On Thu, Nov 08, 2018 at 01:50:31PM -0800, Dave Hansen wrote: > 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. Ya, I meant versus doing an EEXIT on every allocation, i.e. a very naive allocation scheme.
