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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Mar 20, 2019 at 01:38:04PM -0700, Xing, Cedric wrote:
> > > 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,
> > 
> > I vehemently disagree with "inarguably".  IMO, passing data via
> > registers is much more convenient.
> 
> Which is the most convenient approach is always dependent on data size and/or even how the data is produced/consumed. It's kind of a spectrum and we're just talking in the sense of probability. You are right that "inarguably" is arguable if the buffer is small enough to fit in registers, and the producer/consumer also has access to registers.
> 
> > 
> > Even if you qualify your assertion with "data of arbitrary size unknown
> > at build time", I still disagree.  Using the untrusted stack allows for
> > trickery when a debugger is involved, other than that I see no
> > advantages over allocating virtual memory and handing the pointer to the
> > enclave at launch time.  Sure, it requires a few more lines of code to
> > setup, but it's literally ~20 LoC out of thousands required to sign,
> > build and launch an enclave, but it doesn't require playing games with
> > the stack.
> 
> I'm NOT ruling out your approach.
> 
> And like you said, the untrusted stack enables certain trickery that helps
> debugging and also simplifies enclaves (even just a little). Then why are
> you trying to rule that out? Because of 9 LOC in vDSO?

Although its just 9 LOC, consider its impact on someone who is looking at
the kernel's SGX support for the first time.  Questions they may have when
looking at the vDSO code/documentation:

  - What's an exit handler?
  - Why is an exit handler optional?  Don't I always want to handle exits?
  - What value should my exit handler return?
  - What should my exit handler do if it detects an error?
  - Why would I want to preserve %rbp and not %rsp?
  - Isn't it insecure to use the untrusted stack in my enclave?

AFAIK, the only reason to preserve %rbp instead of %rsp, i.e. support an
"exit handler" callback, is to be able to implement an o-call scheme using
the untrusted stack to pass data.  Every idea I came up with for using the
callback, e.g. logging, handling stack corruptiong, testing hooks, etc...
was at worst no more difficult to implement when using a barebones vDSO.

So, given the choice between a) documenting and maintaining all the baggage
that comes with the exit handler and b) saying "go use signals", I chose
option b.