Re: [PATCH] ALSA: pcm: Fix snd_pcm_hw_params struct copy in compat mode

[Takashi Iwai <tiwai@suse.de>]

On Mon, 18 Jan 2016 04:49:18 +0100,
Nicolas Boichat wrote:
> 
> Running a kernel with KASan enabled, we spotted that the following
> read in sound/soc/soc-pcm.c:soc_pcm_hw_params() is out of bounds:
> 
> 		/* copy params for each codec */
> 		codec_params = *params;
> 
> The problem comes from sound/core/pcm_compat.c,
> snd_pcm_ioctl_hw_params_compat, where we're copying from a
> struct snd_pcm_hw_params to a struct snd_pcm_hw_params32:
> 
> 	/* only fifo_size is different, so just copy all */
> 	data = memdup_user(data32, sizeof(*data32));
> 
> It turns out that snd_pcm_hw_params is 4 bytes longer than
> snd_pcm_hw_params32, that's why an out-of-bounds read happens.
> 
> We separate kmalloc and memory copy operations to make sure
> data has the correct length.
> 
> Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>

Thanks for reporting.  This is indeed effectively a revert of the
commit ef44a1ec6eee ('ALSA: sound/core: use memdup_user()'), which is
obviously wrong.  Sigh, it's a really danger of such a "cleanup"
patch.
    
Could you rephrase the changelog mentioning the affecting commit and
resubmit?


Takashi

> ---
>  sound/core/pcm_compat.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/core/pcm_compat.c b/sound/core/pcm_compat.c
> index b48b434..9630e9f 100644
> --- a/sound/core/pcm_compat.c
> +++ b/sound/core/pcm_compat.c
> @@ -255,10 +255,15 @@ static int snd_pcm_ioctl_hw_params_compat(struct snd_pcm_substream *substream,
>  	if (! (runtime = substream->runtime))
>  		return -ENOTTY;
>  
> -	/* only fifo_size is different, so just copy all */
> -	data = memdup_user(data32, sizeof(*data32));
> -	if (IS_ERR(data))
> -		return PTR_ERR(data);
> +	data = kmalloc(sizeof(*data), GFP_KERNEL);
> +	if (!data)
> +		return -ENOMEM;
> +
> +	/* only fifo_size (RO from userspace) is different, so just copy all */
> +	if (copy_from_user(data, data32, sizeof(*data32))) {
> +		err = -EFAULT;
> +		goto error;
> +	}
>  
>  	if (refine)
>  		err = snd_pcm_hw_refine(substream, data);
> -- 
> 2.6.0.rc2.230.g3dd15c0
> 
>