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SubjectRe: [RFC PATCH 4/4] x86/vdso: Add __vdso_sgx_eenter() to wrap SGX enclave transitions
On Wed, Dec 05, 2018 at 03:40:48PM -0800, Andy Lutomirski wrote:
> On Wed, Dec 5, 2018 at 3:20 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> > +notrace long __vdso_sgx_eenter(void *tcs, void *priv,
> > + struct sgx_eenter_fault_info *fault_info)
> > +{
> > + u32 trapnr, error_code;
> > + long leaf;
> > + u64 addr;
> > +
> > + /*
> > + * %eax = EENTER
> > + * %rbx = tcs
> > + * %rcx = do_eresume
> > + * %rdi = priv
> > + * do_eenter:
> > + * enclu
> > + * jmp out
> > + *
> > + * do_eresume:
> > + * enclu
> > + * ud2
>
> Is the only reason for do_eresume to be different from do_eenter so
> that you can do the ud2?

No, it was a holdover from doing fixup via a magic prefix in user code.
The fixup could only skip the ENCLU and so a second ENCLU was needed to
differentiate between EENTER and ERESUME. The need for two ENCLUs got
ingrained in my head. I can't think of anything that will break if we
use a single ENCLU.

> > + *
> > + * out:
> > + * <return to C code>
> > + *
> > + * fault_fixup:
> > + * <extable loads RDI, DSI and RDX with fault info>
> > + * jmp out
> > + */
>
> This has the IMO excellent property that it's extremely awkward to use
> it for a model where the enclave is reentrant. I think it's excellent
> because reentrancy on the same enclave thread is just asking for
> severe bugs. Of course, I fully expect the SDK to emulate reentrancy,
> but then it's 100% their problem :) On the fiip side, it means that
> you can't really recover from a reported fault, even if you want to,
> because there's no way to ask for ERESUME. So maybe the API should
> allow that after all.

Doh. The ability to do ERESUME is an explicit requirement from the SDK
folks. More code that I pulled from my userspace implementation and
didn't revisit.

> I think it might be polite to at least give some out regs, maybe RSI and RDI?

For the outbound path? I was thinking @priv would be used for passing
data out as well as in.

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