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

[John Keeping]

On Sun, Nov 20, 2022 at 03:46:26PM -0500, Alan Stern wrote:
> On Sun, Nov 20, 2022 at 05:22:19PM +0000, John Keeping wrote:
> > On Fri, Nov 18, 2022 at 04:07:24PM -0500, Alan Stern wrote:
> > > On Fri, Nov 18, 2022 at 04:37:32PM +0000, John Keeping wrote:
> > > > I don't think it's at all simple to fix this - I posted a series
> > > > addressing the lifetime issues here a few years ago but didn't chase it
> > > > up and there was no feedback:
> > > > 
> > > > 	https://lore.kernel.org/linux-usb/20191028114228.3679219-1-john@xxxxxxxxxxxx/
> > > > 
> > > > That includes a patch to remove the embedded struct cdev and manage its
> > > > lifetime separately, which I think is needed as there are two different
> > > > struct device objects here and we cannot tie their lifetimes together.
> > > 
> > > I still don't have a clear picture of what the real problem is.  Lee's 
> > > original patch description just said "external references are presently 
> > > not tracked", with no details about what those external references are. 
> > > Why not add just proper cdev_get() and cdev_put() calls to whatever code 
> > > handles those external references, so that they _are_ tracked?
> > > 
> > > What are the two different struct device objects?  Why do their 
> > > lifetimes need to be tied together?  If you do need to tie their 
> > > lifetimes somehow, why not simply make one of them (the one which is 
> > > logically allowed to be shorter-lived) hold a reference to the other?
> > 
> > The problem is that we have a struct cdev embedded in f_hidg but the
> > lifetime of f_hidg is not tied to any kobject so we can't solve this in
> > the right way by setting the parent kobject of the cdev.
> > 
> > While refcounting struct f_hidg is necessary, it's not sufficient
> > because the only way to keep it alive long enough for the final
> > kobject_put() on the embedded cdev is to tie the lifetime to a kobject
> > of its own and there is no suitable object as this is not the model
> > followed by gadget function instances.
> 
> 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.

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.

-- >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<&-

Add a device to f_hidg so that the cdev can properly take a reference to
this and the structure will be freed only when the child device is
released.

[Also fix refcount leak on an error path.]

Signed-off-by: John Keeping <john@xxxxxxxxxxxx>
---
 drivers/usb/gadget/function/f_hid.c | 35 ++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/gadget/function/f_hid.c b/drivers/usb/gadget/function/f_hid.c
index ca0a7d9eaa34..7ae97e5c4d20 100644
--- a/drivers/usb/gadget/function/f_hid.c
+++ b/drivers/usb/gadget/function/f_hid.c
@@ -71,6 +71,7 @@ struct f_hidg {
 	wait_queue_head_t		write_queue;
 	struct usb_request		*req;
 
+	struct device			dev;
 	int				minor;
 	struct cdev			cdev;
 	struct usb_function		func;
@@ -84,6 +85,14 @@ 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);
+}
+
 /*-------------------------------------------------------------------------*/
 /*                           Static descriptors                            */
 
@@ -999,6 +1008,7 @@ static int hidg_bind(struct usb_configuration *c, struct usb_function *f)
 
 	/* create char device */
 	cdev_init(&hidg->cdev, &f_hidg_fops);
+	cdev_set_parent(&hidg->cdev, &hidg->dev.kobj);
 	dev = MKDEV(major, hidg->minor);
 	status = cdev_add(&hidg->cdev, dev, 1);
 	if (status)
@@ -1244,9 +1254,7 @@ 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);
+	device_unregister(&hidg->dev);
 	mutex_lock(&opts->lock);
 	--opts->refcnt;
 	mutex_unlock(&opts->lock);
@@ -1266,6 +1274,7 @@ 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 +1286,27 @@ static struct usb_function *hidg_alloc(struct usb_function_instance *fi)
 	mutex_lock(&opts->lock);
 	++opts->refcnt;
 
+	device_initialize(&hidg->dev);
+	hidg->dev.release = hidg_release;
+	ret = dev_set_name(&hidg->dev, "hidg%d", hidg->minor);
+	if (ret) {
+		--opts->refcnt;
+		mutex_unlock(&opts->lock);
+		return ERR_PTR(ret);
+	}
+
 	hidg->minor = opts->minor;
 	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,
 					    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);
 		}
@@ -1307,6 +1326,12 @@ static struct usb_function *hidg_alloc(struct usb_function_instance *fi)
 	/* this could be made configurable at some point */
 	hidg->qlen	   = 4;
 
+	ret = device_add(&hidg->dev);
+	if (ret) {
+		put_device(&hidg->dev);
+		return ERR_PTR(ret);
+	}
+
 	return &hidg->func;
 }
 
-- 
2.38.1