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
