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SubjectRe: RFC: userspace exception fixups
On Wed, Nov 07, 2018 at 04:27:58PM -0500, Rich Felker wrote:
> On Tue, Nov 06, 2018 at 03:26:16PM -0800, Sean Christopherson wrote:
> > 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@intel.com> 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.
>
> In that case it seems like the only way to use SGX that's not a gaping
> security hole is to run the SGX enclave in its own fully-seccomp (or
> equivalent) process, with no host application in the same address
> space. Since the host application can't see the contents of the
> enclave to make any determination of whether it's safe to run, running
> it in the same address space only makes sense if the cpu provides
> protection against unwanted accesses to the host's memory from the
> enclave -- and according to you, it doesn't.

The enclave's code (and any initial data) isn't encrypted until the
pages are loaded into the Enclave Page Cache (EPC), which can only
be done by the kernel (via ENCLS[EADD]). In other words, both the
kernel and userspace can vet the code/data before running an enclave.

Practically speaking, an enclave will be coupled with an untrusted
userspace runtime, i.e. it's loader. Enclaves are also measured
as part of their build process, and so the enclave loader needs to
know which pages to add to the measurement, and in what order. I
guess technically speaking an enclave could have zero pages added
to its measurement, but that'd probably be a big red flag that said
enclave is up to something fishy.

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