md: fix use-after-free bug when dropping an rdev from an md array

Due to possible deadlock issues we need to use a schedule work to kobject_del
an 'rdev' object from a different thread.

A recent change means that kobject_add no longer gets a refernce, and
kobject_del doesn't put a reference.  Consequently, we need to explicitly hold
a reference to ensure that the last reference isn't dropped before the
scheduled work get a chance to call kobject_del.

Also, rename delayed_delete to md_delayed_delete to that it is more obvious in
a stack trace which code is to blame.

Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 08e0f8d672c3..6cdd1b4faca6 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -1419,10 +1419,11 @@ static int bind_rdev_to_array(mdk_rdev_t * rdev, mddev_t * mddev)
         return err;
 }
 
-static void delayed_delete(struct work_struct *ws)
+static void md_delayed_delete(struct work_struct *ws)
 {
         mdk_rdev_t *rdev = container_of(ws, mdk_rdev_t, del_work);
         kobject_del(&rdev->kobj);
+        kobject_put(&rdev->kobj);
 }
 
 static void unbind_rdev_from_array(mdk_rdev_t * rdev)
@@ -1441,7 +1442,8 @@ static void unbind_rdev_from_array(mdk_rdev_t * rdev)
         /* We need to delay this, otherwise we can deadlock when
          * writing to 'remove' to "dev/state"
          */
-        INIT_WORK(&rdev->del_work, delayed_delete);
+        INIT_WORK(&rdev->del_work, md_delayed_delete);
+        kobject_get(&rdev->kobj);
         schedule_work(&rdev->del_work);
 }
 
@@ -3686,7 +3688,7 @@ static int do_md_stop(mddev_t * mddev, int mode)
                                 sysfs_remove_link(&mddev->kobj, nm);
                         }
 
-                /* make sure all delayed_delete calls have finished */
+                /* make sure all md_delayed_delete calls have finished */
                 flush_scheduled_work();
 
                 export_array(mddev);