Re: [Qemu-devel] snabbswitch integration with QEMU for userspace ethernet I/O

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

On Wed, May 29, 2013 at 04:21:43PM +0200, Stefan Hajnoczi wrote:
> On Wed, May 29, 2013 at 12:08:59PM +0300, Michael S. Tsirkin wrote:
> > On Wed, May 29, 2013 at 09:49:29AM +0200, Stefan Hajnoczi wrote:
> > > On Tue, May 28, 2013 at 08:17:42PM +0300, Michael S. Tsirkin wrote:
> > > > On Tue, May 28, 2013 at 12:00:38PM -0500, Anthony Liguori wrote:
> > > > > Julian Stecklina <jsteckli@os.inf.tu-dresden.de> writes:
> > > > > 
> > > > > > On 05/28/2013 12:10 PM, Luke Gorrie wrote:
> > > > > >> On 27 May 2013 11:34, Stefan Hajnoczi <stefanha@redhat.com
> > > > > >> <mailto:stefanha@redhat.com>> wrote:
> > > > > >> 
> > > > > >>     vhost_net is about connecting the a virtio-net speaking process to a
> > > > > >>     tun-like device.  The problem you are trying to solve is connecting a
> > > > > >>     virtio-net speaking process to Snabb Switch.
> > > > > >> 
> > > > > >> 
> > > > > >> Yep!
> > > > > >
> > > > > > Since I am on a similar path as Luke, let me share another idea.
> > > > > >
> > > > > > What about extending qemu in a way to allow PCI device models to be
> > > > > > implemented in another process.
> > > > > 
> > > > > We aren't going to support any interface that enables out of tree
> > > > > devices.  This is just plugins in a different form with even more
> > > > > downsides.  You cannot easily keep track of dirty info, the guest
> > > > > physical address translation to host is difficult to keep in sync
> > > > > (imagine the complexity of memory hotplug).
> > > > > 
> > > > > Basically, it's easy to hack up but extremely hard to do something that
> > > > > works correctly overall.
> > > > > 
> > > > > There isn't a compelling reason to implement something like this other
> > > > > than avoiding getting code into QEMU.  Best to just submit your device
> > > > > to QEMU for inclusion.
> > > > > 
> > > > > If you want to avoid copying in a vswitch, better to use something like
> > > > > vmsplice as I outlined in another thread.
> > > > > 
> > > > > > This is not as hard as it may sound.
> > > > > > qemu would open a domain socket to this process and map VM memory over
> > > > > > to the other side. This can be accomplished by having file descriptors
> > > > > > in qemu to VM memory (reusing -mem-path code) and passing those over the
> > > > > > domain socket. The other side can then just mmap them. The socket would
> > > > > > also be used for configuration and I/O by the guest on the PCI
> > > > > > I/O/memory regions. You could also use this to do IRQs or use eventfds,
> > > > > > whatever works better.
> > > > > >
> > > > > > To have a zero copy userspace switch, the switch would offer virtio-net
> > > > > > devices to any qemu that wants to connect to it and implement the
> > > > > > complete device logic itself. Since it has access to all guest memory,
> > > > > > it can just do memcpy for packet data. Of course, this only works for
> > > > > > 64-bit systems, because you need vast amounts of virtual address space.
> > > > > > In my experience, doing this in userspace is _way less painful_.
> > > > > >
> > > > > > If you can get away with polling in the switch the overhead of doing all
> > > > > > this in userspace is zero. And as long as you can rate-limit explicit
> > > > > > notifications over the socket even that overhead should be okay.
> > > > > >
> > > > > > Opinions?
> > > > > 
> > > > > I don't see any compelling reason to do something like this.  It's
> > > > > jumping through a tremendous number of hoops to avoid putting code that
> > > > > belongs in QEMU in tree.
> > > > > 
> > > > > Regards,
> > > > > 
> > > > > Anthony Liguori
> > > > > 
> > > > > >
> > > > > > Julian
> > > > 
> > > > OTOH an in-tree device that runs in a separate process would
> > > > be useful e.g. for security.
> > > > For example, we could limit a virtio-net device process
> > > > to only access tap and vhost files.
> > > 
> > > For tap or vhost files only this is good for security.  I'm not sure it
> > > has many advantages over a QEMU process under SELinux though.
> > 
> > At the moment SELinux necessarily gives QEMU rights to
> > e.g. access the filesystem.
> > This process would only get access to tap and vhost.
> > 
> > We can also run it as a different user.
> > Defence in depth.
> > 
> > We can also limit e.g. the CPU of this process aggressively
> > (as it's not doing anything on data path).
> > 
> > I could go on.
> > 
> > And it's really easy too, until you want to use it in production,
> > at which point you need to cover lots of
> > nasty details like hotplug and migration.
> 
> I think there are diminishing returns.  Once QEMU is isolated so it
> cannot open arbitrary files, just has access to the resources granted by
> the management tool on startup, etc then I'm not sure it's worth the
> complexity and performance-cost of splitting the model up into even
> smaller pieces.

Well, this part is network-facing so there is some value,
to isolate it, I don't know how big it is.

> IMO there isn't a trust boundary that's worth isolating
> here (compare to sshd privilege separation where separate uids really
> make sense and are necessary, with QEMU having multiple uids that lack
> capabilities to do much doesn't win much over the SELinux setup).
>
> > > Obviously when the switch process has shared memory access to multiple
> > > guests' RAM, the security is worse than a QEMU process solution but
> > > better than a vhost kernel solution.
> > > So the security story is not a clear win.
> > > 
> > > Stefan
> > 
> > How exactly you pass packets between guest and host is very unlikely to
> > affect your security in a meaningful way.
> > 
> > Except, if you lose networking, orif it's just slow beyond any measure,
> > you are suddenly more secure against network-based attacks.
> 
> The fact that a single switch process has shared memory access to all
> guests' RAM is critical.  If the switch process is exploited, then that
> exposes other guests' data!  (Think of a multi-tenant host with guests
> belonging to different users.)
> 
> Stefan

Well local priveledge escalation bugs are common enough that you
should be very careful in any network facing application,
whether that has access to all guests when well-behaved, or not.

-- 
MST