sctp: sctp_epaddr_lookup_transport should be protected by rcu_read_lock

Since commit 7fda702f9315 ("sctp: use new rhlist interface on sctp transport
rhashtable"), sctp has changed to use rhlist_lookup to look up transport, but
rhlist_lookup doesn't call rcu_read_lock inside, unlike rhashtable_lookup_fast.

It is called in sctp_epaddr_lookup_transport and sctp_addrs_lookup_transport.
sctp_addrs_lookup_transport is always in the protection of rcu_read_lock(),
as __sctp_lookup_association is called in rx path or sctp_lookup_association
which are in the protection of rcu_read_lock() already.

But sctp_epaddr_lookup_transport is called by sctp_endpoint_lookup_assoc, it
doesn't call rcu_read_lock, which may cause "suspicious rcu_dereference_check
usage' in __rhashtable_lookup.

This patch is to fix it by adding rcu_read_lock in sctp_endpoint_lookup_assoc
before calling sctp_epaddr_lookup_transport.

Fixes: 7fda702f9315 ("sctp: use new rhlist interface on sctp transport rhashtable")
Reported-by: Dmitry Vyukov <[e-mail]>
Signed-off-by: Xin Long <[e-mail]>
Acked-by: Marcelo Ricardo Leitner <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sctp/endpointola.c

···
 	 * on this endpoint.
 	 */
 	if (!ep->base.bind_addr.port)
-		goto out;
+		return NULL;
+
+	rcu_read_lock();
 	t = sctp_epaddr_lookup_transport(ep, paddr);
 	if (!t)
 		goto out;
···
 	*transport = t;
 	asoc = t->asoc;
 out:
+	rcu_read_unlock();
 	return asoc;
 }