Re: [PATCH] i2c: core: Fix double-free of fwnode in i2c_unregister_device()

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

On Sat, Jul 19, 2025 at 08:01:04PM +0200, Hans de Goede wrote:
> Before commit df6d7277e552 ("i2c: core: Do not dereference fwnode in struct
> device"), i2c_unregister_device() only called fwnode_handle_put() on
> of_node-s in the form of calling of_node_put(client->dev.of_node).
> 
> But after this commit the i2c_client's fwnode now unconditionally gets
> fwnode_handle_put() on it.
> 
> When the i2c_client has no primary (ACPI / OF) fwnode but it does have
> a software fwnode, the software-node will be the primary node and
> fwnode_handle_put() will put() it.
> 
> But for the software fwnode device_remove_software_node() will also put()
> it leading to a double free:
> 
> [   82.665598] ------------[ cut here ]------------
> [   82.665609] refcount_t: underflow; use-after-free.
> [   82.665808] WARNING: CPU: 3 PID: 1502 at lib/refcount.c:28 refcount_warn_saturate+0xba/0x11
> ...
> [   82.666830] RIP: 0010:refcount_warn_saturate+0xba/0x110
> ...
> [   82.666962]  <TASK>
> [   82.666971]  i2c_unregister_device+0x60/0x90
> 
> Fix this by not calling fwnode_handle_put() when the primary fwnode is
> a software-node.

Thanks for the fix!
It doesn't look elegant, but I think it's all due to the (current) design of
the fwnode linked list.
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

-- 
With Best Regards,
Andy Shevchenko