staging: comedi: disallow COMEDI_DEVCONFIG on non-board minors

Comedi has two sorts of minor devices:
(a) normal board minor devices in the range 0 to
COMEDI_NUM_BOARD_MINORS-1 inclusive; and
(b) special subdevice minor devices in the range COMEDI_NUM_BOARD_MINORS
upwards that are used to open the same underlying comedi device as the
normal board minor devices, but with non-default read and write
subdevices for asynchronous commands.

The special subdevice minor devices get created when a board supporting
asynchronous commands is attached to a normal board minor device, and
destroyed when the board is detached from the normal board minor device.
One way to attach or detach a board is by using the COMEDI_DEVCONFIG
ioctl.  This should only be used on normal board minors as the special
subdevice minors are too ephemeral.  In particular, the change
introduced in commit 7d3135af ("staging:
comedi: prevent auto-unconfig of manually configured devices") breaks
horribly for special subdevice minor devices.

Since there's no legitimate use for the COMEDI_DEVCONFIG ioctl on a
special subdevice minor device node, disallow it and return -ENOTTY.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -1633,6 +1633,11 @@ static long comedi_unlocked_ioctl(struct file *file, unsigned int cmd,
 	/* Device config is special, because it must work on
 	 * an unconfigured device. */
 	if (cmd == COMEDI_DEVCONFIG) {
+		if (minor >= COMEDI_NUM_BOARD_MINORS) {
+			/* Device config not appropriate on non-board minors. */
+			rc = -ENOTTY;
+			goto done;
+		}
 		rc = do_devconfig_ioctl(dev,
 					(struct comedi_devconfig __user *)arg);
 		if (rc == 0)