[PATCH v2 1/2] software node: fix refcount leak in software_node_get_next_child()

[Xu Yang <xu.yang_2@oss.nxp.com>]

From: Xu Yang <xu.yang_2@nxp.com>

When a swnode acts as a secondary fwnode and is participates in child
iteration, a refcount leak occurs for the last child of the primary
fwnode's children.

                   Parent      Child
  (Primary fwnode)   FW:   {FW1, FW2, FW3}
(Secondary fwnode)   SW:   {}

In this case, FW3's refcount is decremented twice during iteration:

 fwnode_get_next_child_node(FW, FW3)
  1. fwnode_call_ptr_op(FW, get_next_child_node, FW3) returns NULL and
     decrements FW3's refcount
  2. fwnode_call_ptr_op(SW, get_next_child_node, FW3) returns NULL and
     decrements FW3's refcount again

The same double-decrement issue occurs when SW has children.

The kernel dump as below:

[   25.435805] OF: ERROR: of_node_release() detected bad of_node_put() on /soc/usb@4c010010/usb@4c100000
[   25.445072] CPU: 0 UID: 0 PID: 617 Comm: sh Not tainted 7.1.0-rc4-next-20260522-00011-g7376b330abca #210 PREEMPT
[   25.445080] Hardware name: NXP i.MX95 19X19 board (DT)
[   25.445083] Call trace:
[   25.445086]  show_stack+0x18/0x30 (C)
[   25.445101]  dump_stack_lvl+0x60/0x80
[   25.445108]  dump_stack+0x18/0x24
[   25.445113]  of_node_release+0x158/0x194
[   25.445122]  kobject_put+0xa0/0x120
[   25.445129]  of_node_put+0x18/0x28
[   25.445134]  of_fwnode_put+0x38/0x58
[   25.445141]  software_node_get_next_child+0x54/0x15c
[   25.445150]  fwnode_get_next_child_node+0x70/0x94
[   25.445156]  fwnode_get_next_available_child_node+0x34/0x88
[   25.445162]  device_links_driver_bound+0x2f4/0x334
[   25.445168]  driver_bound+0x68/0xb0
                ...
[   25.445258] OF: ERROR: next of_node_put() on this node will result in a kobject warning 'refcount_t: underflow; use-after-free.'

Fix this by ensuring software_node_get_next_child() does not decrement
the child's refcount when:
- The parent has no children, OR
- The parent has children but the input child is not a swnode

This prevents the refcount from being incorrectly decremented for
fwnodes that don't belong to the software node hierarchy.

Fixes: fb5ec981adf0 ("media: software_node: Fix refcounts in software_node_get_next_child()")
Cc: stable@vger.kernel.org
Signed-off-by: Xu Yang <xu.yang_2@nxp.com>

---
Changes in v2:
 - no changes
---
 drivers/base/swnode.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/drivers/base/swnode.c b/drivers/base/swnode.c
index 869228a65cb3..507de464d387 100644
--- a/drivers/base/swnode.c
+++ b/drivers/base/swnode.c
@@ -474,18 +474,18 @@ software_node_get_next_child(const struct fwnode_handle *fwnode,
 	struct swnode *p = to_swnode(fwnode);
 	struct swnode *c = to_swnode(child);
 
-	if (!p || list_empty(&p->children) ||
-	    (c && list_is_last(&c->entry, &p->children))) {
-		fwnode_handle_put(child);
+	if (!p || list_empty(&p->children))
 		return NULL;
-	}
 
-	if (c)
+	if (c) {
+		fwnode_handle_put(child);
+		if (list_is_last(&c->entry, &p->children))
+			return NULL;
 		c = list_next_entry(c, entry);
-	else
+	} else {
 		c = list_first_entry(&p->children, struct swnode, entry);
+	}
 
-	fwnode_handle_put(child);
 	return fwnode_handle_get(&c->fwnode);
 }
 

-- 
2.34.1