[RFC PATCH v2 2/5] x86/sgx: Require userspace to define enclave pages' protection bits

[Sean Christopherson]

On Tue, Jun 11, 2019 at 05:09:28PM -0700, Andy Lutomirski wrote:
> 
> On Jun 10, 2019, at 3:28 PM, Xing, Cedric <cedric.xing at intel.com> wrote:
> 
> >> From: Andy Lutomirski [mailto:luto at kernel.org]
> >> Sent: Monday, June 10, 2019 12:15 PM
> >> This seems like an odd workflow.  Shouldn't the #PF return back to
> >> untrusted userspace so that the untrusted user code can make its own
> >> decision as to whether it wants to EAUG a page there as opposed to, say,
> >> killing the enclave or waiting to keep resource usage under control?
> > 
> > This may seem odd to some at the first glance. But if you can think of how
> > static heap (pre-allocated by EADD before EINIT) works, the load parses the
> > "metadata" coming with the enclave to decide the address/size of the heap,
> > EADDs it, and calls it done. In the case of "dynamic" heap (allocated
> > dynamically by EAUG after EINIT), the same thing applies - the loader
> > determines the range of the heap, tells the SGX module about it, and calls
> > it done. Everything else is the between the enclave and the SGX module.
> > 
> > In practice, untrusted code usually doesn't know much about enclaves, just
> > like it doesn't know much about the shared objects loaded into its address
> > space either. Without the necessary knowledge, untrusted code usually just
> > does what it is told (via o-calls, or return value from e-calls), without
> > judging that's right or wrong. 
> > 
> > When it comes to #PF like what I described, of course a signal could be
> > sent to the untrusted code but what would it do then? Usually it'd just
> > come back asking for a page at the fault address. So we figured it'd be
> > more efficient to just have the kernel EAUG at #PF. 
> > 
> > Please don't get me wrong though, as I'm not dictating what the s/w flow
> > shall be. It's just going to be a choice offered to user mode. And that
> > choice was planned to be offered via mprotect() - i.e. a writable vma
> > causes kernel to EAUG while a non-writable vma will result in a signal
> > (then the user mode could decide whether to EAUG). The key point is
> > flexibility - as we want to allow all reasonable s/w flows instead of
> > dictating one over others. We had similar discussions on vDSO API before.
> > And I think you accepted my approach because of its flexibility. Am I
> > right?
> 
> As long as user code can turn this off, I have no real objection. But it
> might make sense to have it be more explicit — have an ioctl set up a range
> as “EAUG-on-demand”.

This was part of the motivation behind changing SGX_IOC_ENCLAVE_ADD_PAGE
to SGX_IOC_ENCLAVE_ADD_REGION and adding a @flags parameter.  E.g. adding
support for "EAUG-on-demand" regions would just be a new flag.

> But this is all currently irrelevant. We can argue about it when the patches
> show up. :)