loop: Avoid updating block size under exclusive owner

Syzbot came up with a reproducer where a loop device block size is
changed underneath a mounted filesystem. This causes a mismatch between
the block device block size and the block size stored in the superblock
causing confusion in various places such as fs/buffer.c. The particular
issue triggered by syzbot was a warning in __getblk_slow() due to
requested buffer size not matching block device block size.

Fix the problem by getting exclusive hold of the loop device to change
its block size. This fails if somebody (such as filesystem) has already
an exclusive ownership of the block device and thus prevents modifying
the loop device under some exclusive owner which doesn't expect it.

Reported-by: syzbot+01ef7a8da81a975e1ccd@syzkaller.appspotmail.com
Signed-off-by: Jan Kara <jack@suse.cz>
Tested-by: syzbot+01ef7a8da81a975e1ccd@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/20250711163202.19623-2-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: jankara

drivers/block/loop.c

@@ -1432,17 +1432,34 @@ static int loop_set_dio(struct loop_device *lo, unsigned long arg)
 	return 0;
 }
 
-static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
+static int loop_set_block_size(struct loop_device *lo, blk_mode_t mode,
+			       struct block_device *bdev, unsigned long arg)
 {
 	struct queue_limits lim;
 	unsigned int memflags;
 	int err = 0;
 
-	if (lo->lo_state != Lo_bound)
-		return -ENXIO;
+	/*
+	 * If we don't hold exclusive handle for the device, upgrade to it
+	 * here to avoid changing device under exclusive owner.
+	 */
+	if (!(mode & BLK_OPEN_EXCL)) {
+		err = bd_prepare_to_claim(bdev, loop_set_block_size, NULL);
+		if (err)
+			return err;
+	}
+
+	err = mutex_lock_killable(&lo->lo_mutex);
+	if (err)
+		goto abort_claim;
+
+	if (lo->lo_state != Lo_bound) {
+		err = -ENXIO;
+		goto unlock;
+	}
 
 	if (lo->lo_queue->limits.logical_block_size == arg)
-		return 0;
+		goto unlock;
 
 	sync_blockdev(lo->lo_device);
 	invalidate_bdev(lo->lo_device);
@@ -1455,6 +1472,11 @@ static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
 	loop_update_dio(lo);
 	blk_mq_unfreeze_queue(lo->lo_queue, memflags);
 
+unlock:
+	mutex_unlock(&lo->lo_mutex);
+abort_claim:
+	if (!(mode & BLK_OPEN_EXCL))
+		bd_abort_claiming(bdev, loop_set_block_size);
 	return err;
 }
 
@@ -1473,9 +1495,6 @@ static int lo_simple_ioctl(struct loop_device *lo, unsigned int cmd,
 	case LOOP_SET_DIRECT_IO:
 		err = loop_set_dio(lo, arg);
 		break;
-	case LOOP_SET_BLOCK_SIZE:
-		err = loop_set_block_size(lo, arg);
-		break;
 	default:
 		err = -EINVAL;
 	}
@@ -1530,9 +1549,12 @@ static int lo_ioctl(struct block_device *bdev, blk_mode_t mode,
 		break;
 	case LOOP_GET_STATUS64:
 		return loop_get_status64(lo, argp);
+	case LOOP_SET_BLOCK_SIZE:
+		if (!(mode & BLK_OPEN_WRITE) && !capable(CAP_SYS_ADMIN))
+			return -EPERM;
+		return loop_set_block_size(lo, mode, bdev, arg);
 	case LOOP_SET_CAPACITY:
 	case LOOP_SET_DIRECT_IO:
-	case LOOP_SET_BLOCK_SIZE:
 		if (!(mode & BLK_OPEN_WRITE) && !capable(CAP_SYS_ADMIN))
 			return -EPERM;
 		fallthrough;