[PATCH 5.10 053/593] can: j1939: j1939_sk_init(): set SOCK_RCU_FREE to call sk_destruct() after RCU is done

[Greg Kroah-Hartman]

From: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>

commit 22c696fed25c63c7f67508309820358b94a96b6d upstream.

Set SOCK_RCU_FREE to let RCU to call sk_destruct() on completion.
Without this patch, we will run in to j1939_can_recv() after priv was
freed by j1939_sk_release()->j1939_sk_sock_destruct()

Fixes: 25fe97cb7620 ("can: j1939: move j1939_priv_put() into sk_destruct callback")
Link: https://lore.kernel.org/r/20210617130623.12705-1-o.rempel@xxxxxxxxxxxxxx
Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
Reported-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
Reported-by: syzbot+bdf710cfc41c186fdff3@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/can/j1939/main.c   |    4 ++++
 net/can/j1939/socket.c |    3 +++
 2 files changed, 7 insertions(+)

--- a/net/can/j1939/main.c
+++ b/net/can/j1939/main.c
@@ -193,6 +193,10 @@ static void j1939_can_rx_unregister(stru
 	can_rx_unregister(dev_net(ndev), ndev, J1939_CAN_ID, J1939_CAN_MASK,
 			  j1939_can_recv, priv);
 
+	/* The last reference of priv is dropped by the RCU deferred
+	 * j1939_sk_sock_destruct() of the last socket, so we can
+	 * safely drop this reference here.
+	 */
 	j1939_priv_put(priv);
 }
 
--- a/net/can/j1939/socket.c
+++ b/net/can/j1939/socket.c
@@ -398,6 +398,9 @@ static int j1939_sk_init(struct sock *sk
 	atomic_set(&jsk->skb_pending, 0);
 	spin_lock_init(&jsk->sk_session_queue_lock);
 	INIT_LIST_HEAD(&jsk->sk_session_queue);
+
+	/* j1939_sk_sock_destruct() depends on SOCK_RCU_FREE flag */
+	sock_set_flag(sk, SOCK_RCU_FREE);
 	sk->sk_destruct = j1939_sk_sock_destruct;
 	sk->sk_protocol = CAN_J1939;