Re: [PATCH] nbd: fix uaf in nbd_open

[Christoph Hellwig]

On Fri, Nov 03, 2023 at 06:13:34PM +0800, Li Lingfeng wrote:
> 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.

I don't think this is the right fix.  nbd needs to move to ->free_disk
to free it's private data.