net: rds: Fix possible null-pointer dereferences in rds_rdma_cm_event_handler_cmn()

In rds_rdma_cm_event_handler_cmn(), there are some if statements to
check whether conn is NULL, such as on lines 65, 96 and 112.
But conn is not checked before being used on line 108:
    trans->cm_connect_complete(conn, event);
and on lines 140-143:
    rdsdebug("DISCONNECT event - dropping connection "
            "%pI6c->%pI6c\n", &conn->c_laddr,
            &conn->c_faddr);
    rds_conn_drop(conn);

Thus, possible null-pointer dereferences may occur.

To fix these bugs, conn is checked before being used.

These bugs are found by a static analysis tool STCheck written by us.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/rdma_transport.c

@@ -105,7 +105,8 @@ static int rds_rdma_cm_event_handler_cmn(struct rdma_cm_id *cm_id,
 		break;
 
 	case RDMA_CM_EVENT_ESTABLISHED:
-		trans->cm_connect_complete(conn, event);
+		if (conn)
+			trans->cm_connect_complete(conn, event);
 		break;
 
 	case RDMA_CM_EVENT_REJECTED:
@@ -137,6 +138,8 @@ static int rds_rdma_cm_event_handler_cmn(struct rdma_cm_id *cm_id,
 		break;
 
 	case RDMA_CM_EVENT_DISCONNECTED:
+		if (!conn)
+			break;
 		rdsdebug("DISCONNECT event - dropping connection "
 			 "%pI6c->%pI6c\n", &conn->c_laddr,
 			 &conn->c_faddr);