Re: [PATCH 1/3] usb: gadget: f_hid: fix f_hidg lifetime vs cdev

[Andrzej Pietrasiewicz <andrzej.p@xxxxxxxxxxxxx>]

Hi John,

W dniu 22.11.2022 o 13:35, John Keeping pisze:
> The embedded struct cdev does not have its lifetime correctly tied to
> the enclosing struct f_hidg, so there is a use-after-free if /dev/hidgN
> is held open while the gadget is deleted.
>
> This can readily be replicated with libusbgx's example programs (for
> conciseness - operating directly via configfs is equivalent):
>
> 	gadget-hid
> 	exec 3<> /dev/hidg0
> 	gadget-vid-pid-remove
> 	exec 3<&-
>
> Pull the existing device up in to struct f_hidg and make use of the
> cdev_device_{add,del}() helpers.  This changes the lifetime of the
> device object to match struct f_hidg, but note that it is still added
> and deleted at the same time.
>
> Fixes: 71adf1189469 ("USB: gadget: add HID gadget driver")
> Signed-off-by: John Keeping <john@xxxxxxxxxxxx>
> ---
>   drivers/usb/gadget/function/f_hid.c | 52 ++++++++++++++++-------------
>   1 file changed, 28 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/usb/gadget/function/f_hid.c b/drivers/usb/gadget/function/f_hid.c
> index ca0a7d9eaa34..8b8bbeaa27cb 100644
> --- a/drivers/usb/gadget/function/f_hid.c
> +++ b/drivers/usb/gadget/function/f_hid.c
> @@ -71,7 +71,7 @@ struct f_hidg {
>   	wait_queue_head_t		write_queue;
>   	struct usb_request		*req;
>   
> -	int				minor;
> +	struct device			dev;
>   	struct cdev			cdev;
>   	struct usb_function		func;
>   
> @@ -84,6 +84,14 @@ static inline struct f_hidg *func_to_hidg(struct usb_function *f)
>   	return container_of(f, struct f_hidg, func);
>   }
>   
> +static void hidg_release(struct device *dev)
> +{
> +	struct f_hidg *hidg = container_of(dev, struct f_hidg, dev);
> +
> +	kfree(hidg->set_report_buf);
> +	kfree(hidg);
> +}
> +

I assume the above is supposed to free the hidg memory as a result of
put_device() and you free two things here ...

>   /*-------------------------------------------------------------------------*/
>   /*                           Static descriptors                            */
>   
> @@ -904,9 +912,7 @@ static int hidg_bind(struct usb_configuration *c, struct usb_function *f)
>   	struct usb_ep		*ep;
>   	struct f_hidg		*hidg = func_to_hidg(f);
>   	struct usb_string	*us;
> -	struct device		*device;
>   	int			status;
> -	dev_t			dev;
>   
>   	/* maybe allocate device-global string IDs, and patch descriptors */
>   	us = usb_gstrings_attach(c->cdev, ct_func_strings,
> @@ -999,21 +1005,11 @@ static int hidg_bind(struct usb_configuration *c, struct usb_function *f)
>   
>   	/* create char device */
>   	cdev_init(&hidg->cdev, &f_hidg_fops);
> -	dev = MKDEV(major, hidg->minor);
> -	status = cdev_add(&hidg->cdev, dev, 1);
> +	status = cdev_device_add(&hidg->cdev, &hidg->dev);
>   	if (status)
>   		goto fail_free_descs;
>   
> -	device = device_create(hidg_class, NULL, dev, NULL,
> -			       "%s%d", "hidg", hidg->minor);
> -	if (IS_ERR(device)) {
> -		status = PTR_ERR(device);
> -		goto del;
> -	}
> -
>   	return 0;
> -del:
> -	cdev_del(&hidg->cdev);
>   fail_free_descs:
>   	usb_free_all_descriptors(f);
>   fail:
> @@ -1244,9 +1240,7 @@ static void hidg_free(struct usb_function *f)
>   
>   	hidg = func_to_hidg(f);
>   	opts = container_of(f->fi, struct f_hid_opts, func_inst);
> -	kfree(hidg->report_desc);
> -	kfree(hidg->set_report_buf);
> -	kfree(hidg);

... while here 3 things used to be freed. What happens to hidg->report_desc?

Regards,

Andrzej