Re: [PATCH] powercap: fix possible name leak while device_register() fails

[Yang Yingliang]

On 2022/11/26 2:45, Rafael J. Wysocki wrote:
> On Thu, Nov 24, 2022 at 3:16 AM Yang Yingliang <yangyingliang@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > > ---
> > > > >    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?
Just move dev_set_name() closer to device_register() is not enough to free
the name, it should call put_device() after device_register() failure. I 
will try
this.
> > > > 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?
Yes, it's right.

Thanks,
Yang
> .