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

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

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.