docs: start a document to describe D-Bus usage

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

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+=====
+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.
+
+The daemon can control which peers can send/recv messages using
+various metadata attributes, however, this is alone is not generally
+sufficient to make the deployment secure.  The semantics of the actual
+methods implemented using D-Bus are just as critical. Peers need to
+carefully validate any information they received from a peer with a
+different trust level.
+
+dbus-daemon policy
+------------------
+
+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.
+
+Depending on the use case, you may choose different scenarios:
+
+ - Everything the same UID
+
+   - Convenient for developers
+   - Improved reliability - crash of one part doens't take
+     out entire VM
+   - No security benefit over traditional QEMU, unless additional
+     unless additional controls such as SELinux or AppArmor are
+     applied
+
+ - Two UIDs, one for QEMU, one for dbus & helpers
+
+   - Moderately improved user based security isolation
+
+ - Many UIDs, one for QEMU one for dbus and one for each helpers
+
+   - Best user based security isolation
+   - Complex to manager distinct UIDs needed for each VM
+
+For example, to allow only ``qemu`` user to talk to ``qemu-helper``
+``org.qemu.Helper1`` service, a dbus-daemon policy may contain:
+
+.. 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``.
+
+See dbus-daemon man page for details.
+
+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 services implemented &
+distributed by the QEMU project.