Re: [Qemu-devel] [PATCH v2 0/2] Add dbus-vmstate

[Dr. David Alan Gilbert]

* Marc-André Lureau (address@hidden) wrote:
> Hi
> 
> On Fri, Aug 23, 2019 at 5:00 PM Dr. David Alan Gilbert
> <address@hidden> wrote:
> >
> > * Daniel P. Berrangé (address@hidden) wrote:
> >
> > <snip>
> >
> > > This means QEMU still has to iterate over every single client
> > > on the bus to identify them. If you're doing that, there's
> > > no point in owning a well known service at all. Just iterate
> > > over the unique bus names and look for the exported object
> > > path /org/qemu/VMState
> > >
> >
> > Not knowing anything about DBus security, I want to ask how do
> > we handle security here?
> 
> First of all, we are talking about cooperative processes, and having a
> specific bus for each qemu instance. So some amount of security/trust
> is already assumed.

Some but we need to keep it as limited as possible; for example two
reasons for having separate processes both come down to security:

  a) vtpm - however screwy the qemu is, you can never get to the keys in
the vtpm

  b) virtio-gpu, loads of complex GPU code that can't break the main
qemu process.

> But if necessary, dbus can enforce policies on who is allowed to own a
> name, or to send/receive message from. As far as I know, this is
> mostly user/group policies.
> 
> But there is also SELinux checks to send_msg and acquire_svc (see
> dbus-daemon(1))

But how does something like SELinux interact with a private dbus 
rather than the system dbus?

> >
> > I want to know that the external device that's giving me migration data
> > is the device I think I'm speaking to, not one of the other devices;
> 
> DBus is not the problem nor the solution here.

Well, if the migration data was squirting down the existing vhost-user
channel then there would be no risk here; so the use of dbus is creating
the problem.

> But what defines that device-service strong relationship? Can you
> generalize it? I don't think so.
> 
> What DBus can guarantee is that the unique-id you are talking to is
> always the same connection (thus the same process).
> 
> > I also dont want different devices chatting to each other over dbus
> > unless we're very careful.
> 
> That's a bus policy job.

OK, as long as you somehow set it up.

Dave

> >
> > Dave
> >
> > > Regards,
> > > Daniel
> > > --
> > > |: https://berrange.com      -o-    
> > > https://www.flickr.com/photos/dberrange :|
> > > |: https://libvirt.org         -o-            
> > > https://fstop138.berrange.com :|
> > > |: https://entangle-photo.org    -o-    
> > > https://www.instagram.com/dberrange :|
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> >
> 
> 
> -- 
> Marc-André Lureau
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK