net: cdc_mbim: reject IP packets on DSS VLANs

DSS VLANs are pseudo network interfaces representing arbitrary
data streams, and specifically not IP. Preventing spurious IP
packets can sometimes be a hassle. The kernel will for example
send an IPv6 Router Solicit when the interface is brought up
unless the user has been careful enough to disable IPv6 first.
Such packets forwared to a MBIM DSS session will look like
spurious noise to the device, and can cause it to log an error
or even malfunction.

Drop all IP packets on the designated DSS VLANs to prevent such
unwanted spurious transmissions.

Cc: Greg Suarez <gsuarez@smithmicro.com>
Reported-by: NArnaud Desmier <adesmier@sequans.com>
Signed-off-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/cdc_mbim.c

@@ -237,6 +237,8 @@ static struct sk_buff *cdc_mbim_tx_fixup(struct usbnet *dev, struct sk_buff *skb
 			c[3] = tci;
 			break;
 		case 0x0100: /* VLAN ID 256 - 511 */
+			if (is_ip)
+				goto error;
 			sign = cpu_to_le32(USB_CDC_MBIM_NDP16_DSS_SIGN);
 			c = (u8 *)&sign;
 			c[3] = tci;