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Message-ID: <20221115111258.GA5373@katalix.com>
Date: Tue, 15 Nov 2022 11:12:58 +0000
From: Tom Parkin <tparkin@...alix.com>
To: Jakub Sitnicki <jakub@...udflare.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Haowei Yan <g1042620637@...il.com>
Subject: Re: [PATCH net v4] l2tp: Serialize access to sk_user_data with
sk_callback_lock
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@...alix.com>
> Fixes: 3557baabf280 ("[L2TP]: PPP over L2TP driver core")
> Reported-by: Haowei Yan <g1042620637@...il.com>
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.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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