[PATCH] software node: make swnodes hold their own references on parents

[Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>]

In swnode_register(), swnode->parent is assigned before calling
kobject_init_and_add(), which normally acquires a reference on
parent->kobj via kobject_add_internal() that is held until after
software_node_release() completes. However, if kobject_init_and_add()
fails before acquiring the parent reference or drops it on error in
kobject_add_internal(), swnode->kobj holds no reference on parent->kobj
while swnode->parent remains set.

With CONFIG_DEBUG_KOBJECT_RELEASE enabled, the random delay in
kobject_delayed_cleanup() may create a situation where - when
unregistering the child node and then the parent - the parent object
gets released before the child who's left with a dangling pointer.

Acquire a refernce to the parent swnode when registering the child and
drop it on child's release.

Cc: stable@vger.kernel.org
Fixes: 59abd83672f7 ("drivers: base: Introducing software nodes to the firmware node framework")
Closes: https://sashiko.dev/#/bug/linux-82933c14-d212-49f1-bd2f-d4ab1e16fed7
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
---
 drivers/base/swnode.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/base/swnode.c b/drivers/base/swnode.c
index 84a77ffa3dc4bbcca62bbe67c3390693500b01ea..986dcb933d2c9274e9000968f115d7fccc2d103f 100644
--- a/drivers/base/swnode.c
+++ b/drivers/base/swnode.c
@@ -843,11 +843,12 @@ static void software_node_free(const struct software_node *node)
 static void software_node_release(struct kobject *kobj)
 {
 	struct swnode *swnode = kobj_to_swnode(kobj);
+	struct swnode *parent = swnode->parent;
 
 	fwnode_links_purge(&swnode->fwnode);
 
-	if (swnode->parent) {
-		ida_free(&swnode->parent->child_ids, swnode->id);
+	if (parent) {
+		ida_free(&parent->child_ids, swnode->id);
 		list_del(&swnode->entry);
 	} else {
 		ida_free(&swnode_root_ids, swnode->id);
@@ -858,6 +859,9 @@ static void software_node_release(struct kobject *kobj)
 
 	ida_destroy(&swnode->child_ids);
 	kfree(swnode);
+
+	if (parent)
+		swnode_put(parent);
 }
 
 static const struct kobj_type software_node_type = {
@@ -885,7 +889,7 @@ swnode_register(const struct software_node *node, struct swnode *parent,
 
 	swnode->id = ret;
 	swnode->node = node;
-	swnode->parent = parent;
+	swnode->parent = parent ? swnode_get(parent) : NULL;
 	swnode->kobj.kset = swnode_kset;
 	fwnode_init(&swnode->fwnode, &software_node_ops);
 

---
base-commit: ddfe51b0a0f5dc9e415cc2c8ad8db17532dd426b
change-id: 20261106-swnode-kobject-parent-ref-3bd9b9b75aeb

Best regards,
-- 
Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>