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>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index aab9e3f29755..69f3799570a7 100644
--- a/net/sctp/transport.c
+++ b/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