[S390] vmur: Invalid allocation sequence for vmur class

The vmur class is allocated after the CCW driver is registered
and it is destroyed before the CCW driver is unregistered.
This is not the correct sequence, because the vmur class can be used
via driver core callbacks that are triggered during the CCW driver
deregistration. For Example:

1. vmur device is online
2. vmur module is unloaded

This leads to the following function call stack:

    <4> [<0000000000387286>] device_destroy+0x36/0x5c
    <4> [<000003e000209714>] ur_set_offline_force+0x9c/0x10c [vmur]
    <4> [<000003e00020a928>] ur_remove+0x64/0xbc [vmur]
    <4> [<00000000003e4d2e>] ccw_device_remove+0x42/0x1ac
    <4> [<000000000038a1aa>] __device_release_driver+0x9a/0xe4
    <4> [<000000000038a2da>] driver_detach+0xe6/0xec
    <4> [<0000000000388ee4>] bus_remove_driver+0xc0/0x108
    <4> [<000003e00020ad5a>] ur_exit+0x52/0x84 [vmur]

In device_destroy() the vmur class is used. Since it is already freed,
this can lead to a kernel panic.

To fix the problem, the vmur class has to be allocated before the CCW
driver is registered and destroyed after the CCW driver has ben unregistered.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/vmur.c

@@ -1026,9 +1026,15 @@ static int __init ur_init(void)
 
 	debug_set_level(vmur_dbf, 6);
 
+	vmur_class = class_create(THIS_MODULE, "vmur");
+	if (IS_ERR(vmur_class)) {
+		rc = PTR_ERR(vmur_class);
+		goto fail_free_dbf;
+	}
+
 	rc = ccw_driver_register(&ur_driver);
 	if (rc)
-		goto fail_free_dbf;
+		goto fail_class_destroy;
 
 	rc = alloc_chrdev_region(&dev, 0, NUM_MINORS, "vmur");
 	if (rc) {
@@ -1038,18 +1044,13 @@ static int __init ur_init(void)
 	}
 	ur_first_dev_maj_min = MKDEV(MAJOR(dev), 0);
 
-	vmur_class = class_create(THIS_MODULE, "vmur");
-	if (IS_ERR(vmur_class)) {
-		rc = PTR_ERR(vmur_class);
-		goto fail_unregister_region;
-	}
 	pr_info("%s loaded.\n", ur_banner);
 	return 0;
 
-fail_unregister_region:
-	unregister_chrdev_region(ur_first_dev_maj_min, NUM_MINORS);
 fail_unregister_driver:
 	ccw_driver_unregister(&ur_driver);
+fail_class_destroy:
+	class_destroy(vmur_class);
 fail_free_dbf:
 	debug_unregister(vmur_dbf);
 	return rc;
@@ -1057,9 +1058,9 @@ fail_free_dbf:
 
 static void __exit ur_exit(void)
 {
-	class_destroy(vmur_class);
 	unregister_chrdev_region(ur_first_dev_maj_min, NUM_MINORS);
 	ccw_driver_unregister(&ur_driver);
+	class_destroy(vmur_class);
 	debug_unregister(vmur_dbf);
 	pr_info("%s unloaded.\n", ur_banner);
 }