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

[ Upstream commit 1b6074112742f65ece71b0f299ca5a6a887d2db6 ]

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
172da89ed0ea ("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: 172da89ed0ea ("s390/cio: avoid excessive path-verification requests")
Fixes: 0c3812c347bf ("s390/cio: derive cdev information only for IO-subchannels")
Cc: <stable@vger.kernel.org> # v5.15
Reported-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/s390/cio/css.c b/drivers/s390/cio/css.c
index ce9e751..2ba9e01 100644 (file)
--- a/drivers/s390/cio/css.c
+++ b/drivers/s390/cio/css.c
@@ -792,13 +792,9 @@ 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;
 }