linux-kernel - Re: [PATCH] usb: Add data checks in usbtmc

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Date:   Tue, 23 Mar 2021 08:16:55 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Lv Yunlong <lyl2019@...l.ustc.edu.cn>
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: Add data checks in usbtmc_disconnect

On Mon, Mar 22, 2021 at 08:47:17PM -0700, Lv Yunlong wrote:
> In usbtmc_disconnect, data is got from intf with the
> initial reference. There is no refcount inc operation
> before usbmc_free_int(data). In usbmc_free_int(data),
> the data may be freed.
> 
> But later in usbtmc_disconnect, there is another put
> function of data. I think it is better to add necessary
> checks to avoid the data being put twice. It could cause
> errors in race.
> 
> Signed-off-by: Lv Yunlong <lyl2019@...l.ustc.edu.cn>
> ---
>  drivers/usb/class/usbtmc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/class/usbtmc.c b/drivers/usb/class/usbtmc.c
> index 74d5a9c5238a..e0438cb46386 100644
> --- a/drivers/usb/class/usbtmc.c
> +++ b/drivers/usb/class/usbtmc.c
> @@ -2494,7 +2494,9 @@ static void usbtmc_disconnect(struct usb_interface *intf)
>  	}
>  	mutex_unlock(&data->io_mutex);
>  	usbtmc_free_int(data);
> -	kref_put(&data->kref, usbtmc_delete);
> +
> +	if (data->iin_ep_present && data->iin_urb)
> +		kref_put(&data->kref, usbtmc_delete);

What protects the data from changing right after the check and right
before the kref_put() call?

krefs need a lock somewhere to protect from races like this, please fix
that logic instead.

thanks,

greg k-h