net: qmi_wwan: prevent duplicate mac address on link (firmware bug workaround)

We normally trust and use the CDC functional descriptors provided by a
number of devices.  But some of these will erroneously list the address
reserved for the device end of the link.  Attempting to use this on
both the device and host side will naturally not work.

Work around this bug by ignoring the functional descriptor and assign a
random address instead in this case.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/qmi_wwan.c

@@ -53,6 +53,9 @@ struct qmi_wwan_state {
 	struct usb_interface *data;
 };
 
+/* default ethernet address used by the modem */
+static const u8 default_modem_addr[ETH_ALEN] = {0x02, 0x50, 0xf3};
+
 /* Make up an ethernet header if the packet doesn't have one.
  *
  * A firmware bug common among several devices cause them to send raw
@@ -317,6 +320,12 @@ next_desc:
 		usb_driver_release_interface(driver, info->data);
 	}
 
+	/* Never use the same address on both ends of the link, even
+	 * if the buggy firmware told us to.
+	 */
+	if (!compare_ether_addr(dev->net->dev_addr, default_modem_addr))
+		eth_hw_addr_random(dev->net);
+
 	/* make MAC addr easily distinguishable from an IP header */
 	if (possibly_iphdr(dev->net->dev_addr)) {
 		dev->net->dev_addr[0] |= 0x02;	/* set local assignment bit */