qmi_wwan: add support for QMAP padding in the RX path

The QMAP code in the qmi_wwan driver is based on the CodeAurora GobiNet
driver which does not process QMAP padding in the RX path correctly.
Add support for QMAP padding to qmimux_rx_fixup() according to the
description of the rmnet driver.

Fixes: c6adf779 ("net: usb: qmi_wwan: add qmap mux protocol support")
Cc: Daniele Palmas <dnlplm@gmail.com>
Signed-off-by: Reinhard Speyerer <rspmn@arcor.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/qmi_wwan.c

@@ -153,7 +153,7 @@ static bool qmimux_has_slaves(struct usbnet *dev)
 
 static int qmimux_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 {
-	unsigned int len, offset = 0;
+	unsigned int len, offset = 0, pad_len, pkt_len;
 	struct qmimux_hdr *hdr;
 	struct net_device *net;
 	struct sk_buff *skbn;
@@ -171,10 +171,16 @@ static int qmimux_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 		if (hdr->pad & 0x80)
 			goto skip;
 
+		/* extract padding length and check for valid length info */
+		pad_len = hdr->pad & 0x3f;
+		if (len == 0 || pad_len >= len)
+			goto skip;
+		pkt_len = len - pad_len;
+
 		net = qmimux_find_dev(dev, hdr->mux_id);
 		if (!net)
 			goto skip;
-		skbn = netdev_alloc_skb(net, len);
+		skbn = netdev_alloc_skb(net, pkt_len);
 		if (!skbn)
 			return 0;
 		skbn->dev = net;
@@ -191,7 +197,7 @@ static int qmimux_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 			goto skip;
 		}
 
-		skb_put_data(skbn, skb->data + offset + qmimux_hdr_sz, len);
+		skb_put_data(skbn, skb->data + offset + qmimux_hdr_sz, pkt_len);
 		if (netif_rx(skbn) != NET_RX_SUCCESS)
 			return 0;