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

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Date:   Mon, 21 Nov 2022 10:00:09 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     netdev@...r.kernel.org, "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 Sat, Nov 19, 2022 at 10:52 PM +09, Tetsuo Handa wrote:
> 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.

You're right. v2 posted:

https://lore.kernel.org/netdev/20221121085426.21315-1-jakub@cloudflare.com/