Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions

[Andy Lutomirski <luto@kernel.org>]

On Fri, Dec 7, 2018 at 8:51 AM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> +Cc: linux-sgx, Haitao, Greg and Jethro
>
> My apologies for neglecting to cc the SGX folks, original thread is here:
>
> https://lkml.kernel.org/r/20181206221922.31012-1-sean.j.christopherson@intel.com
>
> On Thu, Dec 06, 2018 at 02:50:01PM -0800, Andy Lutomirski wrote:
> > On Thu, Dec 6, 2018 at 2:19 PM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > >
> >  +
> > > +       /*
> > > +        * Invoke the caller's exit handler if one was provided.  The return
> > > +        * value tells us whether to re-enter the enclave (EENTER or ERESUME)
> > > +        * or to return (EEXIT).
> > > +        */
> > > +       if (exit_handler) {
> > > +               leaf = exit_handler(exit_info, tcs, priv);
> > > +               if (leaf == SGX_EENTER || leaf == SGX_ERESUME)
> > > +                       goto enter_enclave;
> > > +               if (leaf == SGX_EEXIT)
> > > +                       return 0;
> > > +               return -EINVAL;
> > > +       } else if (leaf != SGX_EEXIT) {
> > > +               return -EFAULT;
> > > +       }
> >
> > This still seems overcomplicated to me.  How about letting the
> > requested leaf (EENTER or ERESUME) be a parameter to the function and
> > then just returning here?  As it stands, you're requiring any ERESUME
> > that gets issued (other than the implicit ones) to be issued in the
> > same call stack, which is very awkward if you're doing something like
> > forwarding the fault to a different task over a socket and then
> > waiting in epoll_wait() or similar before resuming the enclave.
>
> Ah, yeah, wasn't thinking about usage models where the enclave could
> get passed off to a different thread.
>
> What about supporting both, i.e. keep the exit handler but make it 100%
> optional?  And simplify the exit_handler to effectively return a boolean,
> i.e. "exit or continue".
>
> Something like this:
>
> notrace long __vdso_sgx_enter_enclave(u32 op, void *tcs, void *priv,
>                                       struct sgx_enclave_exit_info *exit_info,
>                                       sgx_enclave_exit_handler *exit_handler)
> {
>         u64 rdi, rsi, rdx;
>         u32 leaf;
>         long ret;
>
>         if (!tcs || !exit_info)
>                 return -EINVAL;
>
> enter_enclave:
>         if (op != SGX_EENTER && op != SGX_ERESUME)
>                 return -EINVAL;
>
>         <same core code>
>
>         /*
>          * Invoke the caller's exit handler if one was provided.  The return
>          * value tells us whether to re-enter the enclave (EENTER or ERESUME)
>          * or to return (EEXIT).
>          */
>         if (exit_handler) {
>                 if (exit_handler(exit_info, tcs, priv)) {
>                         op = exit_info->leaf;
>                         goto enter_enclave;
>                 }
>         }
>
>         if (exit_info->leaf == SGX_EEXIT)
>                 return -EFAULT;
>
>         return 0;
> }
>
>
> I like that the exit handler allows userspace to trap/panic with the full
> call stack in place, and in a dedicated path, i.e. outside of the basic
> enter/exit code.  An exit handler probably doesn't fundamentally change
> what userspace can do with respect to debugging/reporting, but I think
> it would actually simplify some userspace implementations, e.g. I'd use
> it in my tests like so:
>
> long fault_handler(struct sgx_enclave_exit_info *exit_info, void *tcs, void *priv)
> {
>         if (exit_info->leaf == SGX_EEXIT)
>                 return 0;
>
>         <report exception and die/hang>
> }
>

Hmm.  That't not totally silly, although you could accomplish almost
the same thing by wrapping the vDSO helper in another function.