Re: RFC: userspace exception fixups

[Dave Hansen]

On 11/6/18 11:02 AM, Andy Lutomirski wrote:
> On Tue, Nov 6, 2018 at 10:41 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>>
>> On 11/6/18 10:20 AM, Andy Lutomirski wrote:
>>> I almost feel like the right solution is to call into SGX on its own
>>> private stack or maybe even its own private address space.
>>
>> Yeah, I had the same gut feeling.  Couldn't the debugger even treat the
>> enclave like its own "thread" with its own stack and its own set of
>> registers and context?  That seems like a much more workable model than
>> trying to weave it together with the EENTER context.
> 
> So maybe the API should be, roughly
> 
> sgx_exit_reason_t sgx_enter_enclave(pointer_to_enclave, struct
> host_state *state);
> sgx_exit_reason_t sgx_resume_enclave(same args);
> 
> where host_state is something like:
> 
> struct host_state {
>   unsigned long bp, sp, ax, bx, cx, dx, si, di;
> };
> 
> and the values in host_state explicitly have nothing to do with the
> actual host registers.  So, if you want to use the outcall mechanism,
> you'd allocate some memory, point sp to that memory, call
> sgx_enter_enclave(), and then read that memory to do the outcall.

Ah, so instead of the enclave rudely "hijacking" the EENTER context, we
have it nicely return and nicely _hint_ to the calling context what it
would like to do.  Then, the EENTER context can make a controlled
transition over to the requested context.

> Actually implementing this would be distinctly nontrivial, and would
> almost certainly need some degree of kernel help to avoid an explosion
> when a signal gets delivered while we have host_state.sp loaded into
> the actual SP register.  Maybe rseq could help with this?

As long as the memory pointed to by host_state.sp is valid and can hold
the signal frame (grows down without clobbering anything), what goes
boom?  The signal handling would push a signal frame and call the
handler.  It would have a shallow-looking stack, but the handler could
just do its normal business and return from the signal where the frame
would get popped and continue with %rsp=host_state.sp, blissfully
unaware of the signal ever having happened.