Re: [PATCH 3/4] software node: handle missing remote endpoint node

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] software node: refcount race in software_node_graph_get_remote_endpoint()
--

commit 248da443d964ed22789819ccc10771552e53780f
Author: Peng Fan <peng.fan@nxp.com>

software node: handle missing remote endpoint node

This commit updates software_node_graph_get_remote_endpoint() to check the
return value of software_node_fwnode() for NULL before passing it to
software_node_get().

> diff --git a/drivers/base/swnode.c b/drivers/base/swnode.c
> index 9095d50ba146f..4d95a59a27f0a 100644
> --- a/drivers/base/swnode.c
> +++ b/drivers/base/swnode.c
> @@ -676,6 +676,7 @@ software_node_graph_get_remote_endpoint(const struct fwnode_handle *fwnode)
>  {
>  	struct swnode *swnode = to_swnode(fwnode);
>  	const struct software_node_ref_args *ref;
> +	struct fwnode_handle *refnode;
>  	const struct property_entry *prop;
>  
>  	if (!swnode)
> @@ -690,7 +691,11 @@ software_node_graph_get_remote_endpoint(const struct fwnode_handle *fwnode)
>  	if (!ref->swnode)
>  		return NULL;
>  
> -	return software_node_get(software_node_fwnode(ref->swnode));
> +	refnode = software_node_fwnode(ref->swnode);
> +	if (!refnode)
> +		return NULL;
> +
> +	return software_node_get(refnode);

[Severity: High]
Can this race with a concurrent unregistration of ref->swnode and cause a
use-after-free or refcount addition on zero?

software_node_fwnode() calls software_node_to_swnode(), which acquires a
reference via software_node_to_swnode_get() and immediately drops it:

drivers/base/swnode.c:software_node_to_swnode() {
	struct swnode *swnode = software_node_to_swnode_get(node);

	if (swnode)
		swnode_put(swnode);

	return swnode;
}

Because the caller of software_node_graph_get_remote_endpoint() only holds
a reference to the local endpoint fwnode and not to ref->swnode, if
ref->swnode is concurrently unregistered after kobject_get_unless_zero()
succeeds in software_node_to_swnode_get(), either swnode_put() inside
software_node_to_swnode() or the concurrent unregistration can drop the
last reference before software_node_get(refnode) runs.

software_node_get(refnode) will then call kobject_get() on a dying or
freed swnode.

Should software_node_graph_get_remote_endpoint() call
software_node_to_swnode_get(ref->swnode) directly and return
&remote->fwnode (or NULL) so that the reference acquired under
swnode_kset->list_lock via kobject_get_unless_zero() is retained?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261104-swnode-race-v1-v1-0-9406a67e25be@nxp.com?part=3