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

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (address@hidden) wrote:
> 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 (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'. .
> 
> Thinking about DBus usage with helpers, as compared to the current state
> with monolithic QEMU, the top priority is to ensure no degradation in
> security vs current practice.
> 
> That is fairly easy from libvirt's POV - we simply need to make sure
> that the dbus daemon and all helpers get given the same SELinux svirt_t
> content as used for QEMU, so each QEMU is still siloed to the same
> extent.
> 
> If SELinux is not enabled, then currently an out of the box libvirt
> config only protects the host from QEMU, it doesn't protect QEMU
> from other QEMUs, since they all run the same user ID.
>
> It is possible to tell libvirt to run each QEMU as a separate user
> ID if the mgmt app has a range of user IDs avalable. In this case,
> we would simply run the helpers/dbus as the same per-QEMU user ID
> to ensure we don't regress.
> 
> 
> Getting an improved security model is obviously the ultimate goal,
> as this modularity needs to offer some benefit to outweight its
> costs.
> 
> In terms of SELinux, this will involve creating distinct SElinux
> contexts for each helper process. (svirt_slirp_t, svirt_swtpm_t,
> etc, etc).
> 
> In terms of DAC, in the per QEMU user ID scenario,  we would need
> to allocate at least 2 UIDs for each QEMU process, so that helpers
> would be separate from the QEMU. To be honest it would be better
> if we had 3 UIDs, to the dbus daemon was separated from both the
> helpers and QEMU.
> 
> This starts to sound like alot of UIDs which is tedious to manage.
> Libvirt already puts QEMU in a separate mount namespace. From a
> DAC POV, to get meaninguful separation will probably want libvirt
> to consider the "user" namespace too. This is quite a bit of work
> to get everything labelled right for  different user namespace,
> but it may well simplify mgmt thereafter. We still have the same
> problem though, of needing to assign a range of user IDs for each
> user namespace.

A separate user namespace might cause problems for things like
virtiofs where it's trying to access the files with particular perms,
or with say a GPU where it needs access to a display.

Dave

> Overall, I can see the possible technical options for securing
> this use of DBus, so I'm not too concerned here.
> 
> Regards,
> Daniel
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK