RE: [PATCH v19,RESEND 24/27] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions

[Xing, Cedric]

> > By requiring preservation of RSP at both AEX and EEXIT, this precludes
> the possibility of using the untrusted stack as temporary storage by
> enclaves. While that looks reasonable at first glance, I'm afraid it
> isn't the case in reality. The untrusted stack is inarguably the most
> convenient way for data exchange between an enclave and its enclosing
> process, and is in fact being used for that purpose by almost all
> existing enclaves to date. Given the expectation that this API will be
> used by all future SGX application, it looks unwise to ban the most
> convenient and commonly used approach for data exchange.
> 
> For reference, here's the code in the Intel toolchain responsible for
> this:
> https://github.com/intel/linux-
> sgx/blob/6a0b5ac71f8d16f04e0376f3b2168e80c773dd23/sdk/trts/trts.cpp#L125
> -L140
> 
> Regarding "almost all existing enclaves to date", enclaves built with
> the Fortanix toolchain don't touch the untrusted stack.
> 
> --
> Jethro Beekman | Fortanix

Thanks for providing the references. Yes, not every enclave touches the untrusted stack so I used the word "almost".

Everything exists for a reason. By bringing up what is done today, I was trying to inspire thinking on the more important question of "why is it done this way today?".