Re: [PATCH] ALSA: usb-audio: fix set_format altsetting sanity check

[Takashi Iwai <tiwai@suse.de>]

On Fri, 20 Dec 2019 10:31:34 +0100,
Johan Hovold wrote:
> 
> Make sure to check the return value of usb_altnum_to_altsetting() to
> avoid dereferencing a NULL pointer when the requested alternate settings
> is missing.
> 
> The format altsetting number may come from a quirk table and there does
> not seem to be any other validation of it (the corresponding index is
> checked however).
> 
> Fixes: b099b9693d23 ("ALSA: usb-audio: Avoid superfluous usb_set_interface() calls")
> Cc: stable <stable@vger.kernel.org>     # 4.18
> Signed-off-by: Johan Hovold <johan@kernel.org>
> ---
>  sound/usb/pcm.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
> index 9c8930bb00c8..73dd9d21bb42 100644
> --- a/sound/usb/pcm.c
> +++ b/sound/usb/pcm.c
> @@ -506,9 +506,9 @@ static int set_format(struct snd_usb_substream *subs, struct audioformat *fmt)
>  	if (WARN_ON(!iface))
>  		return -EINVAL;
>  	alts = usb_altnum_to_altsetting(iface, fmt->altsetting);
> -	altsd = get_iface_desc(alts);
> -	if (WARN_ON(altsd->bAlternateSetting != fmt->altsetting))
> +	if (WARN_ON(!alts))
>  		return -EINVAL;

Do we need WARN_ON() here?  If this may hit on syzbot, it'll stop at
this point because of panic_on_warn.


thanks,

Takashi

> +	altsd = get_iface_desc(alts);
>  
>  	if (fmt == subs->cur_audiofmt)
>  		return 0;
> -- 
> 2.24.1
>