ui: avoid crash if vnc client disconnects with writes pending

The vnc_client_read() function is called from the vnc_client_io()
event handler callback when there is incoming data to process.
If it detects that the client has disconnected, then it will
trigger cleanup and free'ing of the VncState client struct at
a safe time.

Unfortunately, the vnc_client_io() event handler will also call
vnc_client_write() to handle any outgoing data writes. So if
vnc_client_io() was invoked with both G_IO_IN and G_IO_OUT
events set, and the client disconnects, we may try to write to
a client which has just been freed.

https://bugs.launchpad.net/qemu/+bug/1594861Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Message-id: 1467042529-3372-1-git-send-email-berrange@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/vnc.c

@@ -1436,8 +1436,9 @@ static void vnc_jobs_bh(void *opaque)
  * First function called whenever there is more data to be read from
  * the client socket. Will delegate actual work according to whether
  * SASL SSF layers are enabled (thus requiring decryption calls)
+ * Returns 0 on success, -1 if client disconnected
  */
-static void vnc_client_read(VncState *vs)
+static int vnc_client_read(VncState *vs)
 {
     ssize_t ret;
 
@@ -1450,8 +1451,9 @@ static void vnc_client_read(VncState *vs)
     if (!ret) {
         if (vs->disconnecting) {
             vnc_disconnect_finish(vs);
+            return -1;
         }
-        return;
+        return 0;
     }
 
     while (vs->read_handler && vs->input.offset >= vs->read_handler_expect) {
@@ -1461,7 +1463,7 @@ static void vnc_client_read(VncState *vs)
         ret = vs->read_handler(vs, vs->input.buffer, len);
         if (vs->disconnecting) {
             vnc_disconnect_finish(vs);
-            return;
+            return -1;
         }
 
         if (!ret) {
@@ -1470,6 +1472,7 @@ static void vnc_client_read(VncState *vs)
             vs->read_handler_expect = ret;
         }
     }
+    return 0;
 }
 
 gboolean vnc_client_io(QIOChannel *ioc G_GNUC_UNUSED,
@@ -1477,7 +1480,9 @@ gboolean vnc_client_io(QIOChannel *ioc G_GNUC_UNUSED,
 {
     VncState *vs = opaque;
     if (condition & G_IO_IN) {
-        vnc_client_read(vs);
+        if (vnc_client_read(vs) < 0) {
+            return TRUE;
+        }
     }
     if (condition & G_IO_OUT) {
         vnc_client_write(vs);