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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Dec 11, 2018 at 03:10:52PM -0800, Andy Lutomirski wrote:
> On Tue, Dec 11, 2018 at 2:23 PM Sean Christopherson
> <sean.j.christopherson@intel.com> 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@intel.com> 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@joshtriplett.org> 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.

Argh, wasn't thinking about loading other libraries that would also be
using SGX.