s390/cio: fix out-of-bounds access on cio_ignore free

The channel-subsystem-driver scans for newly available devices whenever
device-IDs are removed from the cio_ignore list using a command such as:

  echo free >/proc/cio_ignore

Since an I/O device scan might interfer with running I/Os, commit
172da89e ("s390/cio: avoid excessive path-verification requests")
introduced an optimization to exclude online devices from the scan.

The newly added check for online devices incorrectly assumes that
an I/O-subchannel's drvdata points to a struct io_subchannel_private.
For devices that are bound to a non-default I/O subchannel driver, such
as the vfio_ccw driver, this results in an out-of-bounds read access
during each scan.

Fix this by changing the scan logic to rely on a driver-independent
online indication. For this we can use struct subchannel->config.ena,
which is the driver's requested subchannel-enabled state. Since I/Os
can only be started on enabled subchannels, this matches the intent
of the original optimization of not scanning devices where I/O might
be running.

Fixes: 172da89e ("s390/cio: avoid excessive path-verification requests")
Fixes: 0c3812c3 ("s390/cio: derive cdev information only for IO-subchannels")
Cc: <stable@vger.kernel.org> # v5.15
Reported-by: NAlexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: NVineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: NPeter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: NVasily Gorbik <gor@linux.ibm.com>

drivers/s390/cio/css.c

@@ -753,13 +753,9 @@ static int __unset_online(struct device *dev, void *data)
 {
 	struct idset *set = data;
 	struct subchannel *sch = to_subchannel(dev);
-	struct ccw_device *cdev;
 
-	if (sch->st == SUBCHANNEL_TYPE_IO) {
-		cdev = sch_get_cdev(sch);
-		if (cdev && cdev->online)
-			idset_sch_del(set, sch->schid);
-	}
+	if (sch->st == SUBCHANNEL_TYPE_IO && sch->config.ena)
+		idset_sch_del(set, sch->schid);
 
 	return 0;
 }