Re: RFC: userspace exception fixups

[Jann Horn <jannh@google.com>]

On Fri, Nov 2, 2018 at 11:04 PM Sean Christopherson
<sean.j.christopherson@intel.com> 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@intel.com> 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.