Re: vnc clipboard support

[Daniel P . Berrangé]

On Thu, Jan 28, 2021 at 06:57:24PM +0100, Laszlo Ersek wrote:
> On 01/28/21 18:12, Gerd Hoffmann wrote:
> >   Hi folks,
> > 
> > I'm looking for a good way to implement cut+paste support for vnc.
> > 
> > The vnc core protocol has support for text/plain cut+paste, and there
> > is an extension adding support for other formats.  That'll cover one
> > part of the problem, exchanging cut+paste data between vnc client and
> > qemu vnc server.
> > 
> > The tricky part is the second: the guest <=> qemu communication.
> > I see basically two possible approaches here:
> > 
> >   (1) Have some guest agent (spice does it that way).
> >       Advantage: more flexible, allows more features.
> >       Disadvantage: requires agent in the guest.
> >   
> >   (2) Send text as key events.
> >       Advantage: no guest agent needed.
> >       Disadvantage: is translated by guests keyboard map, so qemu
> >       needs to know the map for proper char -> key event translation.
> >       Only works for text/plain and only for chars you can easily
> >       type, anything needing input methods (emoji 😊 for example)
> >       isn't going to fly.
> > 
> > I think that (1) is clearly the better way.  Given that the agent
> > would need to run in user wayland/xorg session context the existing
> > qemu-guest-agent will not work.  Also linking against some UI library
> > like gtk3 for clipboard handling is not something we want for the
> > qemu-guest-agent.  So we need another one, I'll name it
> > qemu-clipboard-agent for the rest of this mail.  And we need a
> > communication channel.
> > 
> > I'd tend to model the qemu-clipboard-agent simliar to the
> > qemu-guest-agent, i.e. have some stream as communication path and run
> > some stream protocol over it.
> > 
> > Stream options I see are (in order of personal preference):
> > 
> >   (1) New virtio-serial port.  virtio-serial likely is there anyway
> >       for the qemu-guest-agent ...
> > 
> >   (2) Have qemu-clipboard-agent and qemu-guest-agent share the agent
> >       channel, i.e. qemu-clipboard-agent will proxy everything through
> >       qemu-guest-agent (spice does it that way).
> > 
> > Protocol options I see are (not sure yet which to prefer, need to have
> > a closer look at the candidates):
> > 
> >   (1) Add clipboard commands to QMP and use these.
> > 
> >   (2) Reuse the clipboard bits of the vnc protocol (forward
> >       VNC_MSG_CLIENT_CUT_TEXT messages to the guest agent)
> > 
> >   (3) Reuse the clipboard bits of the spice-agent protocol.
> > 
> >   (4) Reuse the clipboard bits of the wayland protocol.
> > 
> > Once we have sorted the qemu <-> guest communication path it should be
> > possible to also hook up other UIs (specifically gtk) without too much
> > effort.  Which probably makes (2) a rather poor choice.
> > 
> > Comments?
> > Suggestions?
> > Other ideas?
> 
> Just a random thought: the guest should not be able to sniff, steal, or
> overwrite host-side clipboard (selection) content, without the host user
> explicitly requesting a clipboard operation. I understand this is
> generally a problem between X11 applications (not so on Wayland, if I
> understand correctly), but I assume it becomes graver with
> virtualization -- there shouldn't be an unintended channel between
> host/guest, or even multiple guests. (Analogy: you can "ssh -X -Y" to
> two remote machines at the same time, and assuming "xauth" etc on both
> of those remote machines, you can run "gedit" on each of those remote
> machines, and cut'n'paste will work between them, *even if* you don't
> ask for it -- that's why ssh has a separate option called "-Y".
> Cut'n'paste does not work without "-Y". So I believe this is something
> to consider here.)

Yes, that's a known risk. SPICE lets you disable clipboard synchronization
in QEMU, and any client app should also support doing the same. Basically
only want clipboard sync if you're using a trustworthy guest. eg where
both host and guest admin are the same person, and even then you might
not want it sometimes.


Regards,
Daniel
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