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

[Daniel P . Berrangé]

On Fri, Aug 23, 2019 at 03:09:48PM +0100, Dr. David Alan Gilbert wrote:
> * 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

Processes connected to dbus can only call the DBus APIs that vtpm
actually exports.  The vtpm should simply *not* export a DBus
API that allows anything to fetch the keys.

If it did want to export APIs for fetching keys, then we would
have to ensure suitable dbus /selinux policy was created to
prevent unwarranted access.

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

That's no problem - virtio-gpu crashes, it disappears from the dbus
bus, but everything else keeps running.

> > 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?

There's already two dbus-daemon's on each host - the system one and
the session one, and they get different selinux contexts,
system_dbus_t and unconfined_dbus_t.

Since libvirt would be responsible for launching these private dbus
daemons it would be easy to make it run  svirt_dbus_t for example.
Actually it would be  svirt_dbus_t:s0:cNNN,cMMM to get uniqueness
per VM.

Will of course require us to talk to the SELinux maintainers to
get some sensible policy rules created.

> > > 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-            
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> > > --
> > > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> > >
> > 
> > 
> > -- 
> > Marc-André Lureau
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK

Regards,
Daniel
-- 
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