Re: [PATCH net v2] l2tp: Serialize access to sk_user_data with sock lock

[Tom Parkin <tparkin@katalix.com>]

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On  Mon, Aug 15, 2022 at 15:01:07 +0200, Jakub Sitnicki wrote:
> sk->sk_user_data has multiple users, which are not compatible with each
> other. To synchronize the users, any check-if-unused-and-set access to the
> pointer has to happen with sock lock held.
> 
> 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.
> 
> v2:
> - update Fixes to point to origin of the bug
> - use real names in Reported/Tested-by tags
> 
> Fixes: 3557baabf280 ("[L2TP]: PPP over L2TP driver core")

This still seems wrong to me.

In 3557baabf280 pppol2tp_connect checks/sets sk_user_data with
lock_sock held.

> Reported-by: Haowei Yan <g1042620637@gmail.com>
> Tested-by: Haowei Yan <g1042620637@gmail.com>
> Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
> ---
> Cc: Tom Parkin <tparkin@katalix.com>
> 
>  net/l2tp/l2tp_core.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index 7499c51b1850..9f5f86bfc395 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -1469,16 +1469,18 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
>  		sock = sockfd_lookup(tunnel->fd, &ret);
>  		if (!sock)
>  			goto err;
> -
> -		ret = l2tp_validate_socket(sock->sk, net, tunnel->encap);
> -		if (ret < 0)
> -			goto err_sock;
>  	}
>  
> +	sk = sock->sk;
> +	lock_sock(sk);
> +
> +	ret = l2tp_validate_socket(sk, net, tunnel->encap);
> +	if (ret < 0)
> +		goto err_sock;
> +
>  	tunnel->l2tp_net = net;
>  	pn = l2tp_pernet(net);
>  
> -	sk = sock->sk;
>  	sock_hold(sk);
>  	tunnel->sock = sk;
>  
> @@ -1504,7 +1506,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
>  
>  		setup_udp_tunnel_sock(net, sock, &udp_cfg);
>  	} else {
> -		sk->sk_user_data = tunnel;
> +		rcu_assign_sk_user_data(sk, tunnel);
>  	}
>  
>  	tunnel->old_sk_destruct = sk->sk_destruct;
> @@ -1518,6 +1520,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
>  	if (tunnel->fd >= 0)
>  		sockfd_put(sock);
>  
> +	release_sock(sk);
>  	return 0;
>  
>  err_sock:
> @@ -1525,6 +1528,8 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
>  		sock_release(sock);
>  	else
>  		sockfd_put(sock);
> +
> +	release_sock(sk);
>  err:
>  	return ret;
>  }
> -- 
> 2.35.3
> 

-- 
Tom Parkin
Katalix Systems Ltd
https://katalix.com
Catalysts for your Embedded Linux software development

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