netdev - Re: [PATCH v2] net: rds: acquire refcount on TCP sockets

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Date:   Tue, 3 May 2022 20:09:36 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     patchwork-bot+netdevbpf@...nel.org,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        syzbot <syzbot+694120e1002c117747ed@...kaller.appspotmail.com>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH v2] net: rds: acquire refcount on TCP sockets

On Tue, May 3, 2022 at 6:04 PM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> On 2022/05/04 7:37, Eric Dumazet wrote:
> >> I think we merged this patch too soon.
> >>
> >> My question is : What prevents rds_tcp_conn_path_connect(), and thus
> >> rds_tcp_tune() to be called
> >> after the netns refcount already reached 0 ?
> >>
> >> I guess we can wait for next syzbot report, but I think that get_net()
> >> should be replaced
> >> by maybe_get_net()
> >
> > Yes, syzbot was fast to trigger this exact issue:
>
> Does maybe_get_net() help?
>
> Since rds_conn_net() returns a net namespace without holding a ref, it is theoretically
> possible that the net namespace returned by rds_conn_net() is already kmem_cache_free()d
> if refcount dropped to 0 by the moment sk_alloc() calls sock_net_set().

Nope. RDS has an exit() handler called from cleanup_net()

(struct pernet_operations)->exit() or exit_batch() :
rds_tcp_exit_net() (rds_tcp_kill_sock())

This exit() handler _has_ to remove all known listeners, and
definitely cancel work queues (synchronous operation)
before the actual "struct net" free can happen later.



>
> rds_tcp_conn_path_connect() {
>   sock_create_kern(net = rds_conn_net(conn)) {
>     __sock_create(net = rds_conn_net(conn), kern = 1) {
>       err = pf->create(net = rds_conn_net(conn), kern = 1) {
>         // pf->create is either inet_create or inet6_create
>         sk_alloc(net = rds_conn_net(conn), kern = 1) {
>           sk->sk_net_refcnt = kern ? 0 : 1;
>           if (likely(sk->sk_net_refcnt)) {
>             get_net_track(net, &sk->ns_tracker, priority);
>             sock_inuse_add(net, 1);
>           }
>           sock_net_set(sk, net);
>         }
>       }
>     }
>   }
>   rds_tcp_tune() {
>     if (!sk->sk_net_refcnt) {
>       sk->sk_net_refcnt = 1;
>       get_net_track(net, &sk->ns_tracker, GFP_KERNEL);
>       sock_inuse_add(net, 1);
>     }
>   }
> }
>
> "struct rds_connection" needs to hold a ref in order to safely allow
> rds_tcp_tune() to call maybe_get_net(), which in turn makes pointless
> to use maybe_get_net() from rds_tcp_tune() because "struct rds_connection"
> must have a ref. Situation where we are protected by maybe_get_net() is
> quite limited if long-lived object is not holding a ref.
>
> Hmm, can we simply use &init_net instead of rds_conn_net(conn) ?

Only if you plan making RDS unavailable for non init netns.