[PATCH net] net: dsa: Ensure validity of dst->ds[0]

[Florian Fainelli <f.fainelli@gmail.com>]

It is perfectly possible to have non zero indexed switches being present
in a DSA switch tree, in such a case, we will be deferencing a NULL
pointer while dsa_cpu_port_ethtool_{setup,restore}. Be more defensive
and ensure that dst->ds[0] is valid before doing anything with it.

Fixes: 0c73c523cf73 ("net: dsa: Initialize CPU port ethtool ops per tree")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
---
 net/dsa/dsa2.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
index 5fff951a0a49..da3862124545 100644
--- a/net/dsa/dsa2.c
+++ b/net/dsa/dsa2.c
@@ -394,9 +394,11 @@ static int dsa_dst_apply(struct dsa_switch_tree *dst)
 			return err;
 	}
 
-	err = dsa_cpu_port_ethtool_setup(dst->ds[0]);
-	if (err)
-		return err;
+	if (dst->ds[0]) {
+		err = dsa_cpu_port_ethtool_setup(dst->ds[0]);
+		if (err)
+			return err;
+	}
 
 	/* If we use a tagging format that doesn't have an ethertype
 	 * field, make sure that all packets from this point on get
@@ -433,7 +435,8 @@ static void dsa_dst_unapply(struct dsa_switch_tree *dst)
 		dsa_ds_unapply(dst, ds);
 	}
 
-	dsa_cpu_port_ethtool_restore(dst->ds[0]);
+	if (dst->ds[0])
+		dsa_cpu_port_ethtool_restore(dst->ds[0]);
 
 	pr_info("DSA: tree %d unapplied\n", dst->tree);
 	dst->applied = false;
-- 
2.9.3