netdev - Re: [PATCH net v3] l2tp: Serialize access to sk_user_data with sk_callback

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Date:   Thu, 25 Aug 2022 12:23:24 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jakub Sitnicki <jakub@...udflare.com>, netdev@...r.kernel.org
Cc:     kernel-team@...udflare.com,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Tom Parkin <tparkin@...alix.com>,
        Haowei Yan <g1042620637@...il.com>
Subject: Re: [PATCH net v3] l2tp: Serialize access to sk_user_data with
 sk_callback_lock

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?

Thanks!

Paolo