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 15:26:51 +0200, Jakub Sitnicki wrote:
> 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.

Yes, you're quite right -- my apologies.

> >
> >> 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
> >> 
> 

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