Re: [RFC] /dev/ioasid uAPI proposal

Jason Gunthorpe
2021-06-04T16:03:51Z

On Fri, Jun 04, 2021 at 05:57:19PM +0200, Paolo Bonzini wrote:
> On 04/06/21 17:50, Jason Gunthorpe wrote:
> > > Extending the scenarios where WBINVD is not a nop is not a problem for me.
> > > If possible I wouldn't mind keeping the existing kvm-vfio connection via the
> > > device, if only because then the decision remains in the VFIO camp (whose
> > > judgment I trust more than mine on this kind of issue).
> > Really the question to answer is what "security proof" do you want
> > before the wbinvd can be enabled
>
> I don't want a security proof myself; I want to trust VFIO to make the right
> judgment and I'm happy to defer to it (via the KVM-VFIO device).
>
> Given how KVM is just a device driver inside Linux, VMs should be a slightly
> more roundabout way to do stuff that is accessible to bare metal; not a way
> to gain extra privilege.

Okay, fine, lets turn the question on its head then.

VFIO should provide a IOCTL VFIO_EXECUTE_WBINVD so that userspace VFIO
application can make use of no-snoop optimizations. The ability of KVM
to execute wbinvd should be tied to the ability of that IOCTL to run
in a normal process context.

So, under what conditions do we want to allow VFIO to giave a process
elevated access to the CPU:

> > 1) User has access to a device that can issue no-snoop TLPS
> > 2) User has access to an IOMMU that can not block no-snoop (today)
> > 3) Require CAP_SYS_RAW_IO
> > 4) Anyone

Jason