can: ucan: Fix infinite loop from zero-length messages

If a broken ucan device gets a message with the message length field set
to 0, then the driver will loop for forever in
ucan_read_bulk_callback(), hanging the system.  If the length is 0, just
skip the message and go on to the next one.

This has been fixed in the kvaser_usb driver in the past in commit
0c73772 ("can: kvaser_usb: leaf: Fix potential infinite loop in
command parsers"), so there must be some broken devices out there like
this somewhere.

Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: Vincent Mailhol <mailhol@kernel.org>
Cc: stable@kernel.org
Assisted-by: gkh_clanker_2000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/2026022319-huff-absurd-6a18@gregkh
Fixes: 9f2d3ea ("can: ucan: add driver for Theobroma Systems UCAN devices")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Author: gregkh

drivers/net/can/usb/ucan.c

@@ -748,7 +748,7 @@ static void ucan_read_bulk_callback(struct urb *urb)
 		len = le16_to_cpu(m->len);
 
 		/* check sanity (length of content) */
-		if (urb->actual_length - pos < len) {
+		if ((len == 0) || (urb->actual_length - pos < len)) {
 			netdev_warn(up->netdev,
 				    "invalid message (short; no data; l:%d)\n",
 				    urb->actual_length);