loop: Don't change loop device under exclusive opener

Loop module allows calling LOOP_SET_FD while there are other openers of
the loop device. Even exclusive ones. This can lead to weird
consequences such as kernel deadlocks like:

mount_bdev()				lo_ioctl()
  udf_fill_super()
    udf_load_vrs()
      sb_set_blocksize() - sets desired block size B
      udf_tread()
        sb_bread()
          __bread_gfp(bdev, block, B)
					  loop_set_fd()
					    set_blocksize()
            - now __getblk_slow() indefinitely loops because B != bdev
              block size

Fix the problem by disallowing LOOP_SET_FD ioctl when there are
exclusive openers of a loop device.

[Deliberately chosen not to CC stable as a user with priviledges to
trigger this race has other means of taking the system down and this
has a potential of breaking some weird userspace setup]

Reported-and-tested-by: syzbot+10007d66ca02b08f0e60@syzkaller.appspotmail.com
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -945,9 +945,20 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	if (!file)
 		goto out;
 
+	/*
+	 * If we don't hold exclusive handle for the device, upgrade to it
+	 * here to avoid changing device under exclusive owner.
+	 */
+	if (!(mode & FMODE_EXCL)) {
+		bdgrab(bdev);
+		error = blkdev_get(bdev, mode | FMODE_EXCL, loop_set_fd);
+		if (error)
+			goto out_putf;
+	}
+
 	error = mutex_lock_killable(&loop_ctl_mutex);
 	if (error)
-		goto out_putf;
+		goto out_bdev;
 
 	error = -EBUSY;
 	if (lo->lo_state != Lo_unbound)
@@ -1012,10 +1023,15 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	mutex_unlock(&loop_ctl_mutex);
 	if (partscan)
 		loop_reread_partitions(lo, bdev);
+	if (!(mode & FMODE_EXCL))
+		blkdev_put(bdev, mode | FMODE_EXCL);
 	return 0;
 
 out_unlock:
 	mutex_unlock(&loop_ctl_mutex);
+out_bdev:
+	if (!(mode & FMODE_EXCL))
+		blkdev_put(bdev, mode | FMODE_EXCL);
 out_putf:
 	fput(file);
 out: