Re: [PATCH v1] PM / devfreq: governor_userspace: Guard against NULL governor_data during switch

Hi Finley,

On 7/23/2026 4:08 PM, Finley Xiao wrote:
> The userspace governor is the only one that uses devfreq->governor_data,
> allocated in its DEVFREQ_GOV_START handler (userspace_init()) and freed
> in its DEVFREQ_GOV_STOP handler (userspace_exit()).
> 
> When switching governors via sysfs (governor_store in devfreq.c), the
> sequence is:
> 
> STOP(old) -> df->governor = userspace -> START(userspace) -> userspace_init
> 
> Between the df->governor assignment and userspace_init() completing,
> governor_data is still NULL (the previous governor never touched it,
> and the core does not clear or pre-allocate it). During that window,
> asynchronous paths that call update_devfreq() outside of the driver's
> control -- the OPP notifier (devfreq.c:686) and the pm_qos notifier
> (devfreq.c:707) -- run devfreq_update_target() under df->lock and reach
> devfreq_userspace_func(), which dereferences governor_data without a
> NULL check, causing:
> 
>   Unable to handle kernel access to user memory outside uaccess routines
>   pc : devfreq_userspace_func+0xc/0x28
>   lr : devfreq_update_target+0x58/0x158
> 
> Add a NULL check in devfreq_userspace_func() and fall back to
> previous_freq, which is the same behaviour as the existing "no user
> frequency specified yet" branch. This closes the race for all callers
> since every path ultimately goes through get_target_freq.
> 
> Additionally, take devfreq->lock around the kfree()/NULL assignment in
> userspace_exit(). Without it, a concurrent OPP/pm_qos notifier running
> under devfreq->lock could read governor_data after it is freed but
> before it is set to NULL, constituting a use-after-free.
> 
> Signed-off-by: Finley Xiao <[email protected]>
> ---
>  drivers/devfreq/governor_userspace.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/devfreq/governor_userspace.c b/drivers/devfreq/governor_userspace.c
> index 3906ebedbae8..f7a485c43fd7 100644
> --- a/drivers/devfreq/governor_userspace.c
> +++ b/drivers/devfreq/governor_userspace.c
> @@ -24,7 +24,7 @@ static int devfreq_userspace_func(struct devfreq *df, unsigned long *freq)
>  {
>  	struct userspace_data *data = df->governor_data;
>  
> -	if (data->valid)
> +	if (data && data->valid)
This one is fine.
>  		*freq = data->user_frequency;
>  	else
>  		*freq = df->previous_freq; /* No user freq specified yet */
> @@ -110,8 +110,10 @@ static void userspace_exit(struct devfreq *devfreq)
>  	if (devfreq->dev.kobj.sd)
>  		sysfs_remove_group(&devfreq->dev.kobj, &dev_attr_group);
>  
> +	mutex_lock(&devfreq->lock);
>  	kfree(devfreq->governor_data);
>  	devfreq->governor_data = NULL;
> +	mutex_unlock(&devfreq->lock);
However, for this one, I'm not sure it's the proper place to add the lock.

Why should we allow the whole governor switching to be concurrent with
frequency switching?  I guess there might be something wrong elsewhere.
The devfreq->lock should be held around governor switching.
Can you have a try?  but be careful of deadlock because the governor
switching currently holds devfreq_list_lock.

Thanks!
Jie
>  }
>  
>  static int devfreq_userspace_handler(struct devfreq *devfreq,