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

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Message-ID: <aEBZkKpKB0uGIgNj@soc-5CG4396X81.clients.intel.com>
Date: Wed, 4 Jun 2025 16:34:56 +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 Wed, Jun 04, 2025 at 03:41:04PM +0200, Antonio Quartulli wrote:
> Hello Larysa and thanks for chiming in,
> 
> On 04/06/2025 15:16, Larysa Zaremba wrote:
> > 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"?
> 
> This is an "interesting" area of the code that required a lot of attention
> during implementation...so having more eyes on it is definitely appreciated.
> 
> In ovpn we have:
> 
> ovpn_socket_new()
>   lock_sock()
>   ovpn_socket_attach()
>     ovpn_udp_socket_attach()
>   rcu_assign_sk_user_data() << sk_user_data is assigned here
>   release_sock()
> 

Oh, I did somehow miss that. Thanks for the clarification!

> The lock takes care of preventing concurrent ovpn_socket_new() invocations,
> with the same sk, to mess things up.
> 
> Upon socket detachment, a similar strategy is implemented to make sure
> concurrent attachment/detachment are properly handled.
> 
> I hope this helps.
> 
> Regards,
> 
> > 
> > > ---
> > >   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
> > > 
> > > 
> 
> -- 
> Antonio Quartulli
> OpenVPN Inc.
>