net: dsa: assert uniqueness of dsa,member properties

The cross-chip notifiers work by comparing each ds->index against the
info->sw_index value from the notifier. The ds->index is retrieved from
the device tree dsa,member property.

If a single tree cross-chip topology does not declare unique switch IDs,
this will result in hard-to-debug issues/voodoo effects such as the
cross-chip notifier for one switch port also matching the port with the
same number from another switch.

Check in dsa_switch_parse_member_of() whether the DSA switch tree
contains a DSA switch with the index we're preparing to add, before
actually adding it.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -1259,6 +1259,13 @@ static int dsa_switch_parse_member_of(struct dsa_switch *ds,
 	if (!ds->dst)
 		return -ENOMEM;
 
+	if (dsa_switch_find(ds->dst->index, ds->index)) {
+		dev_err(ds->dev,
+			"A DSA switch with index %d already exists in tree %d\n",
+			ds->index, ds->dst->index);
+		return -EEXIST;
+	}
+
 	return 0;
 }