netdev - Re: [PATCH net-next v18 20/25] ovpn: implement peer add/get/dump/delete via netlink

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Message-ID: <Z44gwl2d8ThTshzQ@hog>
Date: Mon, 20 Jan 2025 11:09:06 +0100
From: Sabrina Dubroca <sd@...asysnail.net>
To: Antonio Quartulli <antonio@...nvpn.net>
Cc: ryazanov.s.a@...il.com, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Donald Hunter <donald.hunter@...il.com>,
	Shuah Khan <shuah@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>,
	Simon Horman <horms@...nel.org>, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, Xiao Liang <shaw.leon@...il.com>
Subject: Re: [PATCH net-next v18 20/25] ovpn: implement peer
 add/get/dump/delete via netlink

2025-01-19, 14:12:05 +0100, Antonio Quartulli wrote:
> On 17/01/2025 18:12, Sabrina Dubroca wrote:
> > 2025-01-17, 13:59:35 +0100, Antonio Quartulli wrote:
> > > On 17/01/2025 12:48, Sabrina Dubroca wrote:
> > > > 2025-01-13, 10:31:39 +0100, Antonio Quartulli wrote:
> > > > >    int ovpn_nl_peer_new_doit(struct sk_buff *skb, struct genl_info *info)
> > > > >    {
> > > > > -	return -EOPNOTSUPP;
> > > > > +	struct nlattr *attrs[OVPN_A_PEER_MAX + 1];
> > > > > +	struct ovpn_priv *ovpn = info->user_ptr[0];
> > > > > +	struct ovpn_socket *ovpn_sock;
> > > > > +	struct socket *sock = NULL;
> > > > > +	struct ovpn_peer *peer;
> > > > > +	u32 sockfd, peer_id;
> > > > > +	int ret;
> > > > > +
> > > > > +	/* peers can only be added when the interface is up and running */
> > > > > +	if (!netif_running(ovpn->dev))
> > > > > +		return -ENETDOWN;
> > > > 
> > > > Since we're not under rtnl_lock here, the device could go down while
> > > > we're creating this peer, and we may end up with a down device that
> > > > has a peer anyway.
> > > 
> > > hmm, indeed. This means we must hold the rtnl_lock to prevent ending up in
> > > an inconsistent state.
> > > 
> > > > 
> > > > I'm not sure what this (and the peer flushing on NETDEV_DOWN) is
> > > > trying to accomplish. Is it a problem to keep peers when the netdevice
> > > > is down?
> > > 
> > > This is the result of my discussion with Sergey that started in v23 5/23:
> > > 
> > > https://lore.kernel.org/r/netdev/20241029-b4-ovpn-v11-5-de4698c73a25@openvpn.net/
> > > 
> > > The idea was to match operational state with actual connectivity to peer(s).
> > > 
> > > Originally I wanted to simply kee the carrier always on, but after further
> > > discussion (including the meaning of the openvpn option --persist-tun) we
> > > agreed on following the logic where an UP device has a peer connected (logic
> > > is slightly different between MP and P2P).
> > > 
> > > I am not extremely happy with the resulting complexity, but it seemed to be
> > > blocker for Sergey.
> > 
> > [after re-reading that discussion with Sergey]
> > 
> > I don't understand why "admin does 'ip link set tun0 down'" means "we
> > should get rid of all peers. For me the carrier situation goes the
> > other way: no peer, no carrier (as if I unplugged the cable from my
> > ethernet card), and it's independent of what the user does (ip link
> > set XXX up/down). You have that with netif_carrier_{on,off}, but
> > flushing peers when the admin does "ip link set tun0 down" is separate
> > IMO.
> 
> The reasoning was "the user is asking the VPN to go down - it should be
> assumed that from that moment on no VPN traffic whatsoever should flow in
> either direction".
> Similarly to when you bring an Eth interface dwn - the phy link goes down as
> well.
> 
> Does it make sense?

I'm not sure. If I turn the ovpn interface down for a second, the
peers are removed. Will they come back when I bring the interface back
up?  That'd have to be done by userspace (which could also watch for
the DOWN events and tell the kernel to flush the peers) - but some of
the peers could have timed out in the meantime.

If I set the VPN interface down, I expect no packets flowing through
that interface (dropping the peers isn't necessary for that), but all
non-data (key exchange etc sent by openvpn's userspace) should still
go through, and IMO peer keepalive fits in that "non-data" category.


What does openvpn currently do if I do
    ip link set tun0 down ; sleep 5 ; ip link set tun0 up
with a tuntap interface?

-- 
Sabrina