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

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (address@hidden) wrote:
> On Fri, Aug 23, 2019 at 03:26:02PM +0100, Dr. David Alan Gilbert wrote:
> > * Daniel P. Berrangé (address@hidden) wrote:
> > > 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.
> > 
> > This was really just one example of where the security/trust isn't
> > assumed; however a more concrete case is migration of a vtpm, and even
> > though it's probably encrypted blob you still don't want some other
> > device to grab the migration data - or to say reinitialise the vtpm.
> 
> That can be dealt with by the dbus security policies, provided
> you either run the vtpm as a different user ID from the other
> untrustworthy helpers, or use a different selinux context for
> vtpm. You can then express that only the user that QEMU is
> running under can talk to vtpm over dbus.

The need for the extra user ID or selinux context is a pain;
but probably warranted for the vTPM;  in general though some of this
exists because of the choice of DBus and wouldn't be a problem for
something that had a point-to-point socket it sent everything over.

> Where I think you could have problems is if you needed finer
> grainer control with selinux. eg if vstpm exports 2 different
> services, you can't allow access to one service, but forbid
> access to the other service.
> 
> > > >   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.
> > 
> > Crashing is the easy case; assume it's malicious and you don't want it
> > getting to say a storage device provided by another vhost-user device.
> 
> If we assume that the 2 processes can't commnuicate / access each
> other outside DBus, then the attack avenues added by use of dbus
> are most likely either:
> 
>  - invoking some DBus method that should not be allowed due
>    to incomplete dbus security policy. 
> 
>  - finding a crash in a dbus client library that you can somehow
>    exploit to get remote code execution in the separate process
> 
>    I won't claim this is impossible, but I think it helps to be
>    using a standard, widely used battle tested RPC impl, rather
>    than a home grown RPC protocol.

It's only the policy case I worry about; and my point here is if we
decide to use dbus then we have to think properly about security and
defined stuff.

> 
> 
> > > > > 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.
> > 
> > This all relies on SELinux and running privileged qemu/vhost-user pairs;
> > needing to do that purely to enforce security seems wrong.
> 
> Compare to an alternative bus-less solution where each helper has
> a direct UNIX socket connection to QEMU.
> 
> If two helpers are running as the same user ID, then can still
> directly attack each other via things like ptrace or /proc/$PID/mem,
> unless you've used SELinux to isolate them, or run each as a distinct
> user ID.  If you do the latter, then we can still easily isolate
> them using dbus.

You can lock those down pretty easily though.

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