Re: [PATCH] staging: comedi: drivers: Fix memory leak in gsc_hpdi_auto_attach

[Ian Abbott <abbotti@mev.co.uk>]

On 15/12/2019 01:33, Navid Emamdoost wrote:
> In the implementation of gsc_hpdi_auto_attach(), the allocated dma
> description is leaks in case of alignment error, or failure of
> gsc_hpdi_setup_dma_descriptors() or comedi_alloc_subdevices(). Release
> devpriv->dma_desc via dma_free_coherent().
> 
> Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>

Actually, there is no memory leak (although there is another problem 
that I'll mention below).  If the "auto_attach" handler 
gsc_hpdi_auto_attach() returns an error, then the "detach" handler 
gsc_hpdi_detach() will be called automatically to clean up.  (This is 
true for all comedi drivers).  gsc_hpdi_detach() calls 
gsc_hpdi_free_dma() to free the DMA buffers and DMA descriptors.

> ---
>   drivers/staging/comedi/drivers/gsc_hpdi.c | 16 +++++++++++++---
>   1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/staging/comedi/drivers/gsc_hpdi.c b/drivers/staging/comedi/drivers/gsc_hpdi.c
> index 4bdf44d82879..c0c7047a6d1b 100644
> --- a/drivers/staging/comedi/drivers/gsc_hpdi.c
> +++ b/drivers/staging/comedi/drivers/gsc_hpdi.c
> @@ -633,16 +633,17 @@ static int gsc_hpdi_auto_attach(struct comedi_device *dev,
>   	if (devpriv->dma_desc_phys_addr & 0xf) {
>   		dev_warn(dev->class_dev,
>   			 " dma descriptors not quad-word aligned (bug)\n");
> -		return -EIO;
> +		retval = -EIO;
> +		goto release_dma_desc;
>   	}
>   
>   	retval = gsc_hpdi_setup_dma_descriptors(dev, 0x1000);
>   	if (retval < 0)
> -		return retval;
> +		goto release_dma_desc;
>   
>   	retval = comedi_alloc_subdevices(dev, 1);
>   	if (retval)
> -		return retval;
> +		goto release_dma_desc;
>   
>   	/* Digital I/O subdevice */
>   	s = &dev->subdevices[0];
> @@ -660,6 +661,15 @@ static int gsc_hpdi_auto_attach(struct comedi_device *dev,
>   	s->cancel	= gsc_hpdi_cancel;
>   
>   	return gsc_hpdi_init(dev);
> +
> +release_dma_desc:
> +	if (devpriv->dma_desc)
> +		dma_free_coherent(&pcidev->dev,
> +				  sizeof(struct plx_dma_desc) *
> +				NUM_DMA_DESCRIPTORS,
> +				devpriv->dma_desc,
> +				devpriv->dma_desc_phys_addr);
> +	return retval;
>   }
>   
>   static void gsc_hpdi_detach(struct comedi_device *dev)
> 

This patch could actually result in devpriv->dma_desc being freed twice 
- once in the 'release_dma_desc:' code and again when gsc_hpdi_detach() 
is called externally as part of the clean-up.

The real bug in the original code is that it does not check whether any 
of the calls to dma_alloc_coherent() returned NULL.  If any of the calls 
to dma_alloc_coherent() returns NULL, gsc_hpdi_auto_attach() needs to 
return an error (-ENOMEM).  The subsequent call to gsc_hpdi_detach() 
will then free whatever DMA coherent buffers where allocated.

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