ui: place a hard cap on VNC server output buffer size
    
    The previous patches fix problems with throttling of forced framebuffer updates
    and audio data capture that would cause the QEMU output buffer size to grow
    without bound. Those fixes are graceful in that once the client catches up with
    reading data from the server, everything continues operating normally.
    
    There is some data which the server sends to the client that is impractical to
    throttle. Specifically there are various pseudo framebuffer update encodings to
    inform the client of things like desktop resizes, pointer changes, audio
    playback start/stop, LED state and so on. These generally only involve sending
    a very small amount of data to the client, but a malicious guest might be able
    to do things that trigger these changes at a very high rate. Throttling them is
    not practical as missed or delayed events would cause broken behaviour for the
    client.
    
    This patch thus takes a more forceful approach of setting an absolute upper
    bound on the amount of data we permit to be present in the output buffer at
    any time. The previous patch set a threshold for throttling the output buffer
    by allowing an amount of data equivalent to one complete framebuffer update and
    one seconds worth of audio data. On top of this it allowed for one further
    forced framebuffer update to be queued.
    
    To be conservative, we thus take that throttling threshold and multiply it by
    5 to form an absolute upper bound. If this bound is hit during vnc_write() we
    forceably disconnect the client, refusing to queue further data. This limit is
    high enough that it should never be hit unless a malicious client is trying to
    exploit the sever, or the network is completely saturated preventing any sending
    of data on the socket.
    
    This completes the fix for CVE-2017-15124 started in the previous patches.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
    Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
    Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Message-id: 20171218191228.31018-12-berrange@redhat.com
    Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
    (cherry picked from commit f887cf165db20f405cb8805c716bd363aaadf815)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>