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Subject: Re: [PATCH] nbd: fix uaf in nbd_open
To:     Li Lingfeng <lilingfeng@huaweicloud.com>, josef@toxicpanda.com
Cc:     linux-kernel@vger.kernel.org, hch@lst.de,
        linux-block@vger.kernel.org, nbd@other.debian.org, axboe@kernel.dk,
        chaitanya.kulkarni@wdc.com, yukuai1@huaweicloud.com,
        houtao1@huawei.com, yi.zhang@huawei.com, yangerkun@huawei.com,
        lilingfeng3@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
References: <20231103101334.1750094-1-lilingfeng@huaweicloud.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <07045515-6668-19bc-def8-45fb0e9fe2ed@huaweicloud.com>
Date:   Fri, 3 Nov 2023 11:30:28 +0800
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Hi,

?? 2023/11/03 18:13, Li Lingfeng д??:
> From: Li Lingfeng <lilingfeng3@huawei.com>
> 
> 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@huawei.com>
> ---
>   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);
>   
>