md: Free resources in __md_stop

If md_run() fails after ->active_io is initialized, then percpu_ref_exit
is called in error path. However, later md_free_disk will call
percpu_ref_exit again which leads to a panic because of null pointer
dereference. It can also trigger this bug when resources are initialized
but are freed in error path, then will be freed again in md_free_disk.

BUG: kernel NULL pointer dereference, address: 0000000000000038
Oops: 0000 [#1] PREEMPT SMP
Workqueue: md_misc mddev_delayed_delete
RIP: 0010:free_percpu+0x110/0x630
Call Trace:
 <TASK>
 __percpu_ref_exit+0x44/0x70
 percpu_ref_exit+0x16/0x90
 md_free_disk+0x2f/0x80
 disk_release+0x101/0x180
 device_release+0x84/0x110
 kobject_put+0x12a/0x380
 kobject_put+0x160/0x380
 mddev_delayed_delete+0x19/0x30
 process_one_work+0x269/0x680
 worker_thread+0x266/0x640
 kthread+0x151/0x1b0
 ret_from_fork+0x1f/0x30

For creating raid device, md raid calls do_md_run->md_run, dm raid calls
md_run. We alloc those memory in md_run. For stopping raid device, md raid
calls do_md_stop->__md_stop, dm raid calls md_stop->__md_stop. So we can
free those memory resources in __md_stop.

Fixes: 72adae2 ("md: Change active_io to percpu")
Reported-and-tested-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>

Author: XiaoNi87

drivers/md/md.c

@@ -6256,6 +6256,11 @@ static void __md_stop(struct mddev *mddev)
 		mddev->to_remove = &md_redundancy_group;
 	module_put(pers->owner);
 	clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
+
+	percpu_ref_exit(&mddev->writes_pending);
+	percpu_ref_exit(&mddev->active_io);
+	bioset_exit(&mddev->bio_set);
+	bioset_exit(&mddev->sync_set);
 }
 
 void md_stop(struct mddev *mddev)
@@ -6265,10 +6270,6 @@ void md_stop(struct mddev *mddev)
 	 */
 	__md_stop_writes(mddev);
 	__md_stop(mddev);
-	percpu_ref_exit(&mddev->writes_pending);
-	percpu_ref_exit(&mddev->active_io);
-	bioset_exit(&mddev->bio_set);
-	bioset_exit(&mddev->sync_set);
 }
 
 EXPORT_SYMBOL_GPL(md_stop);
@@ -7839,11 +7840,6 @@ static void md_free_disk(struct gendisk *disk)
 {
 	struct mddev *mddev = disk->private_data;
 
-	percpu_ref_exit(&mddev->writes_pending);
-	percpu_ref_exit(&mddev->active_io);
-	bioset_exit(&mddev->bio_set);
-	bioset_exit(&mddev->sync_set);
-
 	mddev_free(mddev);
 }