Re: x86/sgx: uapi change proposal

[Jarkko Sakkinen]

On Wed, Jan 09, 2019 at 08:31:37AM -0800, Sean Christopherson wrote:
> On Tue, Jan 08, 2019 at 02:54:11PM -0800, Andy Lutomirski wrote:
> > On Tue, Jan 8, 2019 at 2:09 PM Sean Christopherson
> > <sean.j.christopherson@xxxxxxxxx> wrote:
> > >
> > > Cleaner in the sense that it's faster to get basic support up and running
> > > since there are fewer touchpoints, but there are long term ramifications
> > > to cramming EPC management in KVM.
> > >
> > > And at this point I'm not stating any absolutes, e.g. how EPC will be
> > > handled by KVM.  What I'm pushing for is to not eliminate the possibility
> > > of having the SGX subsystem own all EPC management, e.g. don't tie
> > > /dev/sgx to a single enclave.
> > 
> > I haven't gone and re-read all the relevant SDM bits, so I'll just
> > ask: what, if anything, are the actual semantics of mapping "raw EPC"
> > like this?  You can't actually do anything with the mapping from user
> > mode unless you actually get an enclave created and initialized in it
> > and have it mapped at the correct linear address, right?  I still
> > think you have the right idea, but it is a bit unusual.
> 
> Correct, the EPC is inaccessible until a range is "mapped" with ECREATE.
> But I'd argue that it's not unusual, just different.  And really it's not
> all that different than userspace mmap'ing /dev/sgx/enclave prior to
> ioctl(ENCLAVE_CREATE).  In that case, userspace can still (attempt to)
> access the "raw" EPC, i.e. generate a #PF, the kernel/driver just happens
> to consider any faulting EPC address without an associated enclave as
> illegal, e.g. signals SIGBUS.
> 
> The /dev/sgx/epc case simply has different semantics for moving pages in
> and out of the EPC, i.e. different fault and eviction semantics.  Yes,
> this allows the guest kernel to directly access the "raw" EPC, but that's
> conceptually in line with hardware where priveleged software can directly
> "access" the EPC (or rather, the abort page for all intents and purposes).
> I.e. it's an argument for requiring certain privileges to open /dev/sgx/epc,
> but IMO it's not unusual.
> 
> Maybe /dev/sgx/epc is a poor name and is causing confusion, e.g.
> /dev/sgx/virtualmachine might be more appropriate.

What do you mean by saying "requiring certain privileges"? Are you
saying that "raw EPC" (lets say /dev/vmsgx, which probably the name I
will use for the device *if* it is required) device would require
differet privileged than /dev/sgx?

/Jarkko