Re: RFC: userspace exception fixups

[Dave Hansen]

On 11/8/18 1:16 PM, Sean Christopherson wrote:
> On Thu, Nov 08, 2018 at 12:10:30PM -0800, Dave Hansen wrote:
>> On 11/8/18 12:05 PM, Andy Lutomirski wrote:
>>> Hmm.  The idea being that the SDK preserves RBP but not RSP.  That's
>>> not the most terrible thing in the world.  But could the SDK live with
>>> something more like my suggestion where the vDSO supplies a normal
>>> function that takes a struct containing registers that are visible to
>>> the enclave?  This would make it extremely awkward for the enclave to
>>> use the untrusted stack per se, but it would make it quite easy (I
>>> think) for the untrusted part of the SDK to allocate some extra memory
>>> and just tell the enclave that *that* memory is the stack.
>>
>> I really think the enclave should keep its grubby mitts off the
>> untrusted stack.  There are lots of ways to get memory, even with
>> stack-like semantics, that don't involve mucking with the stack itself.
>>
>> I have not heard a good, hard argument for why there is an absolute
>> *need* to store things on the actual untrusted stack.
> 
> Convenience and performance are the only arguments I've heard, e.g. so
> that allocating memory doesn't require an extra EEXIT->EENTER round trip.

Well, for the first access, it's going to cost a bunch asynchronous
exits to fault in all the stack pages.  Instead of that, if you had a
single area, or an explicit out-call to allocate and populate the area,
you could do it in a single EEXIT and zero asynchronous exits for demand
page faults.

So, it might be convenient, but I'm rather suspicious of any performance
arguments.