net: dsa: fix 'increment on 0' warning

Setting the refcount to 0 when allocating a tree to match the number of
switch devices it holds may cause an 'increment on 0; use-after-free',
if CONFIG_REFCOUNT_FULL is enabled.

To fix this, do not decrement the refcount of a newly allocated tree,
increment it when an already allocated tree is found, and decrement it
after the probing of a switch, as done with the previous behavior.

At the same time, make dsa_tree_get and dsa_tree_put accept a NULL
argument to simplify callers, and return the tree after incrementation,
as most kref users like of_node_get and of_node_put do.

Fixes: 8e5bf9759a06 ("net: dsa: simplify tree reference counting")
Signed-off-by: Vivien Didelot <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Tested-by: Florian Fainelli <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/dsa/dsa2.c

···
 	INIT_LIST_HEAD(&dst->list);
 	list_add_tail(&dsa_tree_list, &dst->list);
 
-	/* Initialize the reference counter to the number of switches, not 1 */
 	kref_init(&dst->refcount);
-	refcount_set(&dst->refcount.refcount, 0);
 
 	return dst;
 }
···
 	kfree(dst);
 }
 
+static struct dsa_switch_tree *dsa_tree_get(struct dsa_switch_tree *dst)
+{
+	if (dst)
+		kref_get(&dst->refcount);
+
+	return dst;
+}
+
 static struct dsa_switch_tree *dsa_tree_touch(int index)
 {
 	struct dsa_switch_tree *dst;
 
 	dst = dsa_tree_find(index);
-	if (!dst)
-		dst = dsa_tree_alloc(index);
-
-	return dst;
-}
-
-static void dsa_tree_get(struct dsa_switch_tree *dst)
-{
-	kref_get(&dst->refcount);
+	if (dst)
+		return dsa_tree_get(dst);
+	else
+		return dsa_tree_alloc(index);
 }
 
 static void dsa_tree_release(struct kref *ref)
···
 
 static void dsa_tree_put(struct dsa_switch_tree *dst)
 {
-	kref_put(&dst->refcount, dsa_tree_release);
+	if (dst)
+		kref_put(&dst->refcount, dsa_tree_release);
 }
 
 static bool dsa_port_is_dsa(struct dsa_port *port)
···
 
 	mutex_lock(&dsa2_mutex);
 	err = dsa_switch_probe(ds);
+	dsa_tree_put(ds->dst);
 	mutex_unlock(&dsa2_mutex);
 
 	return err;