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Subject: Re: [PATCH] powercap: fix possible name leak while device_register()
 fails
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>
CC:     <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <20221112094048.3614365-1-yangyingliang@huawei.com>
 <CAJZ5v0g6bct5pTruxHjjJPQpwDNR8WgqhLWaZWo8NKeRH3n6iw@mail.gmail.com>
 <Y35zwffl7bUdjs93@kroah.com>
From:   Yang Yingliang <yangyingliang@huawei.com>
Message-ID: <b4fe247b-0980-40ea-bc69-0430e3b18320@huawei.com>
Date:   Thu, 24 Nov 2022 10:16:06 +0800
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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@huawei.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.
>>> 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@huawei.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);
}
>>
>> 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.
> I think we already fixed this in the driver core, so these types of
> patches should not be applied.
driver core free the name by calling put_device(), but
in these two functions, put_device() is not called.

Thanks,
Yang
>
> Yang, can you make sure you respond to all of them and say "this is not
> needed anymore!" and if any got merged, send reverts for them?
>
> thanks,
>
> greg k-h
> .