Re: RFC: userspace exception fixups

[Sean Christopherson <sean.j.christopherson@intel.com>]

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.

> We could quite easily have the untrusted code just promise to allocate a
> stack-sized virtual area (even derived from the stack rlimit size) and
> pass that into the enclave for parameter use.

I agree more and more the further I dig.  AFAIK there is no need to for
the enclave to actually load %rsp.  The initial EENTER can pass in the
base/top of the pseudo-stack and from there the enclave can manage it
purely in software.