usb: gadget: u_ether: Fix MTU size mismatch with RX packet size

commit 0a88fa2 upstream.

Fix the MTU size issue with RX packet size as the host sends the packet
with extra bytes containing ethernet header. This causes failure when
user sets the MTU size to the maximum i.e. 15412. In this case the
ethernet packet received will be of length 15412 plus the ethernet header
length. This patch fixes the issue where there is a check that RX packet
length must not be more than max packet length.

Fixes: bba787a ("usb: gadget: ether: Allow jumbo frames")
Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/1605597215-122027-1-git-send-email-manish.narani@xilinx.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/u_ether.c

@@ -45,9 +45,10 @@
 #define UETH__VERSION	"29-May-2008"
 
 /* Experiments show that both Linux and Windows hosts allow up to 16k
- * frame sizes. Set the max size to 15k+52 to prevent allocating 32k
+ * frame sizes. Set the max MTU size to 15k+52 to prevent allocating 32k
  * blocks and still have efficient handling. */
-#define GETHER_MAX_ETH_FRAME_LEN 15412
+#define GETHER_MAX_MTU_SIZE 15412
+#define GETHER_MAX_ETH_FRAME_LEN (GETHER_MAX_MTU_SIZE + ETH_HLEN)
 
 struct eth_dev {
 	/* lock is held while accessing port_usb
@@ -786,7 +787,7 @@ struct eth_dev *gether_setup_name(struct usb_gadget *g,
 
 	/* MTU range: 14 - 15412 */
 	net->min_mtu = ETH_HLEN;
-	net->max_mtu = GETHER_MAX_ETH_FRAME_LEN;
+	net->max_mtu = GETHER_MAX_MTU_SIZE;
 
 	dev->gadget = g;
 	SET_NETDEV_DEV(net, &g->dev);
@@ -848,7 +849,7 @@ struct net_device *gether_setup_name_default(const char *netname)
 
 	/* MTU range: 14 - 15412 */
 	net->min_mtu = ETH_HLEN;
-	net->max_mtu = GETHER_MAX_ETH_FRAME_LEN;
+	net->max_mtu = GETHER_MAX_MTU_SIZE;
 
 	return net;
 }