Re: [PATCH v6 3/8] docs: start a document to describe D-Bus usage

[Daniel P . Berrangé]

On Wed, Dec 11, 2019 at 05:45:01PM +0400, Marc-André Lureau wrote:
> Signed-off-by: Marc-André Lureau <address@hidden>
> ---
>  MAINTAINERS            |  5 +++
>  docs/interop/dbus.rst  | 99 ++++++++++++++++++++++++++++++++++++++++++
>  docs/interop/index.rst |  1 +
>  3 files changed, 105 insertions(+)
>  create mode 100644 docs/interop/dbus.rst
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 525b4740e8..19faa0e868 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -2199,6 +2199,11 @@ F: tests/migration-test.c
>  F: docs/devel/migration.rst
>  F: qapi/migration.json
>  
> +D-Bus
> +M: Marc-André Lureau <address@hidden>
> +S: Maintained
> +F: docs/interop/dbus.rst
> +
>  Seccomp
>  M: Eduardo Otubo <address@hidden>
>  S: Supported
> diff --git a/docs/interop/dbus.rst b/docs/interop/dbus.rst
> new file mode 100644
> index 0000000000..3d760e4882
> --- /dev/null
> +++ b/docs/interop/dbus.rst
> @@ -0,0 +1,99 @@
> +=====
> +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.


                                 However, because the daemon has
> +controlled who can send/recv messages to who, doesn't magically make
> +this secure. 

This reads a little awkwardly, instead:

 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

In the last point add

    ', unless additional unless additional controls
     such as SELinux or AppArmor are applied'.

> +
> + - Two UIDs, one for QEMU, one for dbus & helpers
> +
> +   - Moderately improved security isolation

s/improved/improved user based/

> +
> + - Many UIDs, one for QEMU one for dbus and one for each helpers
> +
> +   - Best security isolation

s/Best/Best user based/

> +   - 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 the QEMU project.

Perhaps change the last few words to

  'for services implemented & distributed by the QEMU project'

> diff --git a/docs/interop/index.rst b/docs/interop/index.rst
> index 3e33fb5933..ded134ea75 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
>     qemu-ga

Reviewed-by: Daniel P. Berrangé <address@hidden>


Regards,
Daniel
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