net/smc: Fix af_ops of child socket pointing to released memory

Child sockets may inherit the af_ops from the parent listen socket.
When the listen socket is released then the af_ops of the child socket
points to released memory.
Solve that by restoring the original af_ops for child sockets which
inherited the parent af_ops. And clear any inherited user_data of the
parent socket.

Fixes: 8270d9c2 ("net/smc: Limit backlog connections")
Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Reviewed-by: D. Wythe <alibuda@linux.alibaba.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/smc/af_smc.c

@@ -121,6 +121,7 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk,
 					  bool *own_req)
 {
 	struct smc_sock *smc;
+	struct sock *child;
 
 	smc = smc_clcsock_user_data(sk);
 
@@ -134,8 +135,17 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk,
 	}
 
 	/* passthrough to original syn recv sock fct */
-	return smc->ori_af_ops->syn_recv_sock(sk, skb, req, dst, req_unhash,
+	child = smc->ori_af_ops->syn_recv_sock(sk, skb, req, dst, req_unhash,
 					       own_req);
+	/* child must not inherit smc or its ops */
+	if (child) {
+		rcu_assign_sk_user_data(child, NULL);
+
+		/* v4-mapped sockets don't inherit parent ops. Don't restore. */
+		if (inet_csk(child)->icsk_af_ops == inet_csk(sk)->icsk_af_ops)
+			inet_csk(child)->icsk_af_ops = smc->ori_af_ops;
+	}
+	return child;
 
 drop:
 	dst_release(dst);