Re: [RFC] /dev/ioasid uAPI proposal

[Paolo Bonzini]

On 04/06/21 18:03, Jason Gunthorpe wrote:
> On Fri, Jun 04, 2021 at 05:57:19PM +0200, Paolo Bonzini wrote:
> > 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:

Ok, I would definitely not want to tie it *only* to CAP_SYS_RAWIO (i.e. 
#2+#3 would be worse than what we have today), but IIUC the proposal 
(was it yours or Kevin's?) was to keep #2 and add #1 with an 
enable/disable ioctl, which then would be on VFIO and not on KVM.  I 
assumed Alex was more or less okay with it, given he included me in the 
discussion.

If later y'all switch to "it's always okay to issue the enable/disable 
ioctl", I guess the rationale would be documented in the commit message.

Paolo

> > >    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