Re: [Qemu-devel] [PATCH v3 5/6] docs: start a document to describe D-Bus usage

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Sep 16, 2019 at 11:00:35AM +0100, Dr. David Alan Gilbert wrote:
> (Copying in Stefan since he was looking at DBus for virtiofs)
> 
> * Marc-André Lureau (marcandre.lureau@redhat.com) wrote:
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > ---
> >  docs/interop/dbus.rst  | 73 ++++++++++++++++++++++++++++++++++++++++++
> >  docs/interop/index.rst |  1 +
> >  2 files changed, 74 insertions(+)
> >  create mode 100644 docs/interop/dbus.rst
> > 
> > diff --git a/docs/interop/dbus.rst b/docs/interop/dbus.rst
> > new file mode 100644
> > index 0000000000..c08f026edc
> > --- /dev/null
> > +++ b/docs/interop/dbus.rst
> > @@ -0,0 +1,73 @@
> > +=====
> > +D-Bus
> > +=====
> > +
> > +Introduction
> > +============
> > +
> > +QEMU may be running with various helper processes involved:
> > + - vhost-user* processes (gpu, virtfs, input, etc...)
> > + - TPM emulation (or other devices)
> > + - user networking (slirp)
> > + - network services (DHCP/DNS, samba/ftp etc)
> > + - background tasks (compression, streaming etc)
> > + - client UI
> > + - admin & cli
> > +
> > +Having several processes allows stricter security rules, as well as
> > +greater modularity.
> > +
> > +While QEMU itself uses QMP as primary IPC (and Spice/VNC for remote
> > +display), D-Bus is the de facto IPC of choice on Unix systems. The
> > +wire format is machine friendly, good bindings exist for various
> > +languages, and there are various tools available.
> > +
> > +Using a bus, helper processes can discover and communicate with each
> > +other easily, without going through QEMU. The bus topology is also
> > +easier to apprehend and debug than a mesh. However, it is wise to
> > +consider the security aspects of it.
> > +
> > +Security
> > +========
> > +
> > +A QEMU D-Bus bus should be private to a single VM. Thus, only
> > +cooperative tasks are running on the same bus to serve the VM.
> > +
> > +D-Bus, the protocol and standard, doesn't have mechanisms to enforce
> > +security between peers once the connection is established. Peers may
> > +have additional mechanisms to enforce security rules, based for
> > +example on UNIX credentials.
> > +
> > +dbus-daemon can enforce various policies based on the UID/GID of the
> > +processes that are connected to it. It is thus a good idea to run
> > +helpers as different UID from QEMU and set appropriate policies (so
> > +helper processes are only allowed to talk to qemu for example).
> > +
> > +For example, this allows only ``qemu`` user to talk to ``qemu-helper``
> > +``org.qemu.Helper1`` service:
> > +
> > +.. code:: xml
> > +
> > +  <policy user="qemu">
> > +     <allow send_destination="org.qemu.Helper1"/>
> > +     <allow receive_sender="org.qemu.Helper1"/>
> > +  </policy>
> > +
> > +  <policy user="qemu-helper">
> > +     <allow own="org.qemu.Helper1"/>
> > +  </policy>
> > +
> > +
> > +dbus-daemon can also perfom SELinux checks based on the security
> > +context of the source and the target. For example, ``virtiofs_t``
> > +could be allowed to send a message to ``svirt_t``, but ``virtiofs_t``
> > +wouldn't be allowed to send a message to ``virtiofs_t``.
> 
> I think we need to start thinking about this more now rather than
> 'can'. .

virtiofsd has two DBus interfaces:

1. org.qemu.Virtiofsd - the management interface

   We don't expect QEMU to communicate with this.  Administrators or
   management tools will connect to this.

2. dbus-vmstate - we'll probably need this for live migration

   This is for QEMU<->vhost-user device backend communication.

Stefan

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