Re: [PATCH net v4] l2tp: Serialize access to sk_user_data with sk_callback_lock

[Tom Parkin <tparkin@katalix.com>]

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On  Mon, Nov 14, 2022 at 20:16:19 +0100, 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 fields. Fix it by adding appropriate locking.
> 
> We err on the side of safety and grab the sk_callback_lock also inside the
> sk_destruct callback overridden by l2tp, even though there should be no
> refs allowing access to the sock at the time when sk_destruct gets called.
> 
> v4:
> - serialize write to sk_user_data in l2tp sk_destruct
> 
> v3:
> - switch from sock lock to sk_callback_lock
> - document write-protection for sk_user_data
> 
> v2:
> - update Fixes to point to origin of the bug
> - use real names in Reported/Tested-by tags

LGTM, thanks Jakub.

> 
> Cc: Tom Parkin <tparkin@katalix.com>
> Fixes: 3557baabf280 ("[L2TP]: PPP over L2TP driver core")
> Reported-by: Haowei Yan <g1042620637@gmail.com>
> Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
> ---
> 
> This took me forever. Sorry about that.
> 
>  include/net/sock.h   |  2 +-
>  net/l2tp/l2tp_core.c | 19 +++++++++++++------
>  2 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 5db02546941c..e0517ecc6531 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -323,7 +323,7 @@ struct sk_filter;
>    *	@sk_tskey: counter to disambiguate concurrent tstamp requests
>    *	@sk_zckey: counter to order MSG_ZEROCOPY notifications
>    *	@sk_socket: Identd and reporting IO signals
> -  *	@sk_user_data: RPC layer private data
> +  *	@sk_user_data: RPC layer private data. Write-protected by @sk_callback_lock.
>    *	@sk_frag: cached page frag
>    *	@sk_peek_off: current peek_offset value
>    *	@sk_send_head: front of stuff to transmit
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index 7499c51b1850..754fdda8a5f5 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -1150,8 +1150,10 @@ static void l2tp_tunnel_destruct(struct sock *sk)
>  	}
>  
>  	/* Remove hooks into tunnel socket */
> +	write_lock_bh(&sk->sk_callback_lock);
>  	sk->sk_destruct = tunnel->old_sk_destruct;
>  	sk->sk_user_data = NULL;
> +	write_unlock_bh(&sk->sk_callback_lock);
>  
>  	/* Call the original destructor */
>  	if (sk->sk_destruct)
> @@ -1469,16 +1471,18 @@ 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;
> +	write_lock(&sk->sk_callback_lock);
> +
> +	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 +1508,7 @@ 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 +1522,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
>  	if (tunnel->fd >= 0)
>  		sockfd_put(sock);
>  
> +	write_unlock(&sk->sk_callback_lock);
>  	return 0;
>  
>  err_sock:
> @@ -1525,6 +1530,8 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
>  		sock_release(sock);
>  	else
>  		sockfd_put(sock);
> +
> +	write_unlock(&sk->sk_callback_lock);
>  err:
>  	return ret;
>  }
> -- 
> 2.38.1
> 

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

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