linux-kernel - Re: [PATCH 2/3] HID: logitech-hidpp: Fix double free on managed resource

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Date:   Sat, 23 Apr 2022 13:41:30 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     José Expósito <jose.exposito89@...il.com>,
        hadess@...ess.net
Cc:     dmitry.torokhov@...il.com, rydberg@...math.org, lains@...eup.net,
        jikos@...nel.org, benjamin.tissoires@...hat.com,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] HID: logitech-hidpp: Fix double free on managed
 resource

Hi,

On 4/22/22 18:17, José Expósito wrote:
> As described in the documentation for devm_input_allocate_device():
> 
>   Managed input devices do not need to be explicitly unregistered or
>   freed as it will be done automatically when owner device unbinds from
>   its driver (or binding fails).
> 
> However this driver was explicitly freeing the input device, allocated
> using devm_input_allocate_device() through hidpp_allocate_input().
> 
> Remove the call to input_free_device() to avoid a possible double free
> error.

Actually calling input_free_device() on a devm allocated input device
is fine. The input subsystem has chosen to not have a
separate devm_input_free_device(), instead input_free_device() knows
if a device is allocated through devm and then also frees the devres
tied to it:

void input_free_device(struct input_dev *dev)
{
        if (dev) {
                if (dev->devres_managed)
                        WARN_ON(devres_destroy(dev->dev.parent,
                                                devm_input_device_release,
                                                devm_input_device_match,
                                                dev));
                input_put_device(dev);
        }
}


> 
> Fixes: c39e3d5fc9dd3 ("HID: logitech-hidpp: late bind the input device on wireless connection")
> Signed-off-by: José Expósito <jose.exposito89@...il.com>
> ---
>  drivers/hid/hid-logitech-hidpp.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/drivers/hid/hid-logitech-hidpp.c b/drivers/hid/hid-logitech-hidpp.c
> index 81de88ab2ecc..9c00a781ab57 100644
> --- a/drivers/hid/hid-logitech-hidpp.c
> +++ b/drivers/hid/hid-logitech-hidpp.c
> @@ -3957,11 +3957,7 @@ static void hidpp_connect_event(struct hidpp_device *hidpp)
>  	}
>  
>  	hidpp_populate_input(hidpp, input);
> -
> -	ret = input_register_device(input);
> -	if (ret)
> -		input_free_device(input);
> -

The original code does look wrong there though, since the input device
is free-ed it should not be stored in hidpp->delayed_input, so this should be comes:

	ret = input_register_device(input);
	if (ret) {
		input_free_device(input);
		return;
	}


Regards,

Hans


>  	hidpp->delayed_input = input;
>  }
>