ALSA: usb-audio: fix set_format altsetting sanity check

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>
Link: https://lore.kernel.org/r/20191220093134.1248-1-johan@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 96298c7..a11c815 100644 (file)
--- 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;
+       altsd = get_iface_desc(alts);
 
        if (fmt == subs->cur_audiofmt && !subs->need_setup_fmt)
                return 0;