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

Takashi Iwai
2019-12-20T09:46:53Z

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
>