net: dsa: sja1105: fix NULL pointer dereference in sja1105_reload_cbs()

priv->cbs is an array of priv->info->num_cbs_shapers elements of type
struct sja1105_cbs_entry which only get allocated if CONFIG_NET_SCH_CBS
is enabled.

However, sja1105_reload_cbs() is called from sja1105_static_config_reload()
which in turn is called for any of the items in sja1105_reset_reasons,
therefore during the normal runtime of the driver and not just from a
code path which can be triggered by the tc-cbs offload.

The sja1105_reload_cbs() function does not contain a check whether the
priv->cbs array is NULL or not, it just assumes it isn't and proceeds to
iterate through the credit-based shaper elements. This leads to a NULL
pointer dereference.

The solution is to return success if the priv->cbs array has not been
allocated, since sja1105_reload_cbs() has nothing to do.

Fixes: 4d752508 ("net: dsa: sja1105: offload the Credit-Based Shaper qdisc")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/sja1105/sja1105_main.c

@@ -1798,6 +1798,12 @@ static int sja1105_reload_cbs(struct sja1105_private *priv)
 {
 	int rc = 0, i;
 
+	/* The credit based shapers are only allocated if
+	 * CONFIG_NET_SCH_CBS is enabled.
+	 */
+	if (!priv->cbs)
+		return 0;
+
 	for (i = 0; i < priv->info->num_cbs_shapers; i++) {
 		struct sja1105_cbs_entry *cbs = &priv->cbs[i];