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

From: Yang Yingliang
Date: Fri Nov 25 2022 - 21:05:44 EST



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

.