Re: [PATCH v2] params: serialize lookup_or_create_module_kobject()

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 18 Sep 2026 10:07:12 +0000 Jiakai Xu <xujiakai24@mails.ucas.ac.cn> wrote:

> lookup_or_create_module_kobject() first looks up the module kobject with
> kset_find_obj() and, if not found, creates a new one with
> kobject_init_and_add().  The function is called at runtime from
> module_add_driver() since commit f95bbfe18512 ("drivers: base: handle
> module_kobject creation"), which means two concurrent driver
> registrations for the same built-in module name can both miss the
> lookup and race to create the same kobject.
> 
> The loser of the race gets -EEXIST from kobject_init_and_add() and its
> kobject is removed from module_kset by kobject_add_internal() before
> the failure is reported.  The error path then calls kobject_put(),
> which invokes module_kobj_release(), but that only completes
> ->kobj_completion and never frees the dynamically allocated
> module_kobject, leaking it (96 bytes) along with the object having been
> detached from the kset.
> 
> This is triggerable by unprivileged users, e.g. by concurrently issuing
> the RAW_IOCTL_INIT ioctl of the raw-gadget driver, which registers the
> "raw_gadget" driver on the gadget bus:
> 
>   sysfs: cannot create duplicate filename '/module/raw_gadget'
>   ...
>   Adding module 'raw_gadget' to sysfs failed (-17), the system may be
>   unstable.
>   ...
>   unreferenced object 0xffff8880188dacc0 (size 96):
>     backtrace:
>       lookup_or_create_module_kobject+0x47/0x100
>       module_add_driver+0x73/0x1b0
>       bus_add_driver+0x1d9/0x340
>       driver_register+0xde/0x170
> 
> Fix it by serializing the lookup and the creation with a mutex, so that
> the second caller finds the kobject created by the first one instead of
> racing with it.

You're sure we there's no race with module removal as well?

> @@ -20,6 +21,12 @@
>  /* Protects all built-in parameters, modules use their own param_lock */
>  static DEFINE_MUTEX(param_lock);
>  
> +/*
> + * Serializes module kobject lookup and creation in
> + * lookup_or_create_module_kobject()
> + */
> +static DEFINE_MUTEX(mod_kobject_mutex);

minor: this could be static inside lookup_or_create_module_kobject().

>  /* Use the module's mutex, or if built-in use the built-in mutex */
>  #ifdef CONFIG_MODULES
>  #define KPARAM_MUTEX(mod)	((mod) ? &(mod)->param_lock : &param_lock)
> @@ -754,6 +761,13 @@ lookup_or_create_module_kobject(const char *name)
>  	struct kobject *kobj;
>  	int err;
>  
> +	/*
> +	 * The lookup and the creation must be done atomically, otherwise
> +	 * concurrent callers may race to create the same kobject, and the
> +	 * loser of the race gets -EEXIST from kobject_init_and_add().
> +	 */
> +	guard(mutex)(&mod_kobject_mutex);
> +
>  	kobj = kset_find_obj(module_kset, name);
>  	if (kobj)
>  		return to_module_kobject(kobj);