Re: RFC: userspace exception fixups

[Sean Christopherson]

On Tue, Nov 06, 2018 at 06:17:30PM -0500, Rich Felker wrote:
> On Tue, Nov 06, 2018 at 11:02:11AM -0800, 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.
> > 
> > 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?
> > 
> > The ISA here is IMO not well thought through.
> 
> Maybe I'm mistaken about some fundamentals here, but my understanding
> of SGX is that the whole point is that the host application and the
> code running in the enclave are mutually adversarial towards one
> another. Do any or all of the proposed protocols here account for this
> and fully protect the host application from malicious code in the
> enclave? It seems that having control over the register file on exit
> from the enclave is fundamentally problematic but I assume there must
> be some way I'm missing that this is fixed up.

SGX provides protections for the enclave but not the other way around.
The kernel has all of its normal non-SGX protections in place, but the
enclave can certainly wreak havoc on its userspace process.  The basic
design idea is that the enclave is a specialized .so that gets extra
security protections but is still effectively part of the overall
application, e.g. it has full access to its host userspace process'
virtual memory.