rtl8139: limit transmission buffer size in c+ mode

The tx buffer would be re-allocated for tx descriptor with big size
and without LS bit set, this would make guest driver could easily let
qemu to allocate unlimited.

In linux host, a glib failure were easy to be triggered:

GLib-ERROR **: gmem.c:176: failed to allocate 18446744071562067968 bytes

This patch fix this by adding a limit. As the spec didn't tell the maximum size
of buffer allowed, stick it to current CP_TX_BUFFER_SIZE (65536).

Changes from V1:

Drop the while statement and s->cplus_txbuffer check.
Signed-off-by: NJason Wang <jasowang@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/rtl8139.c

@@ -2061,13 +2061,12 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s)
             s->cplus_txbuffer_len);
     }
 
-    while (s->cplus_txbuffer && s->cplus_txbuffer_offset + txsize >= s->cplus_txbuffer_len)
+    if (s->cplus_txbuffer_offset + txsize >= s->cplus_txbuffer_len)
     {
-        s->cplus_txbuffer_len += CP_TX_BUFFER_SIZE;
-        s->cplus_txbuffer = g_realloc(s->cplus_txbuffer, s->cplus_txbuffer_len);
-
-        DPRINTF("+++ C+ mode transmission buffer space changed to %d\n",
-            s->cplus_txbuffer_len);
+        /* The spec didn't tell the maximum size, stick to CP_TX_BUFFER_SIZE */
+        txsize = s->cplus_txbuffer_len - s->cplus_txbuffer_offset;
+        DPRINTF("+++ C+ mode transmission buffer overrun, truncated descriptor"
+                "length to %d\n", txsize);
     }
 
     if (!s->cplus_txbuffer)