sctp: ensure ep is not destroyed before doing the dump

Now before dumping a sock in sctp_diag, it only holds the sock while
the ep may be already destroyed. It can cause a use-after-free panic
when accessing ep->asocs.

This patch is to set sctp_sk(sk)->ep NULL in sctp_endpoint_destroy,
and check if this ep is already destroyed before dumping this ep.

Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdrver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/endpointola.c

@@ -275,6 +275,7 @@ static void sctp_endpoint_destroy(struct sctp_endpoint *ep)
 		if (sctp_sk(sk)->bind_hash)
 			sctp_put_port(sk);
 
+		sctp_sk(sk)->ep = NULL;
 		sock_put(sk);
 	}
 

net/sctp/sctp_diag.c

@@ -278,7 +278,6 @@ out:
 
 static int sctp_sock_dump(struct sock *sk, void *p)
 {
-	struct sctp_endpoint *ep = sctp_sk(sk)->ep;
 	struct sctp_comm_param *commp = p;
 	struct sk_buff *skb = commp->skb;
 	struct netlink_callback *cb = commp->cb;
@@ -287,7 +286,9 @@ static int sctp_sock_dump(struct sock *sk, void *p)
 	int err = 0;
 
 	lock_sock(sk);
-	list_for_each_entry(assoc, &ep->asocs, asocs) {
+	if (!sctp_sk(sk)->ep)
+		goto release;
+	list_for_each_entry(assoc, &sctp_sk(sk)->ep->asocs, asocs) {
 		if (cb->args[4] < cb->args[1])
 			goto next;