Re: RFC: userspace exception fixups

[Jann Horn]

On Sat, Nov 3, 2018 at 12:32 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Fri, Nov 2, 2018 at 4:28 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> > On Fri, Nov 2, 2018 at 11:04 PM Sean Christopherson
> > <sean.j.christopherson@xxxxxxxxx> wrote:
> > > On Fri, Nov 02, 2018 at 08:02:23PM +0100, Jann Horn wrote:
> > > > On Fri, Nov 2, 2018 at 7:27 PM Sean Christopherson
> > > > <sean.j.christopherson@xxxxxxxxx> wrote:
> > > > > On Fri, Nov 02, 2018 at 10:48:38AM -0700, Andy Lutomirski wrote:
> > > > > > This whole mechanism seems very complicated, and it's not clear
> > > > > > exactly what behavior user code wants.
> > > > >
> > > > > No argument there.  That's why I like the approach of dumping the
> > > > > exception to userspace without trying to do anything intelligent in
> > > > > the kernel.  Userspace can then do whatever it wants AND we don't
> > > > > have to worry about mucking with stacks.
> > > > >
> > > > > One of the hiccups with the VDSO approach is that the enclave may
> > > > > want to use the untrusted stack, i.e. the stack that has the VDSO's
> > > > > stack frame.  For example, Intel's SDK uses the untrusted stack to
> > > > > pass parameters for EEXIT, which means an AEX might occur with what
> > > > > is effectively a bad stack from the VDSO's perspective.
> > > >
> > > > What exactly does "uses the untrusted stack to pass parameters for
> > > > EEXIT" mean? I guess you're saying that the enclave is writing to
> > > > RSP+[0...some_positive_offset], and the written data needs to be
> > > > visible to the code outside the enclave afterwards?
> > >
> > > As is, they actually do it the other way around, i.e. negative offsets
> > > relative to the untrusted %RSP.  Going into the enclave there is no
> > > reserved space on the stack.  The SDK uses EEXIT like a function call,
> > > i.e. pushing parameters on the stack and making an call outside of the
> > > enclave, hence the name out-call.  This allows the SDK to handle any
> > > reasonable out-call without a priori knowledge of the application's
> > > maximum out-call "size".
> >
> > But presumably this is bounded to be at most 128 bytes (the red zone
> > size), right? Otherwise this would be incompatible with
> > non-sigaltstack signal delivery.
>
>
> I think Sean is saying that the enclave also updates RSP.

Ah, bleh, of course.