Re: [PATCH net v3] l2tp: Serialize access to sk_user_data with sk_callback_lock

[Jakub Sitnicki <jakub@cloudflare.com>]

On Thu, Aug 25, 2022 at 12:23 PM +02, Paolo Abeni wrote:
> hello,
>
> On Tue, 2022-08-23 at 12:14 +0200, Jakub Sitnicki wrote:
>> sk->sk_user_data has multiple users, which are not compatible with each
>> other. Writers must synchronize by grabbing the sk->sk_callback_lock.
>> 
>> l2tp currently fails to grab the lock when modifying the underlying tunnel
>> socket. Fix it by adding appropriate locking.
>> 
>> We don't to grab the lock when l2tp clears sk_user_data, because it happens
>> only in sk->sk_destruct, when the sock is going away.
>
> l2tp can additionally clears sk_user_data in sk->sk_prot->close() via 
> udp_lib_close() -> sk_common_release() -> sk->sk_prot->destroy() -> 
> udp_destroy_sock() -> up->encap_destroy() -> l2tp_udp_encap_destroy().
>
> That still happens at socket closing time, but when network has still
> access to the sock itself. It should be safe as the other sk_user_data
> users touch it only via fd, but perhaps a 'better safe the sorry'
> approach could be relevant there?

Fair point. Let me add that.