md: check command validity early in md_ioctl().

Verify that the cmd parameter passed to md_ioctl() is valid before
doing anything.

This fixes mddev->hold_active being set to 0 when an invalid ioctl
command is passed to md_ioctl() before the array has been configured.

Clearing mddev->hold_active in that case can lead to a livelock
situation when an invalid ioctl number is given to md_ioctl() by a
process when the mddev is currently being opened by another process:

Process 1				Process 2
---------				---------

md_alloc()
  mddev_find()
  -> returns a new mddev with
     hold_active == UNTIL_IOCTL
  add_disk()
  -> sends KOBJ_ADD uevent

					(sees KOBJ_ADD uevent for device)
                    			md_open()
                    			md_ioctl(INVALID_IOCTL)
                    			-> returns ENODEV and clears
                       			   mddev->hold_active
                    			md_release()
                      			md_put()
                      			-> deletes the mddev as
                         		   hold_active is 0

md_open()
  mddev_find()
  -> returns a newly
    allocated mddev with
    mddev->gendisk == NULL
-> returns with ERESTARTSYS
   (kernel restarts the open syscall)

Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -6356,6 +6356,32 @@ static int md_getgeo(struct block_device *bdev, struct hd_geometry *geo)
 	return 0;
 }
 
+static inline bool md_ioctl_valid(unsigned int cmd)
+{
+	switch (cmd) {
+	case ADD_NEW_DISK:
+	case BLKROSET:
+	case GET_ARRAY_INFO:
+	case GET_BITMAP_FILE:
+	case GET_DISK_INFO:
+	case HOT_ADD_DISK:
+	case HOT_REMOVE_DISK:
+	case PRINT_RAID_DEBUG:
+	case RAID_AUTORUN:
+	case RAID_VERSION:
+	case RESTART_ARRAY_RW:
+	case RUN_ARRAY:
+	case SET_ARRAY_INFO:
+	case SET_BITMAP_FILE:
+	case SET_DISK_FAULTY:
+	case STOP_ARRAY:
+	case STOP_ARRAY_RO:
+		return true;
+	default:
+		return false;
+	}
+}
+
 static int md_ioctl(struct block_device *bdev, fmode_t mode,
 			unsigned int cmd, unsigned long arg)
 {
@@ -6364,6 +6390,9 @@ static int md_ioctl(struct block_device *bdev, fmode_t mode,
 	struct mddev *mddev = NULL;
 	int ro;
 
+	if (!md_ioctl_valid(cmd))
+		return -ENOTTY;
+
 	switch (cmd) {
 	case RAID_VERSION:
 	case GET_ARRAY_INFO: