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Re: [libvirt] NFS over AF_VSOCK in <filesystem>



On Mon, May 16, 2016 at 04:58:48PM -0700, Stefan Hajnoczi wrote:
> On Wed, May 11, 2016 at 03:42:34PM +0100, Daniel P. Berrange wrote:
> > On Tue, May 10, 2016 at 01:03:48PM +0100, Stefan Hajnoczi wrote:
> > > On Mon, May 09, 2016 at 05:18:42PM +0100, Daniel P. Berrange wrote:
> > > > On Mon, May 09, 2016 at 04:57:17PM +0100, Stefan Hajnoczi wrote:
> > > > > virtio-vsock support has been added to the nfs-ganesha NFS server.  I'm
> > > > > currently working on upstreaming virtio-vsock into Linux and QEMU.  I
> > > > > also have patches for the Linux NFS client and server.
> > > > > 
> > > > > Users wishing to share a file system with the guest will need to
> > > > > configure the NFS server.  Perhaps libvirt could handle that given that
> > > > > it already has <filesystem> syntax.
> > > > > 
> > > > > The basic task is setting up either the kernel nfsd or nfs-ganesha for
> > > > > the VM to access the NFS export(s).  When the VM is destroy the NFS
> > > > > server can be shut down.
> > > > 
> > > > Can you elaborate on the interaction between QEMU and the NFS server
> > > > on the host ? What actually needed changing in nfs-ganesha to support
> > > > virtio-vsock ?  I thought that on the host side we wouldn't need any
> > > > changes, because QEMU would just talk to a regular NFS server over
> > > > TCP, and the only virtio-vsock changes would be in QEMU and the guest
> > > > kernel.
> > > 
> > > The NFS protocol (and SUNRPC) is aware of the transport its running
> > > over.  In order to fully support the protocol it needs to know about
> > > AF_VSOCK and addressing.
> > > 
> > > The NFS server changes allow specifying an AF_VSOCK listen port.  The
> > > machine name format in /etc/exports or equivalent config also needs to
> > > support vsock.
> > 
> > So from host POV, in our current model of exposing host FS to the guest
> > where libvirt wants control over managing exports, I don't think we
> > would be inclined to use the in-kernel NFS server at all, nor would we
> > likely use the main system ganesha server instance.
> > 
> > Instead what I think we'd want is to run an isolated instance of ganesha
> > for each QEMU VM that requires filesystem passthrough. First of all this
> > removes any unpredictability around setup, as arbitrary admin config
> > changes to the default system ganesha server would not conflict with
> > settings libvirt needed to make for QEMU. Second it would let us place
> > the ganesha server associated with a VM in the same cgroup, so we can
> > ensure resources limits associated with the VM get applied. Third it
> > would let us apply the per-VM svirt MCS level to each ganesha, to
> > ensure that there's no risk of cross-VM attack vectors via the
> > ganesha services. Ideally QEMU would talk to ganesha over a private
> > UNIX domain socket though it looks like ganesha only has the ability
> > to use TCP or UDP right now, so that'd be something we need to add
> > support for.
> 
> virtio-vsock uses a vhost kernel module so that traffic comes from the
> host's network stack, not from QEMU.  So there wouldn't be a unix domain
> socket connecting the VM to ganesha (although something along those
> lines could be implemented in the future).

Hmm, are there any docs explaining the virtio-vsock architecture/setup
in a bit more detail ? It feels some undesirable for the vsock backend
to be directly connected to the host network - feels like it would be
opening avenues for attacking the host network services.

> One issue we need to consider is that the host's port namespace is
> global.  If there is an isolated ganesha for each VM then either each
> ganesha must bind to a separate port (2049, 2050, etc) or there needs to
> be some namespace/firewall magic to send traffic for 2049 to different
> ganesha processes.

Regards,
Daniel
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