USB: serial: io_ti: fix information leak in completion handler

commit 654b404f upstream.

Add missing sanity check to the bulk-in completion handler to avoid an
integer underflow that can be triggered by a malicious device.

This avoids leaking 128 kB of memory content from after the URB transfer
buffer to user space.

Fixes: 8c209e67 ("USB: make actual_length in struct urb field u32")
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/serial/io_ti.c

@@ -1766,7 +1766,7 @@ static void edge_bulk_in_callback(struct urb *urb)
 
 	port_number = edge_port->port->number - edge_port->port->serial->minor;
 
-	if (edge_port->lsr_event) {
+	if (urb->actual_length > 0 && edge_port->lsr_event) {
 		edge_port->lsr_event = 0;
 		dbg("%s ===== Port %u LSR Status = %02x, Data = %02x ======",
 		     __func__, port_number, edge_port->lsr_mask, *data);