linux-kernel - [PATCH 3.2 047/101] md: ensure md devices are freed before module is unloaded.

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Date:   Thu, 01 Jun 2017 16:40:55 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Guoqing Jiang" <gqjiang@...e.com>,
        "Shaohua Li" <shli@...com>, "NeilBrown" <neilb@...e.com>
Subject: [PATCH 3.2 047/101] md: ensure md devices are freed before module
 is unloaded.

3.2.89-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: NeilBrown <neilb@...e.com>

commit 9356863c9409efc79029c01a85d015efae977e69 upstream.

Commit: cbd199837750 ("md: Fix unfortunate interaction with evms")
change mddev_put() so that it would not destroy an md device while
->ctime was non-zero.

Unfortunately, we didn't make sure to clear ->ctime when unloading
the module, so it is possible for an md device to remain after
module unload.  An attempt to open such a device will trigger
an invalid memory reference in:
  get_gendisk -> kobj_lookup -> exact_lock -> get_disk

when tring to access disk->fops, which was in the module that has
been removed.

So ensure we clear ->ctime in md_exit(), and explain how that is useful,
as it isn't immediately obvious when looking at the code.

Fixes: cbd199837750 ("md: Fix unfortunate interaction with evms")
Tested-by: Guoqing Jiang <gqjiang@...e.com>
Signed-off-by: NeilBrown <neilb@...e.com>
Signed-off-by: Shaohua Li <shli@...com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/md/md.c | 7 +++++++
 1 file changed, 7 insertions(+)

--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -8292,7 +8292,14 @@ static __exit void md_exit(void)
 	remove_proc_entry("mdstat", NULL);
 	for_each_mddev(mddev, tmp) {
 		export_array(mddev);
+		mddev->ctime = 0;
 		mddev->hold_active = 0;
+		/*
+		 * for_each_mddev() will call mddev_put() at the end of each
+		 * iteration.  As the mddev is now fully clear, this will
+		 * schedule the mddev for destruction by a workqueue, and the
+		 * destroy_workqueue() below will wait for that to complete.
+		 */
 	}
 	destroy_workqueue(md_misc_wq);
 	destroy_workqueue(md_wq);