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

[Andy Lutomirski]

On Tue, Dec 11, 2018 at 2:23 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Tue, Dec 11, 2018 at 09:58:19AM -0800, Andy Lutomirski wrote:
> > > 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.
>
> Are you concerned about userspace shooting themselves in the foot, e.g.
> unknowingly overwriting their handler?  Requiring unregister->register
> to change the handler would mitigate that issue for the most part.  Or
> we could even say it's a write-once property.
>
> That obviously doesn't solve the issue of a userspace application
> deliberately using two different libraries to run enclaves in a single
> process, but I have a hard time envisioning a scenario where someone
> would want to use two different *SGX* libraries in a single process.
> Don't most of the signal issue arise due to loading multiple libraries
> that provide *different* services needing to handle signals?

I can easily imagine two SGX libraries that know nothing about each
other running in the same process.  One or both could be PKCS#11
modules, for example.

I suspect that Linux will eventually want the ability for libraries to
register exception handlers, but that's not going to get designed and
implemented quickly enough for SGX's initial Linux rollout.  A vDSO
helper like in your earlier series should solve most of the problem
without any contention issues.