dm: don't allow ioctls to targets that don't map to whole devices

.. at least for unprivileged users.  Before we called into the SCSI
ioctl code to allow excemptions for a few SCSI passthrough ioctls,
but this is pretty unsafe and except for this call dm knows nothing
about SCSI ioctls.

As the SCSI ioctl code is now optional, we really don't want to
drag it in for DM, and the exception is not very useful anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/md/dm.c

@@ -465,13 +465,16 @@ static int dm_blk_ioctl(struct block_device *bdev, fmode_t mode,
 
 	if (r > 0) {
 		/*
-		 * Target determined this ioctl is being issued against
-		 * a logical partition of the parent bdev; so extra
-		 * validation is needed.
+		 * Target determined this ioctl is being issued against a
+		 * subset of the parent bdev; require extra privileges.
 		 */
-		r = scsi_verify_blk_ioctl(NULL, cmd);
-		if (r)
+		if (!capable(CAP_SYS_RAWIO)) {
+			DMWARN_LIMIT(
+	"%s: sending ioctl %x to DM device without required privilege.",
+				current->comm, cmd);
+			r = -ENOIOCTLCMD;
 			goto out;
+		}
 	}
 
 	r =  __blkdev_driver_ioctl(bdev, mode, cmd, arg);