On Mon, Nov 21, 2022 at 12:12 AM -05, Sasha Levin wrote:

This is a note to let you know that I've just added the patch titled

l2tp: Serialize access to sk_user_data with sk_callback_lock

to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: l2tp-serialize-access-to-sk_user_data-with-sk_callba.patch and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

Please don't add the commit below to stable tree, yet. We have a fix-up for it under review:

https://lore.kernel.org/netdev/20221121085426.21315-1-jakub@cloudflare.com/

commit 92aa1132edc6e6e912efd056c698cd6e52b83c6f Author: Jakub Sitnicki jakub@cloudflare.com Date: Mon Nov 14 20:16:19 2022 +0100

l2tp: Serialize access to sk_user_data with sk_callback_lock

[ Upstream commit b68777d54fac21fc833ec26ea1a2a84f975ab035 ] 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 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 Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Sasha Levin sashal@kernel.org

diff --git a/include/net/sock.h b/include/net/sock.h index e1a303e4f0f7..3e9db5146765 100644 --- a/include/net/sock.h +++ b/include/net/sock.h @@ -323,7 +323,7 @@ struct bpf_local_storage;

• @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 93271a2632b8..c77032638a06 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) @@ -1471,16 +1473,18 @@ 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;

@@ -1506,7 +1510,7 @@ 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; @@ -1520,6 +1524,7 @@ 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: @@ -1527,6 +1532,8 @@ 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; }