Re: [RFC] /dev/ioasid uAPI proposal

Paolo Bonzini
2021-06-04T16:11:01Z

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
>