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

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 15 Aug 2022 15:26:51 +0200

On Mon, Aug 15, 2022 at 02:21 PM +01, Tom Parkin wrote:
> [[PGP Signed Part:Undecided]]
> 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.

I think you are referring to the PPP-over-L2TP socket, not the UDP
socket. In pppol2tp_prepare_tunnel_socket() @ 3557baabf280 we're not
holding the sock lock over the UDP socket, AFAICT.

>
>> Reported-by: Haowei Yan <[email-masked]>
>> Tested-by: Haowei Yan <[email-masked]>
>> Signed-off-by: Jakub Sitnicki <[email-masked]>
>> ---
>> Cc: Tom Parkin <[email-masked]>
>> 
>>  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
>>