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

[Dr. David Alan Gilbert]

(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'. .

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