Subject: Re: [PATCH] nbd: restrict sockets to TCP and UDP

From: "Richard W.M. Jones" <[email-masked]>
Date: Tue, 9 Sep 2025 14:32:32 +0100

On Tue, Sep 09, 2025 at 01:22:43PM +0000, Eric Dumazet wrote:
> Recently, syzbot started to abuse NBD with all kinds of sockets.
> 
> Commit cf1b2326b734 ("nbd: verify socket is supported during setup")
> made sure the socket supported a shutdown() method.
> 
> Explicitely accept TCP and UNIX stream sockets.

I'm not clear what the actual problem is, but I will say that libnbd &
nbdkit (which are another NBD client & server, interoperable with the
kernel) we support and use NBD over vsock[1].  And we could support
NBD over pretty much any stream socket (Infiniband?) [2].

[1] https://libguestfs.org/nbd_aio_connect_vsock.3.html
    https://libguestfs.org/nbdkit-service.1.html#AF_VSOCK
[2] https://libguestfs.org/nbd_connect_socket.3.html

TCP and Unix domain sockets are by far the most widely used, but I
don't think it's fair to exclude other socket types.

Rich.

> Fixes: cf1b2326b734 ("nbd: verify socket is supported during setup")
> Reported-by: [email-masked]
> Closes: https://lore.kernel.org/netdev/[email-masked]/T/#m081036e8747cd7e2626c1da5d78c8b9d1e55b154
> Signed-off-by: Eric Dumazet <[email-masked]>
> Cc: Mike Christie <[email-masked]>
> Cc: Richard W.M. Jones <[email-masked]>
> Cc: Jens Axboe <[email-masked]>
> Cc: Yu Kuai <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> ---
>  drivers/block/nbd.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 6463d0e8d0cef71e73e67fecd16de4dec1c75da7..87b0b78249da3325023949585f4daf40486c9692 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -1217,6 +1217,14 @@ static struct socket *nbd_get_socket(struct nbd_device *nbd, unsigned long fd,
>  	if (!sock)
>  		return NULL;
>  
> +	if (!sk_is_tcp(sock->sk) &&
> +	    !sk_is_stream_unix(sock->sk)) {
> +		dev_err(disk_to_dev(nbd->disk), "Unsupported socket: should be TCP or UNIX.\n");
> +		*err = -EINVAL;
> +		sockfd_put(sock);
> +		return NULL;
> +	}
> +
>  	if (sock->ops->shutdown == sock_no_shutdown) {
>  		dev_err(disk_to_dev(nbd->disk), "Unsupported socket: shutdown callout must be supported.\n");
>  		*err = -EINVAL;
> -- 
> 2.51.0.384.g4c02a37b29-goog

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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