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Message-ID: <Y3y4u21VaBgBdP9V@donbot>
Date: Tue, 22 Nov 2022 11:55:39 +0000
From: John Keeping <john@...anate.com>
To: Lee Jones <lee@...nel.org>
Cc: Alan Stern <stern@...land.harvard.edu>,
Greg KH <gregkh@...uxfoundation.org>, balbi@...nel.org,
linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH 1/1] usb: gadget: f_hid: Conduct proper refcounting on
shared f_hidg pointer
On Tue, Nov 22, 2022 at 08:31:08AM +0000, Lee Jones wrote:
> On Mon, 21 Nov 2022, John Keeping wrote:
> > 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@...anate.com>
> > ---
> > 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.
That will reduce the size of struct f_hidg, but we'll have an extra
allocation for the device object, so I don't think that's a real
benefit.
It seems simpler to keep a single allocation and embed the device.
> > + 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);
Thanks, I'll change this.
> > + 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?
This makes sense. I'll respin this as a series and include a patch
tidying up the error handling as a final step.
> > + 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.
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