ALSA: seq: Add sanity check for user-space pointer delivery

The sequencer event may contain a user-space pointer with its
SNDRV_SEQ_EXT_USRPTR bit, and we assure that its delivery is limited
with non-atomic mode.  Otherwise the copy_from_user() may hit the
fault and cause a problem.  Although the core code doesn't set such a
flag (only set at snd_seq_write()), any wild driver may set it
mistakenly and lead to an unexpected crash.

This patch adds a sanity check of such events at the delivery core
code to filter out the invalid invocation in the atomic mode.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_clientmgr.c

@@ -802,6 +802,10 @@ static int snd_seq_deliver_event(struct snd_seq_client *client, struct snd_seq_e
 		return -EMLINK;
 	}
 
+	if (snd_seq_ev_is_variable(event) &&
+	    snd_BUG_ON(atomic && (event->data.ext.len & SNDRV_SEQ_EXT_USRPTR)))
+		return -EINVAL;
+
 	if (event->queue == SNDRV_SEQ_ADDRESS_SUBSCRIBERS ||
 	    event->dest.client == SNDRV_SEQ_ADDRESS_SUBSCRIBERS)
 		result = deliver_to_subscribers(client, event, atomic, hop);