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

[Dr. David Alan Gilbert]

* Marc-André Lureau (address@hidden) wrote:
> Hi
> 
> On Mon, Sep 16, 2019 at 2:02 PM Dr. David Alan Gilbert
> <address@hidden> wrote:
> >
> > (Copying in Stefan since he was looking at DBus for virtiofs)
> >
> > * Marc-André Lureau (address@hidden) wrote:
> > > Signed-off-by: Marc-André Lureau <address@hidden>
> > > ---
> > >  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'. .
> 
> Do you have a specific question we can answer or guide for qemu? Is
> there something we have to document or implement?
> 
> Since qemu is not managing the extra processes or applying policies, I
> don't know what else could be done. From qemu pov, it can rely on
> management layer to trust the bus and the helpers, similar to trusting
> the system in general.

Well pretty much the same questions I asked in the discussion on v2;
what is the supported configuration to ensure that one helper that's
been compromised can't attack the others and qemu?

Dave

> > Dave
> >
> > > +Guidelines
> > > +==========
> > > +
> > > +When implementing new D-Bus interfaces, it is recommended to follow
> > > +the "D-Bus API Design Guidelines":
> > > +https://dbus.freedesktop.org/doc/dbus-api-design.html
> > > +
> > > +The "org.qemu*" prefix is reserved for the QEMU project.
> > > diff --git a/docs/interop/index.rst b/docs/interop/index.rst
> > > index b4bfcab417..fa4478ce2e 100644
> > > --- a/docs/interop/index.rst
> > > +++ b/docs/interop/index.rst
> > > @@ -13,6 +13,7 @@ Contents:
> > >     :maxdepth: 2
> > >
> > >     bitmaps
> > > +   dbus
> > >     live-block-operations
> > >     pr-helper
> > >     vhost-user
> > > --
> > > 2.23.0
> > >
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> >
> 
> 
> -- 
> Marc-André Lureau
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK