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From 52fe55e2bf9df408ebe127a670ee698642d3fcb4 Mon Sep 17 00:00:00 2001
From: "Daniel P. Berrange" <>
Date: Thu, 8 Feb 2018 17:50:38 +0100
Subject: [PATCH 24/27] ui: place a hard cap on VNC server output buffer size
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RH-Author: Daniel P. Berrange <>
Message-id: <>
Patchwork-id: 78957
O-Subject: [RHEL-7.5 qemu-kvm PATCH v1 24/27] ui: place a hard cap on VNC server output buffer size
Bugzilla: 1527405
RH-Acked-by: Laszlo Ersek <>
RH-Acked-by: Gerd Hoffmann <>
RH-Acked-by: Miroslav Rezanina <>

From: "Daniel P. Berrange" <>

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

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 <>
Reviewed-by: Darren Kenny <>
Reviewed-by: Marc-André Lureau <>
Signed-off-by: Gerd Hoffmann <>
(cherry picked from commit f887cf165db20f405cb8805c716bd363aaadf815)

	ui/vnc.c - context differences and no 'vs->disconnecting' flag.
	Using share_mode as a better check for the disconnecting state
	than csock == -1, because the worker thread calls vnc_write()
	with a fake VncState that has csock == -1.

Signed-off-by: Miroslav Rezanina <>
 ui/vnc.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/ui/vnc.c b/ui/vnc.c
index 96b6caf..61fbec2 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -1460,8 +1460,37 @@ void vnc_client_read(void *opaque)
+ * 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.
+ */
 void vnc_write(VncState *vs, const void *data, size_t len)
+    if (vs->share_mode == VNC_SHARE_MODE_DISCONNECTED) {
+        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->csock != -1 && buffer_empty(&vs->output)) {