netdev - Re: [PATCH net] l2tp: Don't sleep and disable BH under writer-side sk_callback

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Message-ID: <f8e54710-6889-5c27-2b3c-333537495ecd@I-love.SAKURA.ne.jp>
Date:   Sat, 19 Nov 2022 22:52:30 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Jakub Sitnicki <jakub@...udflare.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Tom Parkin <tparkin@...alix.com>,
        syzbot+703d9e154b3b58277261@...kaller.appspotmail.com,
        syzbot+50680ced9e98a61f7698@...kaller.appspotmail.com,
        syzbot+de987172bb74a381879b@...kaller.appspotmail.com
Subject: Re: [PATCH net] l2tp: Don't sleep and disable BH under writer-side
 sk_callback_lock

On 2022/11/19 22:03, Jakub Sitnicki wrote:
> When holding a reader-writer spin lock we cannot sleep. Calling
> setup_udp_tunnel_sock() with write lock held violates this rule, because we
> end up calling percpu_down_read(), which might sleep, as syzbot reports
> [1]:
> 
>  __might_resched.cold+0x222/0x26b kernel/sched/core.c:9890
>  percpu_down_read include/linux/percpu-rwsem.h:49 [inline]
>  cpus_read_lock+0x1b/0x140 kernel/cpu.c:310
>  static_key_slow_inc+0x12/0x20 kernel/jump_label.c:158
>  udp_tunnel_encap_enable include/net/udp_tunnel.h:187 [inline]
>  setup_udp_tunnel_sock+0x43d/0x550 net/ipv4/udp_tunnel_core.c:81
>  l2tp_tunnel_register+0xc51/0x1210 net/l2tp/l2tp_core.c:1509
>  pppol2tp_connect+0xcdc/0x1a10 net/l2tp/l2tp_ppp.c:723
> 
> Trim the writer-side critical section for sk_callback_lock down to the
> minimum, so that it covers only operations on sk_user_data.

This patch does not look correct.

Since l2tp_validate_socket() checks that sk->sk_user_data == NULL with
sk->sk_callback_lock held, you need to call rcu_assign_sk_user_data(sk, tunnel)
before releasing sk->sk_callback_lock.