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Message-ID: <Y3vJfwtH3fniy5ep@donbot>
Date: Mon, 21 Nov 2022 18:54:55 +0000
From: John Keeping <john@...anate.com>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Lee Jones <lee@...nel.org>, 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 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.
[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);
+
+ 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);
+ 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;
+ 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,
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);
}
--
2.38.1
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