-fault.
>>
>> It is completely nuts, but sometimes completely nuts is actually useful.
>> It is more complexity, to be sure, but it doesn't seem completely out
>> of the realm of reason, and avoids having to unwind the ministack except
>> in the normally-fatal #DF handler. #DFs are documented as not
>> recoverable, but we might be able to do something here.
>>
>> The only real disadvantage I see is the need for more bookkeeping
>> metadata. Basically the bitmask in espfix_64.c now needs to turn into
>> an array, plus we need a second percpu variable. Given that if
>> CONFIG_NR_CPUS=8192 the array has 128 entries I think we can survive that.
>
> Doing something in #DF needs percpu data? What am I missing?

You need the second percpu variable in the espfix setup code so you have
both the write address and the target rsp (read address).

-hpa