ui/vnc: limit client_cut_text msg payload size

currently a malicious client could define a payload
size of 2^32 - 1 bytes and send up to that size of
data to the vnc server. The server would allocated
that amount of memory which could easily create an
out of memory condition.

This patch limits the payload size to 1MB max.

Please note that client_cut_text messages are currently
silently ignored.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/vnc.c

@@ -2165,13 +2165,20 @@ static int protocol_client_msg(VncState *vs, uint8_t *data, size_t len)
         pointer_event(vs, read_u8(data, 1), read_u16(data, 2), read_u16(data, 4));
         break;
     case VNC_MSG_CLIENT_CUT_TEXT:
-        if (len == 1)
+        if (len == 1) {
             return 8;
-
+        }
         if (len == 8) {
             uint32_t dlen = read_u32(data, 4);
-            if (dlen > 0)
+            if (dlen > (1 << 20)) {
+                error_report("vnc: client_cut_text msg payload has %u bytes"
+                             " which exceeds our limit of 1MB.", dlen);
+                vnc_client_error(vs);
+                break;
+            }
+            if (dlen > 0) {
                 return 8 + dlen;
+            }
         }
 
         client_cut_text(vs, read_u32(data, 4), data + 8);