Re: RFC: userspace exception fixups

[Dave Hansen]

On 11/6/18 12:12 PM, Andy Lutomirski wrote:
> True, but what if we have a nasty enclave that writes to memory just
> below SP *before* decrementing SP?

Yeah, that would be unfortunate.  If an enclave did this (roughly):

	1. EENTER
	2. Hardware sets eenter_hwframe->sp = %sp
	3. Enclave runs... wants to do out-call
	4. Enclave sets up parameters:
		memcpy(&eenter_hwframe->sp[-offset], arg1, size);
		...
	5. Enclave sets eenter_hwframe->sp -= offset

If we got a signal between 4 and 5, we'd clobber the copy of 'arg1' that
was on the stack.  The enclave could easily fix this by moving ->sp first.

But, this is one of those "fun" parts of the ABI that I think we need to
talk about.  If we do this, we also basically require that the code
which handles asynchronous exits must *not* write to the stack.  That's
not hard because it's typically just a single ERESUME instruction, but
it *is* a requirement.

It means fun stuff like that you absolutely can't just async-exit to C code.