block: genhd: fix double kfree() in __alloc_disk_node()

syzbot is reporting use-after-free read at bdev_free_inode() [1], for
kfree() from __alloc_disk_node() is called before bdev_free_inode()
(which is called after RCU grace period) reads bdev->bd_disk and calls
kfree(bdev->bd_disk).

Fix use-after-free read followed by double kfree() problem
by making sure that bdev->bd_disk is NULL when calling iput().

Link: https://syzkaller.appspot.com/bug?extid=8281086e8a6fbfbd952a [1]
Reported-by: syzbot <syzbot+8281086e8a6fbfbd952a@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/e6dd13c5-8db0-4392-6e78-a42ee5d2a1c4@i-love.sakura.ne.jp
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bdev.c

@@ -490,7 +490,6 @@ struct block_device *bdev_alloc(struct gendisk *disk, u8 partno)
 	bdev = I_BDEV(inode);
 	mutex_init(&bdev->bd_fsfreeze_mutex);
 	spin_lock_init(&bdev->bd_size_lock);
-	bdev->bd_disk = disk;
 	bdev->bd_partno = partno;
 	bdev->bd_inode = inode;
 	bdev->bd_stats = alloc_percpu(struct disk_stats);
@@ -498,6 +497,7 @@ struct block_device *bdev_alloc(struct gendisk *disk, u8 partno)
 		iput(inode);
 		return NULL;
 	}
+	bdev->bd_disk = disk;
 	return bdev;
 }
 

block/genhd.c

@@ -1268,6 +1268,7 @@ struct gendisk *__alloc_disk_node(struct request_queue *q, int node_id,
 
 out_destroy_part_tbl:
 	xa_destroy(&disk->part_tbl);
+	disk->part0->bd_disk = NULL;
 	iput(disk->part0->bd_inode);
 out_free_bdi:
 	bdi_put(disk->bdi);