Re: [PATCH v2] usb: gadget: f_hid: fixed NULL pointer dereference

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Jul 23, 2021 at 06:59:30PM +0300, Maxim Devaev wrote:
> From: Phil Elwell <phil@raspberrypi.com>
> 
> Disconnecting and reconnecting the USB cable can lead to crashes
> and a variety of kernel log spam.
> 
> The problem was found and reproduced on the Raspberry Pi [1]
> and the original fix was created in Raspberry's own fork [2].
> 
> Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
> Signed-off-by: Phil Elwell <phil@raspberrypi.com>
> Link: https://github.com/raspberrypi/linux/issues/3870 [1]
> Link: https://github.com/raspberrypi/linux/commit/a6e47d5f4efbd2ea6a0b6565cd2f9b7bb217ded5 [2]
> ---
>  drivers/usb/gadget/function/f_hid.c | 26 ++++++++++++++++++++------
>  1 file changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_hid.c b/drivers/usb/gadget/function/f_hid.c
> index 02683ac07..08e73e812 100644
> --- a/drivers/usb/gadget/function/f_hid.c
> +++ b/drivers/usb/gadget/function/f_hid.c
> @@ -338,6 +338,11 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
>  
>  	spin_lock_irqsave(&hidg->write_spinlock, flags);
>  
> +	if (!hidg->req) {
> +		spin_unlock_irqrestore(&hidg->write_spinlock, flags);
> +		return -ESHUTDOWN;
> +	}
> +
>  #define WRITE_COND (!hidg->write_pending)
>  try_again:
>  	/* write queue */
> @@ -358,8 +363,14 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
>  	count  = min_t(unsigned, count, hidg->report_length);
>  
>  	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
> -	status = copy_from_user(req->buf, buffer, count);
>  
> +	if (!req) {
> +		ERROR(hidg->func.config->cdev, "hidg->req is NULL\n");
> +		status = -ESHUTDOWN;
> +		goto release_write_pending;
> +	}
> +
> +	status = copy_from_user(req->buf, buffer, count);
>  	if (status != 0) {
>  		ERROR(hidg->func.config->cdev,
>  			"copy_from_user error\n");
> @@ -387,14 +398,17 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
>  
>  	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
>  
> +	if (!hidg->in_ep->enabled) {
> +		ERROR(hidg->func.config->cdev, "in_ep is disabled\n");
> +		status = -ESHUTDOWN;
> +		goto release_write_pending;

What prevents "enabled" from changing right after you test this?  You do
not have the lock held so could it change then?

> +	}
> +
>  	status = usb_ep_queue(hidg->in_ep, req, GFP_ATOMIC);
> -	if (status < 0) {
> -		ERROR(hidg->func.config->cdev,
> -			"usb_ep_queue error on int endpoint %zd\n", status);
> +	if (status < 0)
>  		goto release_write_pending;

No error message if queueing up failed?

thanks,

greg k-h