sctp: fix the transport dead race check by using atomic_add_unless on refcnt

Now when __sctp_lookup_association is running in BH, it will try to
check if t->dead is set, but meanwhile other CPUs may be freeing this
transport and this assoc and if it happens that
__sctp_lookup_association checked t->dead a bit too early, it may think
that the association is still good while it was already freed.

So we fix this race by using atomic_add_unless in sctp_transport_hold.
After we get one transport from hashtable, we will hold it only when
this transport's refcnt is not 0, so that we can make sure t->asoc
cannot be freed before we hold the asoc again.

Note that sctp association is not freed using RCU so we can't use
atomic_add_unless() with it as it may just be too late for that either.

Fixes: 4f0087812648 ("sctp: apply rhashtable api to send/recv path")
Reported-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/structs.h

@@ -955,7 +955,7 @@ void sctp_transport_route(struct sctp_transport *, union sctp_addr *,
 void sctp_transport_pmtu(struct sctp_transport *, struct sock *sk);
 void sctp_transport_free(struct sctp_transport *);
 void sctp_transport_reset_timers(struct sctp_transport *);
-void sctp_transport_hold(struct sctp_transport *);
+int sctp_transport_hold(struct sctp_transport *);
 void sctp_transport_put(struct sctp_transport *);
 void sctp_transport_update_rto(struct sctp_transport *, __u32);
 void sctp_transport_raise_cwnd(struct sctp_transport *, __u32, __u32);

net/sctp/input.c

@@ -935,15 +935,22 @@ static struct sctp_association *__sctp_lookup_association(
 					struct sctp_transport **pt)
 {
 	struct sctp_transport *t;
+	struct sctp_association *asoc = NULL;
 
+	rcu_read_lock();
 	t = sctp_addrs_lookup_transport(net, local, peer);
-	if (!t || t->dead)
-		return NULL;
+	if (!t || !sctp_transport_hold(t))
+		goto out;
 
-	sctp_association_hold(t->asoc);
+	asoc = t->asoc;
+	sctp_association_hold(asoc);
 	*pt = t;
 
-	return t->asoc;
+	sctp_transport_put(t);
+
+out:
+	rcu_read_unlock();
+	return asoc;
 }
 
 /* Look up an association. protected by RCU read lock */
@@ -955,9 +962,7 @@ struct sctp_association *sctp_lookup_association(struct net *net,
 {
 	struct sctp_association *asoc;
 
-	rcu_read_lock();
 	asoc = __sctp_lookup_association(net, laddr, paddr, transportp);
-	rcu_read_unlock();
 
 	return asoc;
 }

net/sctp/transport.c

@@ -296,9 +296,9 @@ void sctp_transport_route(struct sctp_transport *transport,
 }
 
 /* Hold a reference to a transport.  */
-void sctp_transport_hold(struct sctp_transport *transport)
+int sctp_transport_hold(struct sctp_transport *transport)
 {
-	atomic_inc(&transport->refcnt);
+	return atomic_add_unless(&transport->refcnt, 1, 0);
 }
 
 /* Release a reference to a transport and clean up