Re: [PATCH 1/1] usb: gadget: f_hid: Conduct proper refcounting on shared f_hidg pointer

[Lee Jones]

On Mon, 21 Nov 2022, John Keeping wrote:
65;6999;1c
> On Mon, Nov 21, 2022 at 11:18:34AM -0500, Alan Stern wrote:
> > On Mon, Nov 21, 2022 at 12:38:37PM +0000, John Keeping wrote:
> > > On Sun, Nov 20, 2022 at 03:46:26PM -0500, Alan Stern wrote:
> > > > I see.  The solution is simple: Embed a struct device in struct f_hidg, 
> > > > and call cdev_device_add() to add the device and the cdev.  This will 
> > > > automatically make the device the parent of the cdev, so the device's 
> > > > refcount won't go to 0 until the cdev's refcount does.  Then you can tie 
> > > > the f_hidg's lifetime to the device's, so the device's release routine 
> > > > can safely deallocate the entire f_hidg structure.
> > > > 
> > > > The parent of the new struct device should be set to &gadget->dev.  If 
> > > > you can't think of a better name for the device, you could simply append 
> > > > ":I" to the parent's name, where I is the interface number, or even 
> > > > append ":C.I" where C is the config number (like we do on the host 
> > > > side).
> > > 
> > > There is no gadget->dev at the time struct f_hidg is allocated.
> > > 
> > > AFAICT the device is the UDC, which is only associated with the gadget
> > > when it is bound.  The functions are allocated earlier than this and I
> > > can't see any device associated with struct usb_function_instance.
> > 
> > Ah, that's a shame.  All right, so be it.
> > 
> > > The patch below does fix the issue, but I'm wondering if there's a
> > > deeper problem here that can only be properly solved by adding some
> > > device/kobject hierarchy to the config side of things.
> > 
> > I don't believe there's any deeper problem.  If someone wants to have an 
> > fhidg device as a child of the gadget, I think it could be added and 
> > removed in the ->set_alt() and ->disable() callbacks.  Or maybe the 
> > ->bind() and ->unbind() callbacks (I've never had to work with configfs 
> > so I'm not clear on the details).
> 
> It turns out there's already a device being created here, just not
> associated with the structure.  Your suggestions around
> cdev_device_add() made me spot what's going on with that so the actual
> fix is to pull its lifetime up to match struct f_hidg.
> 
> -- >8 --
> Subject: [PATCH] usb: gadget: f_hid: fix f_hidg lifetime vs cdev
> 
> 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.

This is much better, thanks for re-spinning.

> [Also fix refcount leak on an error path.]
> 
> Signed-off-by: John Keeping <john@xxxxxxxxxxxx>
> ---
>  drivers/usb/gadget/function/f_hid.c | 50 ++++++++++++++++-------------
>  1 file changed, 28 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_hid.c b/drivers/usb/gadget/function/f_hid.c
> index ca0a7d9eaa34..0b94668a3812 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);

Could we store/fetch this with dev_{set,get}_drvdata(), and make
hidg->dev a pointer reducing the size of the struct f_hidg.

> +	kfree(hidg->set_report_buf);
> +	kfree(hidg);
> +}
> +
>  /*-------------------------------------------------------------------------*/
>  /*                           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 +1005,12 @@ 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);
> +	cdev_set_parent(&hidg->cdev, &hidg->dev.kobj);

cdev_device_add() should take care of this, so long as:

    if (dev->devt)
        dev_set_parent(cdev, &dev->kobj);

> +	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 +1241,7 @@ 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);
> +	put_device(&hidg->dev);
>  	mutex_lock(&opts->lock);
>  	--opts->refcnt;
>  	mutex_unlock(&opts->lock);
> @@ -1256,8 +1251,7 @@ static void hidg_unbind(struct usb_configuration *c, struct usb_function *f)
>  {
>  	struct f_hidg *hidg = func_to_hidg(f);
>  
> -	device_destroy(hidg_class, MKDEV(major, hidg->minor));
> -	cdev_del(&hidg->cdev);
> +	cdev_device_del(&hidg->cdev);
>  
>  	usb_free_all_descriptors(f);
>  }
> @@ -1266,6 +1260,7 @@ static struct usb_function *hidg_alloc(struct usb_function_instance *fi)
>  {
>  	struct f_hidg *hidg;
>  	struct f_hid_opts *opts;
> +	int ret;
>  
>  	/* allocate and initialize one new instance */
>  	hidg = kzalloc(sizeof(*hidg), GFP_KERNEL);
> @@ -1277,17 +1272,28 @@ static struct usb_function *hidg_alloc(struct usb_function_instance *fi)
>  	mutex_lock(&opts->lock);
>  	++opts->refcnt;
>  
> -	hidg->minor = opts->minor;
> +	device_initialize(&hidg->dev);
> +	hidg->dev.release = hidg_release;
> +	hidg->dev.class = hidg_class;
> +	hidg->dev.devt = MKDEV(major, opts->minor);
> +	ret = dev_set_name(&hidg->dev, "hidg%d", opts->minor);
> +	if (ret) {
> +		--opts->refcnt;

Since we're holding the opts lock at this point, is there anything
preventing us from incrementing the refcnt at the end, just before
giving up the lock, thus saving 2 decrements in the error paths?

> +		mutex_unlock(&opts->lock);
> +		return ERR_PTR(ret);
> +	}
> +
>  	hidg->bInterfaceSubClass = opts->subclass;
>  	hidg->bInterfaceProtocol = opts->protocol;
>  	hidg->report_length = opts->report_length;
>  	hidg->report_desc_length = opts->report_desc_length;
>  	if (opts->report_desc) {
> -		hidg->report_desc = kmemdup(opts->report_desc,
> +		hidg->report_desc = devm_kmemdup(&hidg->dev, opts->report_desc,

Nice.

>  					    opts->report_desc_length,
>  					    GFP_KERNEL);
>  		if (!hidg->report_desc) {
> -			kfree(hidg);
> +			put_device(&hidg->dev);
> +			--opts->refcnt;
>  			mutex_unlock(&opts->lock);
>  			return ERR_PTR(-ENOMEM);
>  		}

Thanks for doing this John, your work is appreciated.

-- 
Lee Jones [李琼斯]