Re: [RFC PATCH 03/23] x86/sgx: Introduce virtual EPC for use by KVM guests

Sean Christopherson
2021-01-15T21:46:27Z

On Sat, Jan 16, 2021, Kai Huang wrote:
> On Fri, 15 Jan 2021 07:39:44 -0800 Dave Hansen wrote:
> > On 1/15/21 6:07 AM, Kai Huang wrote:
> > >>From virtual EPC's perspective, if we don't force this in kernel, then
> > > *theoretically*, userspace can use fork() to make multiple VMs map to the
> > > same physical EPC, which will potentially cause enclaves in all VMs to behave
> > > abnormally. So to me, from this perspective, it's better to enforce in kernel
> > > so that only first VM can use this virtual EPC instance, because EPC by
> > > architectural design cannot be shared.
> > >
> > > But as Sean said, KVM doesn't support VM across multiple mm structs. And if I
> > > read code correctly, KVM doesn't support userspace to use fork() to create new
> > > VM. For instance, when creating VM, KVM grabs current->mm and keeps it in
> > > 'struct kvm' for bookkeeping, and kvm_vcpu_ioctl() and kvm_device_ioctl() will
> > > refuse to work if kvm->mm doesn't equal to current->mm. So in practice, I
> > > believe w/o enforcing this in kernel, we should also have no problem here.
> > >
> > > Sean, please correct me if I am wrong.
> > >
> > > Dave, if above stands, do you think it is reasonable to keep current->mm in
> > > epc->mm and enforce in sgx_virt_epc_mmap()?
> >
> > Everything you wrote above tells me the kernel should not be enforcing
> > the behavior. You basically said that it's only a theoretical problem,
> > and old if someone goes and does something with KVM that's nobody can do
> > today.
> >
> > You've 100% convinced me that having the kernel enforce this is
> > *un*reasonable.
>
> Sean, I'll remove epc->mm, unless I see your further objection.

It's probably ok. I guess worst case scenario, to avoid the mm tracking
nightmare for oversubscription, you could prevent attaching KVM to a virtual EPC
if there is already a mm associated with the EPC, or if there are already EPC
pages "in" the virt EPC.