generic: mt7530: fix null pointer dereferencing in port5 setup

The 2nd gmac of mediatek soc ethernet may not be connected to a PHY
and a phy-handle isn't always available.
Unfortunately, mt7530 dsa driver assumes that the 2nd gmac is always
connected to switch port 5 and setup mt7530 according to phy address
of 2nd gmac node, causing null pointer dereferencing when phy-handle
isn't defined in dts.
This commit fix this setup code by checking return value of
of_parse_phandle before using it.

Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>

target/linux/generic/backport-5.4/746-stable-net-dsa-mt7530-fix-null-pointer-dereferencing-in-por.patch

@@ -0,0 +1,39 @@
+From 0452800f6db4ed0a42ffb15867c0acfd68829f6a Mon Sep 17 00:00:00 2001
+From: Chuanhong Guo <gch981213@gmail.com>
+Date: Fri, 3 Apr 2020 19:28:24 +0800
+Subject: net: dsa: mt7530: fix null pointer dereferencing in port5 setup
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+The 2nd gmac of mediatek soc ethernet may not be connected to a PHY
+and a phy-handle isn't always available.
+Unfortunately, mt7530 dsa driver assumes that the 2nd gmac is always
+connected to switch port 5 and setup mt7530 according to phy address
+of 2nd gmac node, causing null pointer dereferencing when phy-handle
+isn't defined in dts.
+This commit fix this setup code by checking return value of
+of_parse_phandle before using it.
+
+Fixes: 38f790a80560 ("net: dsa: mt7530: Add support for port 5")
+Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
+Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
+Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
+Tested-by: René van Dorst <opensource@vdorst.com>
+Signed-off-by: David S. Miller <davem@davemloft.net>
+---
+ drivers/net/dsa/mt7530.c | 3 +++
+ 1 file changed, 3 insertions(+)
+
+--- a/drivers/net/dsa/mt7530.c
++++ b/drivers/net/dsa/mt7530.c
+@@ -1411,6 +1411,9 @@ mt7530_setup(struct dsa_switch *ds)
+ 				continue;
+ 
+ 			phy_node = of_parse_phandle(mac_np, "phy-handle", 0);
++			if (!phy_node)
++				continue;
++
+ 			if (phy_node->parent == priv->dev->of_node->parent) {
+ 				interface = of_get_phy_mode(mac_np);
+ 				id = of_mdio_parse_addr(ds->dev, phy_node);