ALSA: pcm: oss: Fix potential out-of-bounds shift

syzbot spotted a potential out-of-bounds shift in the PCM OSS layer
where it calculates the buffer size with the arbitrary shift value
given via an ioctl.

Add a range check for avoiding the undefined behavior.
As the value can be treated by a signed integer, the max shift should
be 30.

Reported-by: syzbot+df7dc146ebdd6435eea3@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20201209084552.17109-2-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/oss/pcm_oss.c

@@ -1935,11 +1935,15 @@ static int snd_pcm_oss_set_subdivide(struct snd_pcm_oss_file *pcm_oss_file, int
 static int snd_pcm_oss_set_fragment1(struct snd_pcm_substream *substream, unsigned int val)
 {
 	struct snd_pcm_runtime *runtime;
+	int fragshift;
 
 	runtime = substream->runtime;
 	if (runtime->oss.subdivision || runtime->oss.fragshift)
 		return -EINVAL;
-	runtime->oss.fragshift = val & 0xffff;
+	fragshift = val & 0xffff;
+	if (fragshift >= 31)
+		return -EINVAL;
+	runtime->oss.fragshift = fragshift;
 	runtime->oss.maxfrags = (val >> 16) & 0xffff;
 	if (runtime->oss.fragshift < 4)		/* < 16 */
 		runtime->oss.fragshift = 4;