Re: [PATCH v2] usb: raw_gadget: fix use-after-free when UDC is removed

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Aug 04, 2026 at 12:39:05PM +0530, Anuj Bolewar via B4 Relay wrote:
> From: Anuj Bolewar <bolewara@gmail.com>
> 
> When the UDC is removed (e.g. dummy_hcd unbind via sysfs) while the raw
> gadget fd is still open, usb_del_gadget() destroys the gadget device
> and its name. raw_gadget keeps a dangling pointer in dev->gadget, and
> ioctls dereference it after releasing dev->lock, leading to a
> use-after-free in dev_err() when usb_ep_queue() fails.
> 
> Take a gadget reference in gadget_bind() and drop it in dev_free() so
> the gadget device and its name stay alive for as long as the fd is open
> and an ioctl may still dereference dev->gadget. gadget_unbind() only
> marks the device as failed under dev->lock; the reference is dropped in
> dev_free(), which runs only after the fd is closed and no ioctl can be
> in flight.
> 
> Reported-by: syzbot+9aacea11bc70c3ddaff2@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=9aacea11bc70c3ddaff2
> Fixes: f2c2e717642c ("usb: gadget: add raw-gadget interface")
> Signed-off-by: Anuj Bolewar <bolewara@gmail.com>
> ---
> The gadget device embedded in the UDC is destroyed when the UDC is
> removed while the raw gadget fd is still open (e.g. unbinding dummy_hcd
> via sysfs). raw_gadget keeps a dangling pointer in dev->gadget and
> ioctls dereference it after dropping dev->lock, which KASAN reports as a
> slab use-after-free in raw_process_ep0_io() (dev_err with a freed device
> name).
> 
> Fix it by holding a gadget reference for the raw device lifetime:
> gadget_bind() takes it, gadget_unbind() only marks the device as failed
> under dev->lock, and the reference is dropped in dev_free() once the fd
> is closed and no ioctl can still be in flight.
> ---
> Changes in v2:
> - Reworked per review: the gadget reference is now held for the raw
>   device lifetime (bind until dev_free()) instead of the bind/unbind
>   window, since ioctls dereference dev->gadget after releasing dev->lock
>   and the UDC core owns the gadget's lifetime during bind/unbind.
> - gadget_unbind() now only marks the device as failed under dev->lock;
>   the reference is dropped in dev_free() after the fd is closed.
> - Dropped the now-unneeded comment and switched the spinlock to guard().
> - Link to v1: https://patch.msgid.link/20260804-raw-gadget-ep0-uaf-v1-1-07878773da15@gmail.com
> ---
>  drivers/usb/gadget/legacy/raw_gadget.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/gadget/legacy/raw_gadget.c b/drivers/usb/gadget/legacy/raw_gadget.c
> index 4febf8dac7c..d6341823f90 100644
> --- a/drivers/usb/gadget/legacy/raw_gadget.c
> +++ b/drivers/usb/gadget/legacy/raw_gadget.c
> @@ -226,6 +226,7 @@ static void dev_free(struct kref *kref)
>  		kfree(dev->eps[i].ep->desc);
>  		dev->eps[i].state = STATE_EP_DISABLED;
>  	}
> +	usb_put_gadget(dev->gadget);
>  	kfree(dev);
>  }
>  
> @@ -302,7 +303,7 @@ static int gadget_bind(struct usb_gadget *gadget,
>  	dev->req = req;
>  	dev->req->context = dev;
>  	dev->req->complete = gadget_ep0_complete;
> -	dev->gadget = gadget;
> +	dev->gadget = usb_get_gadget(gadget);
>  	gadget_for_each_ep(ep, dev->gadget) {
>  		dev->eps[i].ep = ep;
>  		dev->eps[i].addr = get_ep_addr(ep->name);
> @@ -329,6 +330,10 @@ static void gadget_unbind(struct usb_gadget *gadget)
>  {
>  	struct raw_dev *dev = get_gadget_data(gadget);
>  
> +	{
> +		guard(spinlock_irqsave)(&dev->lock);
> +		dev->state = STATE_DEV_FAILED;
> +	}

Please look at how scoped_guard() works.

And again, are you _SURE_ you need to call get/put on the device?  That
still feels wrong...

And you did not answer my question about the need for the Assisted-by:
tag.

greg k-h