netdev - Re: [PATCH net] l2tp: use rcu_dereference_sk_user_data() in l2tp_udp_encap

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Date:   Wed, 24 Apr 2019 11:58:11 +0200
From:   Guillaume Nault <gnault@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        James Chapman <jchapman@...alix.com>
Subject: Re: [PATCH net] l2tp: use rcu_dereference_sk_user_data() in
 l2tp_udp_encap_recv()

On Tue, Apr 23, 2019 at 09:43:26AM -0700, Eric Dumazet wrote:
> Canonical way to fetch sk_user_data from an encap_rcv() handler called
> from UDP stack in rcu protected section is to use rcu_dereference_sk_user_data(),
> otherwise compiler might read it multiple times.
> 
That reminds me the more general problem we have with ->sk_user_data:
https://lore.kernel.org/netdev/20180117.142538.1972806008716856078.davem@davemloft.net/

We're not even guarateed that ->sk_user_data points to a struct l2tp_tunnel
(some external modules can still probably override it).

There were some locking rules defined for setting ->sk_user_data:
https://lore.kernel.org/netdev/20180124203541.3172-3-tom@quantonium.net/
Converting all users to either avoid using ->sk_user_data or using it
under the proper pre-conditions has been on my TODO list for a while...
I'll try to revive this effort.

> Fixes: d00fa9adc528 ("il2tp: fix races with tunnel socket close")
                        ^
Spurious "i". Vi user? :)

> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: James Chapman <jchapman@...alix.com>
> ---
>  net/l2tp/l2tp_core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index fed6becc5daf86afa2ad9188bb28e151244bb5a6..aee33d1320184e411dbedff72b5bf5199481e53f 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -909,7 +909,7 @@ int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
>  {
>  	struct l2tp_tunnel *tunnel;
>  
> -	tunnel = l2tp_tunnel(sk);
> +	tunnel = rcu_dereference_sk_user_data(sk);
>  	if (tunnel == NULL)
>  		goto pass_up;
>  
> -- 
> 2.21.0.593.g511ec345e18-goog
>