Re: [PATCH 2/5] staging: comedi: usbdux: Split a condition check in usbdux_alloc_usb_buffers()

[Ian Abbott]

On 08/12/16 11:34, SF Markus Elfring wrote:
> From: Markus Elfring <elfring@xxxxxxxxxxxxxxxxxxxxx>
> Date: Thu, 8 Dec 2016 10:01:54 +0100
>
> The functions "kcalloc" and "kzalloc" were called in four cases by the
> function "usbdux_alloc_usb_buffers" without checking immediately
> if they succeded.
> This issue was detected by using the Coccinelle software.
>
> Allocated memory was also not released if one of these function
> calls failed.
>
> * Split a condition check for memory allocation failures.
>
> * Add more exception handling.
>
> Fixes: ef1e3c4a3b383c6da3979670fcb5c6e9c7de4741 ("staging: comedi: usbdux: tidy up usbdux_alloc_usb_buffers()")
>
> Signed-off-by: Markus Elfring <elfring@xxxxxxxxxxxxxxxxxxxxx>
> ---
>  drivers/staging/comedi/drivers/usbdux.c | 53 ++++++++++++++++++++++++++-------
>  1 file changed, 43 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/staging/comedi/drivers/usbdux.c b/drivers/staging/comedi/drivers/usbdux.c
> index f4f05d29d30d..d7d683bd669c 100644
> --- a/drivers/staging/comedi/drivers/usbdux.c
> +++ b/drivers/staging/comedi/drivers/usbdux.c
> @@ -1449,24 +1449,35 @@ static int usbdux_alloc_usb_buffers(struct comedi_device *dev)
>  	struct usb_device *usb = comedi_to_usb_dev(dev);
>  	struct usbdux_private *devpriv = dev->private;
>  	struct urb *urb;
> -	int i;
> +	int i, x;
>
>  	devpriv->dux_commands = kzalloc(SIZEOFDUXBUFFER, GFP_KERNEL);
> +	if (!devpriv->dux_commands)
> +		return -ENOMEM;
> +
>  	devpriv->in_buf = kzalloc(SIZEINBUF, GFP_KERNEL);
> +	if (!devpriv->in_buf)
> +		goto free_commands;
> +
>  	devpriv->insn_buf = kzalloc(SIZEINSNBUF, GFP_KERNEL);
> +	if (!devpriv->insn_buf)
> +		goto free_in_buf;
> +
>  	devpriv->ai_urbs = kcalloc(devpriv->n_ai_urbs, sizeof(void *),
>  				   GFP_KERNEL);
> +	if (!devpriv->ai_urbs)
> +		goto free_insn_buf;
> +
>  	devpriv->ao_urbs = kcalloc(devpriv->n_ao_urbs, sizeof(void *),
>  				   GFP_KERNEL);
> -	if (!devpriv->dux_commands || !devpriv->in_buf || !devpriv->insn_buf ||
> -	    !devpriv->ai_urbs || !devpriv->ao_urbs)
> -		return -ENOMEM;
> +	if (!devpriv->ao_urbs)
> +		goto free_ai_urbs;
>
>  	for (i = 0; i < devpriv->n_ai_urbs; i++) {
>  		/* one frame: 1ms */
>  		urb = usb_alloc_urb(1, GFP_KERNEL);
>  		if (!urb)
> -			return -ENOMEM;
> +			goto free_n_ai_urbs;
>  		devpriv->ai_urbs[i] = urb;
>
>  		urb->dev = usb;
> @@ -1475,7 +1486,7 @@ static int usbdux_alloc_usb_buffers(struct comedi_device *dev)
>  		urb->transfer_flags = URB_ISO_ASAP;
>  		urb->transfer_buffer = kzalloc(SIZEINBUF, GFP_KERNEL);
>  		if (!urb->transfer_buffer)
> -			return -ENOMEM;
> +			goto free_n_ai_urbs;
>
>  		urb->complete = usbduxsub_ai_isoc_irq;
>  		urb->number_of_packets = 1;
> @@ -1488,7 +1499,7 @@ static int usbdux_alloc_usb_buffers(struct comedi_device *dev)
>  		/* one frame: 1ms */
>  		urb = usb_alloc_urb(1, GFP_KERNEL);
>  		if (!urb)
> -			return -ENOMEM;
> +			goto free_n_ao_urbs;
>  		devpriv->ao_urbs[i] = urb;
>
>  		urb->dev = usb;
> @@ -1497,7 +1508,7 @@ static int usbdux_alloc_usb_buffers(struct comedi_device *dev)
>  		urb->transfer_flags = URB_ISO_ASAP;
>  		urb->transfer_buffer = kzalloc(SIZEOUTBUF, GFP_KERNEL);
>  		if (!urb->transfer_buffer)
> -			return -ENOMEM;
> +			goto free_n_ao_urbs;
>
>  		urb->complete = usbduxsub_ao_isoc_irq;
>  		urb->number_of_packets = 1;
> @@ -1514,17 +1525,39 @@ static int usbdux_alloc_usb_buffers(struct comedi_device *dev)
>  	if (devpriv->pwm_buf_sz) {
>  		urb = usb_alloc_urb(0, GFP_KERNEL);
>  		if (!urb)
> -			return -ENOMEM;
> +			goto free_n_ao_urbs;
>  		devpriv->pwm_urb = urb;
>
>  		/* max bulk ep size in high speed */
>  		urb->transfer_buffer = kzalloc(devpriv->pwm_buf_sz,
>  					       GFP_KERNEL);
>  		if (!urb->transfer_buffer)
> -			return -ENOMEM;
> +			goto free_pwm_urb;
>  	}
>
>  	return 0;
> +free_pwm_urb:
> +	usb_free_urb(urb);
> +free_n_ao_urbs:
> +	for (x = 0; x < i; ++x) {
> +		kfree(devpriv->ao_urbs[x]->transfer_buffer);
> +		usb_free_urb(devpriv->ao_urbs[x]);
> +	}
> +free_n_ai_urbs:
> +	for (x = 0; x < i; ++x) {
> +		kfree(devpriv->ai_urbs[x]->transfer_buffer);
> +		usb_free_urb(devpriv->ai_urbs[x]);
> +	}
> +	kfree(devpriv->ao_urbs);
> +free_ai_urbs:
> +	kfree(devpriv->ai_urbs);
> +free_insn_buf:
> +	kfree(devpriv->insn_buf);
> +free_in_buf:
> +	kfree(devpriv->in_buf);
> +free_commands:
> +	kfree(devpriv->dux_commands);
> +	return -ENOMEM;
>  }
>
>  static void usbdux_free_usb_buffers(struct comedi_device *dev)

Actually, the original code worked fine, and these changes will result 
in an Oops if the allocations fail.  I'll explain why, since it isn't 
obvious without some knowledge of the clean-up strategy used by comedi 
drivers:

1. usbdux_alloc_usb_buffers() is called from usbdux_auto_attach().
2. usbdux_auto_attach() will return an error if 
usbdux_alloc_usb_buffers() fails.
3. If usbdux_auto_attach() returns an error to the core comedi module, 
the core comedi module will call usbdux_detach().
4. usbdux_detach() calls usbdux_free_usb_buffers().
5. usbdux_free_usb_buffers() frees any of the buffers that were 
successfully allocated by usbdux_alloc_usb_buffers().

The net result is that devpriv->dux_commands and the others will be 
passed to kfree() twice, leading to the oops.  That could be prevented 
by settting devpriv->dux_commands and the others to NULL in the error 
handling of usbdux_alloc_usb_buffers(), but it really isn't necessary as 
the existing code works, and all the other comedi drivers follow the 
same strategy of leaving clean-up to their comedi 'detach' handler.

--
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@xxxxxxxxx> )=-
-=(                          Web: http://www.mev.co.uk/  )=-