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.
