netdev - Re: [PATCH v2] net: fix a concurrency bug in l2tp_tunnel

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Date:   Fri, 16 Apr 2021 10:21:12 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Sishuai Gong <sishuai@...due.edu>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, tparkin@...alix.com,
        Matthias Schiffer <mschiffer@...verse-factory.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH v2] net: fix a concurrency bug in l2tp_tunnel_register()

On Thu, Apr 15, 2021 at 7:18 AM Sishuai Gong <sishuai@...due.edu> wrote:
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index 203890e378cb..879f1264ec3c 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -1478,6 +1478,9 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
>         tunnel->l2tp_net = net;
>         pn = l2tp_pernet(net);
>
> +       sk = sock->sk;
> +       tunnel->sock = sk;
> +
>         spin_lock_bh(&pn->l2tp_tunnel_list_lock);
>         list_for_each_entry(tunnel_walk, &pn->l2tp_tunnel_list, list) {
>                 if (tunnel_walk->tunnel_id == tunnel->tunnel_id) {
> @@ -1490,9 +1493,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
>         list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
>         spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
>
> -       sk = sock->sk;
>         sock_hold(sk);
> -       tunnel->sock = sk;

I think you have to hold this refcnt before making tunnel->sock visible
to others.

Why not just move this together and simply release the refcnt on error
path?

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 203890e378cb..8eb805ee18d4 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1478,11 +1478,15 @@ int l2tp_tunnel_register(struct l2tp_tunnel
*tunnel, struct net *net,
        tunnel->l2tp_net = net;
        pn = l2tp_pernet(net);

+       sk = sock->sk;
+       sock_hold(sk);
+       tunnel->sock = sk;
+
        spin_lock_bh(&pn->l2tp_tunnel_list_lock);
        list_for_each_entry(tunnel_walk, &pn->l2tp_tunnel_list, list) {
                if (tunnel_walk->tunnel_id == tunnel->tunnel_id) {
                        spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
-
+                       sock_put(sk);
                        ret = -EEXIST;
                        goto err_sock;
                }
@@ -1490,10 +1494,6 @@ int l2tp_tunnel_register(struct l2tp_tunnel
*tunnel, struct net *net,
        list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
        spin_unlock_bh(&pn->l2tp_tunnel_list_lock);

-       sk = sock->sk;
-       sock_hold(sk);
-       tunnel->sock = sk;
-
        if (tunnel->encap == L2TP_ENCAPTYPE_UDP) {
                struct udp_tunnel_sock_cfg udp_cfg = {
                        .sk_user_data = tunnel,