linux-kernel - Re: [PATCH] powercap: fix possible name leak while device

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Date:   Fri, 25 Nov 2022 19:45:21 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Yang Yingliang <yangyingliang@...wei.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powercap: fix possible name leak while device_register() fails

On Thu, Nov 24, 2022 at 3:16 AM Yang Yingliang <yangyingliang@...wei.com> wrote:
>
>
> On 2022/11/24 3:25, Greg Kroah-Hartman wrote:
> > On Wed, Nov 23, 2022 at 08:00:14PM +0100, Rafael J. Wysocki wrote:
> >> On Sat, Nov 12, 2022 at 10:42 AM Yang Yingliang
> >> <yangyingliang@...wei.com> wrote:
> >>> If device_register() returns error, the name allocated by
> Sorry,
> I didn't describe clearly here, it's not only after device_register()
> failure, but also in the error path before register, the name is not
> freed, see description below.

So you would need to update the changelog at least.  But see below.

> >>> dev_set_name() need be freed. In technical, we should call
> >>> put_device() to give up the reference and free the name in
> >>> driver core, but in some cases the device is not intizalized,
> >>> put_device() can not be called, so don't complicate the code,
> >>> just call kfree_const() to free name in the error path.
> >>>
> >>> Fixes: 75d2364ea0ca ("PowerCap: Add class driver")
> >>> Signed-off-by: Yang Yingliang <yangyingliang@...wei.com>
> >>> ---
> >>>   drivers/powercap/powercap_sys.c | 2 ++
> >>>   1 file changed, 2 insertions(+)
> >>>
> >>> diff --git a/drivers/powercap/powercap_sys.c b/drivers/powercap/powercap_sys.c
> >>> index f0654a932b37..11e742dc83b9 100644
> >>> --- a/drivers/powercap/powercap_sys.c
> >>> +++ b/drivers/powercap/powercap_sys.c
> >>> @@ -572,6 +572,7 @@ struct powercap_zone *powercap_register_zone(
> >>>   err_name_alloc:
> >>>          idr_remove(power_zone->parent_idr, power_zone->id);
> >>>   err_idr_alloc:
> >>> +       kfree_const(dev_name(&power_zone->dev));
> >>>          if (power_zone->allocated)
> >>>                  kfree(power_zone);
> >>>          mutex_unlock(&control_type->lock);
> >>> @@ -622,6 +623,7 @@ struct powercap_control_type *powercap_register_control_type(
> >>>          dev_set_name(&control_type->dev, "%s", name);
> >>>          result = device_register(&control_type->dev);
> >>>          if (result) {
> >>> +               kfree_const(dev_name(&control_type->dev));
> >> Why is it necessary to free a device name explicitly after a failing
> >> device_register()?
> powercap_register_zone()
> {
>      ...
>      dev_set_name() // allocate name
>      ...
>      if (!power_zone->constraints)
>          goto err_const_alloc; //the name is leaked in this path
>      ...
>      if (!power_zone->zone_dev_attrs)
>          goto err_attr_alloc; //the name is leaked in this path
>      ...
>      if (result)
>          goto err_dev_ret; //the name is leaked in this path
>
>      result = device_register(&power_zone->dev);
>      if (result)
>          goto err_dev_ret;//put_device() is not called, the name is
> leaked in this path
>      ...
> err_dev_ret:
>      kfree(power_zone->zone_dev_attrs);
> err_attr_alloc:
>      kfree(power_zone->constraints);
> err_const_alloc:
>      kfree(power_zone->name);
> err_name_alloc:
>      idr_remove(power_zone->parent_idr, power_zone->id);
> err_idr_alloc:
>      if (power_zone->allocated)
>          kfree(power_zone);
> }

So can't the dev_set_name() be reordered closer to device_register(),
so it is not necessary to worry about freeing the name?

> >>
> >> If it is really necessary, then there is a problem in
> >> device_register() itself AFAICS, because it uses dev_set_name() at
> >> least in the dev->init_name present case.
> When the dev_set_name() called in device_register(), if register fails, the
> name is freed in its error path. But in this case, dev_set_name() is called
> outside the register, it needs call put_device() to free the name.

In any case, device_register() needs to take care of it anyway,
because it uses dev_set_name() itself in the dev->init_name case,
doesn't it?