[PATCH 4.18 033/197] sctp: hold transport before accessing its asoc in sctp_transport_get_next

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Xin Long <lucien.xin@xxxxxxxxx>

[ Upstream commit bab1be79a5169ac748d8292b20c86d874022d7ba ]

As Marcelo noticed, in sctp_transport_get_next, it is iterating over
transports but then also accessing the association directly, without
checking any refcnts before that, which can cause an use-after-free
Read.

So fix it by holding transport before accessing the association. With
that, sctp_transport_hold calls can be removed in the later places.

Fixes: 626d16f50f39 ("sctp: export some apis or variables for sctp_diag and reuse some for proc")
Reported-by: syzbot+fe62a0c9aa6a85c6de16@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/sctp/proc.c   |    4 ----
 net/sctp/socket.c |   22 +++++++++++++++-------
 2 files changed, 15 insertions(+), 11 deletions(-)

--- a/net/sctp/proc.c
+++ b/net/sctp/proc.c
@@ -260,8 +260,6 @@ static int sctp_assocs_seq_show(struct s
 	}
 
 	transport = (struct sctp_transport *)v;
-	if (!sctp_transport_hold(transport))
-		return 0;
 	assoc = transport->asoc;
 	epb = &assoc->base;
 	sk = epb->sk;
@@ -318,8 +316,6 @@ static int sctp_remaddr_seq_show(struct
 	}
 
 	transport = (struct sctp_transport *)v;
-	if (!sctp_transport_hold(transport))
-		return 0;
 	assoc = transport->asoc;
 
 	list_for_each_entry_rcu(tsp, &assoc->peer.transport_addr_list,
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -4881,9 +4881,14 @@ struct sctp_transport *sctp_transport_ge
 			break;
 		}
 
+		if (!sctp_transport_hold(t))
+			continue;
+
 		if (net_eq(sock_net(t->asoc->base.sk), net) &&
 		    t->asoc->peer.primary_path == t)
 			break;
+
+		sctp_transport_put(t);
 	}
 
 	return t;
@@ -4893,13 +4898,18 @@ struct sctp_transport *sctp_transport_ge
 					      struct rhashtable_iter *iter,
 					      int pos)
 {
-	void *obj = SEQ_START_TOKEN;
+	struct sctp_transport *t;
+
+	if (!pos)
+		return SEQ_START_TOKEN;
 
-	while (pos && (obj = sctp_transport_get_next(net, iter)) &&
-	       !IS_ERR(obj))
-		pos--;
+	while ((t = sctp_transport_get_next(net, iter)) && !IS_ERR(t)) {
+		if (!--pos)
+			break;
+		sctp_transport_put(t);
+	}
 
-	return obj;
+	return t;
 }
 
 int sctp_for_each_endpoint(int (*cb)(struct sctp_endpoint *, void *),
@@ -4958,8 +4968,6 @@ again:
 
 	tsp = sctp_transport_get_idx(net, &hti, *pos + 1);
 	for (; !IS_ERR_OR_NULL(tsp); tsp = sctp_transport_get_next(net, &hti)) {
-		if (!sctp_transport_hold(tsp))
-			continue;
 		ret = cb(tsp, p);
 		if (ret)
 			break;