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

[Jakub Sitnicki <jakub@cloudflare.com>]

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.

Fixes: fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Reported-by: van fantasy <g1042620637@gmail.com>
Tested-by: van fantasy <g1042620637@gmail.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.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