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

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Wed, Jun 03, 2026 at 04:44:31PM +0800, Xu Yang wrote:

> 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.

...

>  	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);

Wouldn't be better to use swnode_get() / swnode_put() instead?
*Yes, we might need to add some NULL checks there.

> +	if (!p || list_empty(&p->children))
>  		return NULL;
> -	}

-- 
With Best Regards,
Andy Shevchenko