usbnet: ipheth: drop RX URBs with no payload

BugLink: https://bugs.launchpad.net/bugs/2086138

[ Upstream commit 94d7eeb6c0ef0310992944f0d0296929816a2cb0 ]

On iPhone 15 Pro Max one can observe periodic URBs with no payload
on the "bulk in" (RX) endpoint. These don't seem to do anything
meaningful. Reproduced on iOS 17.5.1 and 17.6.

This behaviour isn't observed on iPhone 11 on the same iOS version. The
nature of these zero-length URBs is so far unknown.

Drop RX URBs with no payload.

Signed-off-by: Foster Snowhill <forst@pen.gy>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>

drivers/net/usb/ipheth.c

@@ -286,6 +286,12 @@ static void ipheth_rcvbulk_callback(struct urb *urb)
 		return;
 	}
 
+	/* iPhone may periodically send URBs with no payload
+	 * on the "bulk in" endpoint. It is safe to ignore them.
+	 */
+	if (urb->actual_length == 0)
+		goto rx_submit;
+
 	/* RX URBs starting with 0x00 0x01 do not encapsulate Ethernet frames,
 	 * but rather are control frames. Their purpose is not documented, and
 	 * they don't affect driver functionality, okay to drop them.