Re: [PATCH] nbd: fix uaf in nbd_open

[Yu Kuai]

Hi,

在 2023/11/03 18:13, Li Lingfeng 写道:
> From: Li Lingfeng <lilingfeng3@xxxxxxxxxx>
>
> Commit 4af5f2e03013 ("nbd: use blk_mq_alloc_disk and
> blk_cleanup_disk") cleans up disk by blk_cleanup_disk() and it won't set
> disk->private_data as NULL as before. UAF may be triggered in nbd_open()
> if someone tries to open nbd device right after nbd_put() since refcount
> of nbd device is zero and private_data is not NULL.
Do you mean that nbd_open concurrent with nbd_dev_remove?

nbd_open		nbd_dev_remove
			 del_gendisk
			 kfree(nbd)
 mutex_lock
 nbd = disk->private_data
 -> UAF
 refcount_inc_not_zero

Looks like it's possible, but you should use READ/WRITE_ONCE() here,
because disk->pravate_data can be accessed concurrently.

Thanks,
Kuai

> Fixes: 4af5f2e03013 ("nbd: use blk_mq_alloc_disk and blk_cleanup_disk")
> Signed-off-by: Li Lingfeng <lilingfeng3@xxxxxxxxxx>
> ---
>   drivers/block/nbd.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 800f131222fc..aab93b836e84 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -250,6 +250,7 @@ static void nbd_dev_remove(struct nbd_device *nbd)
>   	struct gendisk *disk = nbd->disk;
>   
>   	del_gendisk(disk);
> +	disk->private_data = NULL;
>   	put_disk(disk);
>   	blk_mq_free_tag_set(&nbd->tag_set);
>