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

From: Andy Lutomirski
Date: Tue Dec 11 2018 - 12:58:36 EST

> On Dec 11, 2018, at 8:52 AM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>> 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.

Right. The problem is that user libraries have a remarkably hard time
agreeing on where their one copy of anything lives.

>> 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.