netdev - Re: [PATCH net 1/5] ovpn: properly deconfigure UDP-tunnel

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Message-ID: <aEBHNVFvthKTUWuO@soc-5CG4396X81.clients.intel.com>
Date: Wed, 4 Jun 2025 15:16:37 +0200
From: Larysa Zaremba <larysa.zaremba@...el.com>
To: Antonio Quartulli <antonio@...nvpn.net>
CC: <netdev@...r.kernel.org>, Michal Swiatkowski
	<michal.swiatkowski@...ux.intel.com>, Sabrina Dubroca <sd@...asysnail.net>,
	"David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, "Oleksandr
 Natalenko" <oleksandr@...alenko.name>
Subject: Re: [PATCH net 1/5] ovpn: properly deconfigure UDP-tunnel

On Tue, Jun 03, 2025 at 01:11:06PM +0200, Antonio Quartulli wrote:
> When deconfiguring a UDP-tunnel from a socket, we cannot
> call setup_udp_tunnel_sock() with an empty config, because
> this helper is expected to be invoked only during setup.
> 
> Get rid of the call to setup_udp_tunnel_sock() and just
> revert what it did during socket initialization..
> 
> Note that the global udp_encap_needed_key and the GRO state
> are left untouched: udp_destroy_socket() will eventually
> take care of them.
> 
> Cc: Sabrina Dubroca <sd@...asysnail.net>
> Cc: Oleksandr Natalenko <oleksandr@...alenko.name>
> Fixes: ab66abbc769b ("ovpn: implement basic RX path (UDP)")
> Reported-by: Paolo Abeni <pabeni@...hat.com>
> Closes: https://lore.kernel.org/netdev/1a47ce02-fd42-4761-8697-f3f315011cc6@redhat.com
> Signed-off-by: Antonio Quartulli <antonio@...nvpn.net>

I do not think MC_LOOP is necessarily set before attaching the socket, but 1 is 
the default value, so I guess restoring to it should be fine.

Reviewed-by: Larysa Zaremba <larysa.zaremba@...el.com>

Another less related thing is more concerning to me: ovpn_udp_socket_attach() 
checks if rcu_dereference_sk_user_data(sock->sk) is NULL to determine if socket 
is available to configure, but a lot of callers of e.g. setup_udp_tunnel_sock(), 
including ovpn, set the user data to NULL by not providing it in the config.

In such case, is checking rcu_dereference_sk_user_data() actually enough to say 
that "socket is currently unused"?

> ---
>  drivers/net/ovpn/udp.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ovpn/udp.c b/drivers/net/ovpn/udp.c
> index aef8c0406ec9..f4d3bd070f11 100644
> --- a/drivers/net/ovpn/udp.c
> +++ b/drivers/net/ovpn/udp.c
> @@ -442,8 +442,16 @@ int ovpn_udp_socket_attach(struct ovpn_socket *ovpn_sock,
>   */
>  void ovpn_udp_socket_detach(struct ovpn_socket *ovpn_sock)
>  {
> -	struct udp_tunnel_sock_cfg cfg = { };
> +	struct sock *sk = ovpn_sock->sock->sk;
>  
> -	setup_udp_tunnel_sock(sock_net(ovpn_sock->sock->sk), ovpn_sock->sock,
> -			      &cfg);
> +	/* Re-enable multicast loopback */
> +	inet_set_bit(MC_LOOP, sk);
> +	/* Disable CHECKSUM_UNNECESSARY to CHECKSUM_COMPLETE conversion */
> +	inet_dec_convert_csum(sk);
> +
> +	WRITE_ONCE(udp_sk(sk)->encap_type, 0);
> +	WRITE_ONCE(udp_sk(sk)->encap_rcv, NULL);
> +	WRITE_ONCE(udp_sk(sk)->encap_destroy, NULL);
> +
> +	rcu_assign_sk_user_data(sk, NULL);
>  }
> -- 
> 2.49.0
> 
>