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

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Message-ID: <a3b7d8cd-0c72-8e6b-78f2-71b92e70360f@I-love.SAKURA.ne.jp>
Date:   Tue, 22 Nov 2022 20:14:33 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Jakub Sitnicki <jakub@...udflare.com>
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 2022/11/22 19:46, Jakub Sitnicki wrote:
>> https://syzkaller.appspot.com/bug?extid=94cc2a66fc228b23f360 is the one
>> where changing lockdep class is concurrently done on pre-existing sockets.
>>
>> I think we need to always create a new socket inside l2tp_tunnel_register(),
>> rather than trying to serialize setting of sk_user_data under sk_callback_lock.
> 
> While that would be easier to handle, I don't see how it can be done in
> a backward-compatible way. User-space is allowed to pass a socket to
> l2tp today [1].

What is the expected usage of the socket which was passed to l2tp_tunnel_register() ?
Is the userspace supposed to just close() that socket? Or, is the userspace allowed to
continue using the socket?

If the userspace might continue using the socket, we would

  create a new socket, copy required attributes (the source and destination addresses?) from
  the socket fetched via sockfd_lookup(), and call replace_fd() like e.g. umh_pipe_setup() does

inside l2tp_tunnel_register(). i-node number of the socket would change, but I assume that
the process which called l2tp_tunnel_register() is not using that i-node number.

Since the socket is a datagram socket, I think we can copy required attributes. But since
I'm not familiar with networking code, I don't know what attributes need to be copied. Thus,
I leave implementing it to netdev people.