Re: [RFC PATCH v3 0/4] x86: Add exception fixup for SGX ENCLU

From: Sean Christopherson
Date: Tue Dec 11 2018 - 11:52:56 EST

On Tue, Dec 11, 2018 at 07:41:27AM -0800, Andy Lutomirski wrote:
> > On Dec 10, 2018, at 3:24 PM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
> >
> >> On Mon, Dec 10, 2018 at 03:21:37PM -0800, Sean Christopherson wrote:
> >> At that point I realized it's a hell of a lot easier to simply provide
> >> an IOCTL via /dev/sgx that allows userspace to register a per-process
> >> ENCLU exception handler. At a high level, the basic idea is the same
> >> as the vDSO approach: provide a hardcoded fixup handler for ENCLU and
> >> attempt to fixup select unhandled exceptions that occurred in user code.
> >
> > So, on the one hand, this is *absolutely* much cleaner than the VDSO
> > approach. On the other hand, this is global process state and has some
> > of the same problems as a signal handler as a result.
> I liked the old version better for this reason

This isn't fundamentally different than forcing all EENTER calls through
the vDSO, which is also per-process. Technically this is more flexible
in that regard since userspace gets to choose where their one ENCLU gets
to reside. Userspace can have per-enclave entry flows so long as the
actual ENLU[EENTER] is common, same as vDSO.

> and for another reason:
> while this new one looks very very simple, it still has the hidden
> complexity that the magic values written to registers in the event of an
> exception are very much Linux specific.

Definitely more magical, but not necessarily more difficult to document.
It'd essentially be an extension of hardware's AEE/AEP behavior.

> OTOH, the old approach clobbered more regs than needed, but thatâs a easy fix.