Re: Plan for /dev/ioasid RFC v2

Alex Williamson
2021-06-09T16:27:32Z

On Wed, 9 Jun 2021 10:15:32 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Wed, 9 Jun 2021 17:51:26 +0200
> Joerg Roedel <joro@8bytes.org> wrote:
>
> > On Wed, Jun 09, 2021 at 12:00:09PM -0300, Jason Gunthorpe wrote:
> > > Only *drivers* know what the actual device is going to do, devices do
> > > not. Since the group doesn't have drivers it is the wrong layer to be
> > > making choices about how to configure the IOMMU.
> >
> > Groups don't carry how to configure IOMMUs, that information is
> > mostly in the IOMMU domains. And those (or an abstraction of them) is
> > configured through /dev/ioasid. So not sure what you wanted to say with
> > the above.
> >
> > All a group carries is information about which devices are not
> > sufficiently isolated from each other and thus need to always be in the
> > same domain.
> >
> > > The device centric approach is my attempt at this, and it is pretty
> > > clean, I think.
> >
> > Clean, but still insecure.
> >
> > > All ACS does is prevent P2P operations, if you assign all the group
> > > devices into the same /dev/iommu then you may not care about that
> > > security isolation property. At the very least it is policy for user
> > > to decide, not kernel.
> >
> > It is a kernel decision, because a fundamental task of the kernel is to
> > ensure isolation between user-space tasks as good as it can. And if a
> > device assigned to one task can interfer with a device of another task
> > (e.g. by sending P2P messages), then the promise of isolation is broken.
>
> AIUI, the IOASID model will still enforce IOMMU groups, but it's not an
> explicit part of the interface like it is for vfio. For example the
> IOASID model allows attaching individual devices such that we have
> granularity to create per device IOASIDs, but all devices within an
> IOMMU group are required to be attached to an IOASID before they can be
> used. It's not entirely clear to me yet how that last bit gets
> implemented though, ie. what barrier is in place to prevent device
> usage prior to reaching this viable state.
>
> > > Groups should be primarily about isolation security, not about IOASID
> > > matching.
> >
> > That doesn't make any sense, what do you mean by 'IOASID matching'?
>
> One of the problems with the vfio interface use of groups is that we
> conflate the IOMMU group for both isolation and granularity. I think
> what Jason is referring to here is that we still want groups to be the
> basis of isolation, but we don't want a uAPI that presumes all devices
> within the group must use the same IOASID. For example, if a user owns
> an IOMMU group consisting of non-isolated functions of a multi-function
> device, they should be able to create a vIOMMU VM where each of those
> functions has its own address space. That can't be done today, the
> entire group would need to be attached to the VM under a PCIe-to-PCI
> bridge to reflect the address space limitation imposed by the vfio
> group uAPI model. Thanks,

Hmm, likely discussed previously in these threads, but I can't come up
with the argument that prevents us from making the BIND interface
at the group level but the ATTACH interface at the device level? For
example:

- VFIO_GROUP_BIND_IOASID_FD
- VFIO_DEVICE_ATTACH_IOASID

AFAICT that makes the group ownership more explicit but still allows
the device level IOASID granularity. Logically this is just an
internal iommu_group_for_each_dev() in the BIND ioctl. Thanks,

Alex