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

Sabrina Dubroca
2025-01-17T11:48:58Z

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.

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?

> +
> + if (GENL_REQ_ATTR_CHECK(info, OVPN_A_PEER))
> + return -EINVAL;
> +
> + ret = nla_parse_nested(attrs, OVPN_A_PEER_MAX, info->attrs[OVPN_A_PEER],
> + ovpn_peer_nl_policy, info->extack);
> + if (ret)
> + return ret;
> +
> + ret = ovpn_nl_peer_precheck(ovpn, info, attrs);
> + if (ret < 0)
> + return ret;
> +
> + if (NL_REQ_ATTR_CHECK(info->extack, info->attrs[OVPN_A_PEER], attrs,
> + OVPN_A_PEER_SOCKET))
> + return -EINVAL;
> +
> + peer_id = nla_get_u32(attrs[OVPN_A_PEER_ID]);
> + peer = ovpn_peer_new(ovpn, peer_id);
> + if (IS_ERR(peer)) {
> + NL_SET_ERR_MSG_FMT_MOD(info->extack,
> + "cannot create new peer object for peer %u: %ld",
> + peer_id, PTR_ERR(peer));
> + return PTR_ERR(peer);
> + }
> +
> + /* lookup the fd in the kernel table and extract the socket object */
> + sockfd = nla_get_u32(attrs[OVPN_A_PEER_SOCKET]);
> + /* sockfd_lookup() increases sock's refcounter */
> + sock = sockfd_lookup(sockfd, &ret);
> + if (!sock) {
> + NL_SET_ERR_MSG_FMT_MOD(info->extack,
> + "cannot lookup peer socket (fd=%u): %d",
> + sockfd, ret);
> + return -ENOTSOCK;

All those returns should be "goto peer_release" (and setting ret) so
that we don't leak peer.

> + }
> +
> + /* Only when using UDP as transport protocol the remote endpoint
> + * can be configured so that ovpn knows where to send packets to.
> + *
> + * In case of TCP, the socket is connected to the peer and ovpn
> + * will just send bytes over it, without the need to specify a
> + * destination.
> + */
> + if (sock->sk->sk_protocol != IPPROTO_UDP &&
> + (attrs[OVPN_A_PEER_REMOTE_IPV4] ||
> + attrs[OVPN_A_PEER_REMOTE_IPV6])) {
> + NL_SET_ERR_MSG_FMT_MOD(info->extack,
> + "unexpected remote IP address for non UDP socket");
> + sockfd_put(sock);
> + return -EINVAL;

goto peer_release

> + }
> +
> + ovpn_sock = ovpn_socket_new(sock, peer);
> + if (IS_ERR(ovpn_sock)) {
> + NL_SET_ERR_MSG_FMT_MOD(info->extack,
> + "cannot encapsulate socket: %ld",
> + PTR_ERR(ovpn_sock));
> + sockfd_put(sock);
> + return -ENOTSOCK;

goto peer_release

> + }
> +
> + peer->sock = ovpn_sock;
> +
> + ret = ovpn_nl_peer_modify(peer, info, attrs);
> + if (ret < 0)
> + goto peer_release;
> +
> + ret = ovpn_peer_add(ovpn, peer);
> + if (ret < 0) {
> + NL_SET_ERR_MSG_FMT_MOD(info->extack,
> + "cannot add new peer (id=%u) to hashtable: %d\n",
> + peer->id, ret);
> + goto peer_release;
> + }
> +
> + return 0;
> +
> +peer_release:
> + /* release right away because peer is not used in any context */
> + ovpn_peer_release(peer);
> +
> + return ret;
> }

[...]
> int ovpn_nl_peer_del_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_peer *peer;
> + u32 peer_id;
> + int ret;
> +
> + if (GENL_REQ_ATTR_CHECK(info, OVPN_A_PEER))
> + return -EINVAL;
> +
> + ret = nla_parse_nested(attrs, OVPN_A_PEER_MAX, info->attrs[OVPN_A_PEER],
> + ovpn_peer_nl_policy, info->extack);
> + if (ret)
> + return ret;
> +
> + if (NL_REQ_ATTR_CHECK(info->extack, info->attrs[OVPN_A_PEER], attrs,
> + OVPN_A_PEER_ID))
> + return -EINVAL;
> +
> + peer_id = nla_get_u32(attrs[OVPN_A_PEER_ID]);
> + peer = ovpn_peer_get_by_id(ovpn, peer_id);
> + if (!peer) {
> + NL_SET_ERR_MSG_FMT_MOD(info->extack,
> + "cannot find peer with id %u", peer_id);
> + return -ENOENT;
> + }
> +
> + netdev_dbg(ovpn->dev, "del peer %u\n", peer->id);
> + ret = ovpn_peer_del(peer, OVPN_DEL_PEER_REASON_USERSPACE);

With the delayed socket release (which is similar to what was in v11,
but now with refcounting on the netdevice which should make
rtnl_link_unregister in ovpn_cleanup wait [*]), we may return to
userspace as if the peer was gone, but the socket hasn't been detached
yet.

A userspace application that tries to remove the peer and immediately
re-create it with the same socket could get EBUSY if the workqueue
hasn't done its job yet. That would be quite confusing to the
application.

So I would add a completion to wait here until the socket has been
fully detached. Something like below.

[*] I don't think the current refcounting fully protects against that,
I'll comment on 05/25

-------- 8< --------

diff --git a/drivers/net/ovpn/netlink.c b/drivers/net/ovpn/netlink.c
index 72357bb5f30b..19aa4ee6d468 100644
--- a/drivers/net/ovpn/netlink.c
+++ b/drivers/net/ovpn/netlink.c
@@ -733,6 +733,9 @@ int ovpn_nl_peer_del_doit(struct sk_buff *skb, struct genl_info *info)

netdev_dbg(ovpn->dev, "del peer %u\n", peer->id);
ret = ovpn_peer_del(peer, OVPN_DEL_PEER_REASON_USERSPACE);
+ if (ret >= 0 && peer->sock)
+ wait_for_completion(&peer->sock_detach);
+
ovpn_peer_put(peer);

return ret;
diff --git a/drivers/net/ovpn/peer.c b/drivers/net/ovpn/peer.c
index b032390047fe..6120521d0c32 100644
--- a/drivers/net/ovpn/peer.c
+++ b/drivers/net/ovpn/peer.c
@@ -92,6 +92,7 @@ struct ovpn_peer *ovpn_peer_new(struct ovpn_priv *ovpn, u32 id)
ovpn_peer_stats_init(&peer->vpn_stats);
ovpn_peer_stats_init(&peer->link_stats);
INIT_WORK(&peer->keepalive_work, ovpn_peer_keepalive_send);
+ init_completion(&peer->sock_detach);

ret = dst_cache_init(&peer->dst_cache, GFP_KERNEL);
if (ret < 0) {
diff --git a/drivers/net/ovpn/peer.h b/drivers/net/ovpn/peer.h
index 7a062cc5a5a4..8c54bf5709ef 100644
--- a/drivers/net/ovpn/peer.h
+++ b/drivers/net/ovpn/peer.h
@@ -112,6 +112,7 @@ struct ovpn_peer {
struct rcu_head rcu;
struct work_struct remove_work;
struct work_struct keepalive_work;
+ struct completion sock_detach;
};

/**
diff --git a/drivers/net/ovpn/socket.c b/drivers/net/ovpn/socket.c
index a5c3bc834a35..7cefac42c3be 100644
--- a/drivers/net/ovpn/socket.c
+++ b/drivers/net/ovpn/socket.c
@@ -31,6 +31,8 @@ static void ovpn_socket_release_kref(struct kref *kref)

sockfd_put(sock->sock);
kfree_rcu(sock, rcu);
+
+ complete(&sock->peer->sock_detach);
}

/**
@@ -181,12 +183,12 @@ struct ovpn_socket *ovpn_socket_new(struct socket *sock, struct ovpn_peer *peer)

ovpn_sock->sock = sock;
kref_init(&ovpn_sock->refcount);
+ ovpn_sock->peer = peer;

/* TCP sockets are per-peer, therefore they are linked to their unique
* peer
*/
if (sock->sk->sk_protocol == IPPROTO_TCP) {
- ovpn_sock->peer = peer;
ovpn_peer_hold(peer);
} else if (sock->sk->sk_protocol == IPPROTO_UDP) {
/* in UDP we only link the ovpn instance since the socket is
diff --git a/drivers/net/ovpn/socket.h b/drivers/net/ovpn/socket.h
index 15827e347f53..3f5a35fd9048 100644
--- a/drivers/net/ovpn/socket.h
+++ b/drivers/net/ovpn/socket.h
@@ -28,12 +28,12 @@ struct ovpn_peer;
* @rcu: member used to schedule RCU destructor callback
*/
struct ovpn_socket {
+ struct ovpn_peer *peer;
union {
struct {
struct ovpn_priv *ovpn;
netdevice_tracker dev_tracker;
};
- struct ovpn_peer *peer;
};

struct socket *sock;

--
Sabrina