Re: [Qemu-devel] [virtio-dev] [PATCH v3 0/7] Vhost-pci for inter-VM communication

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 07, 2017 at 04:29:45PM +0000, Stefan Hajnoczi wrote:
> On Thu, Dec 7, 2017 at 2:02 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Thu, Dec 07, 2017 at 01:08:04PM +0000, Stefan Hajnoczi wrote:
> >> Instead of responding individually to these points, I hope this will
> >> explain my perspective.  Let me know if you do want individual
> >> responses, I'm happy to talk more about the points above but I think
> >> the biggest difference is our perspective on this:
> >>
> >> Existing vhost-user slave code should be able to run on top of
> >> vhost-pci.  For example, QEMU's
> >> contrib/vhost-user-scsi/vhost-user-scsi.c should work inside the guest
> >> with only minimal changes to the source file (i.e. today it explicitly
> >> opens a UNIX domain socket and that should be done by libvhost-user
> >> instead).  It shouldn't be hard to add vhost-pci vfio support to
> >> contrib/libvhost-user/ alongside the existing UNIX domain socket code.
> >>
> >> This seems pretty easy to achieve with the vhost-pci PCI adapter that
> >> I've described but I'm not sure how to implement libvhost-user on top
> >> of vhost-pci vfio if the device doesn't expose the vhost-user
> >> protocol.
> >>
> >> I think this is a really important goal.  Let's use a single
> >> vhost-user software stack instead of creating a separate one for guest
> >> code only.
> >>
> >> Do you agree that the vhost-user software stack should be shared
> >> between host userspace and guest code as much as possible?
> >
> >
> >
> > The sharing you propose is not necessarily practical because the security goals
> > of the two are different.
> >
> > It seems that the best motivation presentation is still the original rfc
> >
> > http://virtualization.linux-foundation.narkive.com/A7FkzAgp/rfc-vhost-user-enhancements-for-vm2vm-communication
> >
> > So comparing with vhost-user iotlb handling is different:
> >
> > With vhost-user guest trusts the vhost-user backend on the host.
> >
> > With vhost-pci we can strive to limit the trust to qemu only.
> > The switch running within a VM does not have to be trusted.
> 
> Can you give a concrete example?
> 
> I have an idea about what you're saying but it may be wrong:
> 
> Today the iotlb mechanism in vhost-user does not actually enforce
> memory permissions.  The vhost-user slave has full access to mmapped
> memory regions even when iotlb is enabled.  Currently the iotlb just
> adds an indirection layer but no real security.  (Is this correct?)

Not exactly. iotlb protects against malicious drivers within guest.
But yes, not against a vhost-user driver on the host.

> Are you saying the vhost-pci device code in QEMU should enforce iotlb
> permissions so the vhost-user slave guest only has access to memory
> regions that are allowed by the iotlb?

Yes.

> This is a weak way to enforce memory permissions.  If the guest is
> able to exploit a bug in QEMU then it has full memory access.

That's par for the course though. We don't have many of these.  If you
assume qemu is insecure, using a theoretical kernel-based mechanism does
not add much since kernel exploits are pretty common too :).

>  It's a
> security problem waiting to happen

It's better security than running the switch on host though.

> and QEMU generally doesn't
> implement things this way.

Not sure what does this mean.

> A stronger solution is for the vhost-user master to control memory
> protection and to disallow the vhost-user slave from changing memory
> protection.  I think the kernel mechanism to support this does not
> exist today.  Such a mechanism would also make the vhost-user host
> userspace use case secure.  The kernel mechanism to do this would
> definitely be useful outside of virtualization too.
> 
> Stefan

In theory, maybe. But I'm not up to implementing this, it is very far
from trivial. We can do a QEMU based one and then add the kernel
based one on top when it surfaces.

Also I forgot - this has some performance advantages too.

-- 
MST