[Xen-changelog] [qemu-xen master] ui: place a hard cap on VNC server output buffer size

[patchbot@xxxxxxx]

commit 0c85a40e710d4b71656b28f1f5e1ae5e3780d369
Author:     Daniel P. Berrange <berrange@xxxxxxxxxx>
AuthorDate: Mon Dec 18 19:12:26 2017 +0000
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Mon Feb 12 18:34:26 2018 -0600

    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@xxxxxxxxxx>
    Reviewed-by: Darren Kenny <darren.kenny@xxxxxxxxxx>
    Reviewed-by: Marc-André Lureau <marcandre.lureau@xxxxxxxxxx>
    Message-id: 20171218191228.31018-12-berrange@xxxxxxxxxx
    Signed-off-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
    (cherry picked from commit f887cf165db20f405cb8805c716bd363aaadf815)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 ui/vnc.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/ui/vnc.c b/ui/vnc.c
index 4021c011..a4f0279 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -1521,8 +1521,37 @@ gboolean vnc_client_io(QIOChannel *ioc G_GNUC_UNUSED,
 }
 
 
+/*
+ * Scale factor to apply to vs->throttle_output_offset when checking for
+ * hard limit. Worst case normal usage could be x2, if we have a complete
+ * incremental update and complete forced update in the output buffer.
+ * So x3 should be good enough, but we pick x5 to be conservative and thus
+ * (hopefully) never trigger incorrectly.
+ */
+#define VNC_THROTTLE_OUTPUT_LIMIT_SCALE 5
+
 void vnc_write(VncState *vs, const void *data, size_t len)
 {
+    if (vs->disconnecting) {
+        return;
+    }
+    /* Protection against malicious client/guest to prevent our output
+     * buffer growing without bound if client stops reading data. This
+     * should rarely trigger, because we have earlier throttling code
+     * which stops issuing framebuffer updates and drops audio data
+     * if the throttle_output_offset value is exceeded. So we only reach
+     * this higher level if a huge number of pseudo-encodings get
+     * triggered while data can't be sent on the socket.
+     *
+     * NB throttle_output_offset can be zero during early protocol
+     * handshake, or from the job thread's VncState clone
+     */
+    if (vs->throttle_output_offset != 0 &&
+        vs->output.offset > (vs->throttle_output_offset *
+                             VNC_THROTTLE_OUTPUT_LIMIT_SCALE)) {
+        vnc_disconnect_start(vs);
+        return;
+    }
     buffer_reserve(&vs->output, len);
 
     if (vs->ioc != NULL && buffer_empty(&vs->output)) {
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#master

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