net: usb: ax88179_178a: fix packet alignment padding

BugLink: https://bugs.launchpad.net/bugs/1885932

[ Upstream commit e869e7a1 ]

Using a AX88179 device (0b95:1790), I see two bytes of appended data on
every RX packet. For example, this 48-byte ping, using 0xff as a
payload byte:

  04:20:22.528472 IP 192.168.1.1 > 192.168.1.2: ICMP echo request, id 2447, seq 1, length 64
	0x0000:  000a cd35 ea50 000a cd35 ea4f 0800 4500
	0x0010:  0054 c116 4000 4001 f63e c0a8 0101 c0a8
	0x0020:  0102 0800 b633 098f 0001 87ea cd5e 0000
	0x0030:  0000 dcf2 0600 0000 0000 ffff ffff ffff
	0x0040:  ffff ffff ffff ffff ffff ffff ffff ffff
	0x0050:  ffff ffff ffff ffff ffff ffff ffff ffff
	0x0060:  ffff 961f

Those last two bytes - 96 1f - aren't part of the original packet.

In the ax88179 RX path, the usbnet rx_fixup function trims a 2-byte
'alignment pseudo header' from the start of the packet, and sets the
length from a per-packet field populated by hardware. It looks like that
length field *includes* the 2-byte header; the current driver assumes
that it's excluded.

This change trims the 2-byte alignment header after we've set the packet
length, so the resulting packet length is correct. While we're moving
the comment around, this also fixes the spelling of 'pseudo'.
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

drivers/net/usb/ax88179_178a.c

@@ -1396,10 +1396,10 @@ static int ax88179_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 		}
 
 		if (pkt_cnt == 0) {
-			/* Skip IP alignment psudo header */
-			skb_pull(skb, 2);
 			skb->len = pkt_len;
-			skb_set_tail_pointer(skb, pkt_len);
+			/* Skip IP alignment pseudo header */
+			skb_pull(skb, 2);
+			skb_set_tail_pointer(skb, skb->len);
 			skb->truesize = pkt_len + sizeof(struct sk_buff);
 			ax88179_rx_checksum(skb, pkt_hdr);
 			return 1;
@@ -1408,8 +1408,9 @@ static int ax88179_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 		ax_skb = skb_clone(skb, GFP_ATOMIC);
 		if (ax_skb) {
 			ax_skb->len = pkt_len;
-			ax_skb->data = skb->data + 2;
-			skb_set_tail_pointer(ax_skb, pkt_len);
+			/* Skip IP alignment pseudo header */
+			skb_pull(ax_skb, 2);
+			skb_set_tail_pointer(ax_skb, ax_skb->len);
 			ax_skb->truesize = pkt_len + sizeof(struct sk_buff);
 			ax88179_rx_checksum(ax_skb, pkt_hdr);
 			usbnet_skb_return(dev, ax_skb);