ALSA: seq: Fix OSS sysex delivery in OSS emulation

The SYSEX event delivery in OSS sequencer emulation assumed that the
event is encoded in the variable-length data with the straight
buffering.  This was the normal behavior in the past, but during the
development, the chained buffers were introduced for carrying more
data, while the OSS code was left intact.  As a result, when a SYSEX
event with the chained buffer data is passed to OSS sequencer port,
it may end up with the wrong memory access, as if it were having a too
large buffer.

This patch addresses the bug, by applying the buffer data expansion by
the generic snd_seq_dump_var_event() helper function.
Reported-by: syzbot <syzkaller@googlegroups.com>
Reported-by: Mark Salyzyn <salyzyn@android.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/oss/seq_oss_midi.c

@@ -612,9 +612,7 @@ send_midi_event(struct seq_oss_devinfo *dp, struct snd_seq_event *ev, struct seq
 	if (!dp->timer->running)
 		len = snd_seq_oss_timer_start(dp->timer);
 	if (ev->type == SNDRV_SEQ_EVENT_SYSEX) {
-		if ((ev->flags & SNDRV_SEQ_EVENT_LENGTH_MASK) == SNDRV_SEQ_EVENT_LENGTH_VARIABLE)
-			snd_seq_oss_readq_puts(dp->readq, mdev->seq_device,
-					       ev->data.ext.ptr, ev->data.ext.len);
+		snd_seq_oss_readq_sysex(dp->readq, mdev->seq_device, ev);
 	} else {
 		len = snd_midi_event_decode(mdev->coder, msg, sizeof(msg), ev);
 		if (len > 0)

sound/core/seq/oss/seq_oss_readq.c

@@ -117,6 +117,35 @@ snd_seq_oss_readq_puts(struct seq_oss_readq *q, int dev, unsigned char *data, in
 	return 0;
 }
 
+/*
+ * put MIDI sysex bytes; the event buffer may be chained, thus it has
+ * to be expanded via snd_seq_dump_var_event().
+ */
+struct readq_sysex_ctx {
+	struct seq_oss_readq *readq;
+	int dev;
+};
+
+static int readq_dump_sysex(void *ptr, void *buf, int count)
+{
+	struct readq_sysex_ctx *ctx = ptr;
+
+	return snd_seq_oss_readq_puts(ctx->readq, ctx->dev, buf, count);
+}
+
+int snd_seq_oss_readq_sysex(struct seq_oss_readq *q, int dev,
+			    struct snd_seq_event *ev)
+{
+	struct readq_sysex_ctx ctx = {
+		.readq = q,
+		.dev = dev
+	};
+
+	if ((ev->flags & SNDRV_SEQ_EVENT_LENGTH_MASK) != SNDRV_SEQ_EVENT_LENGTH_VARIABLE)
+		return 0;
+	return snd_seq_dump_var_event(ev, readq_dump_sysex, &ctx);
+}
+
 /*
  * copy an event to input queue:
  * return zero if enqueued

sound/core/seq/oss/seq_oss_readq.h

@@ -44,6 +44,8 @@ void snd_seq_oss_readq_delete(struct seq_oss_readq *q);
 void snd_seq_oss_readq_clear(struct seq_oss_readq *readq);
 unsigned int snd_seq_oss_readq_poll(struct seq_oss_readq *readq, struct file *file, poll_table *wait);
 int snd_seq_oss_readq_puts(struct seq_oss_readq *readq, int dev, unsigned char *data, int len);
+int snd_seq_oss_readq_sysex(struct seq_oss_readq *q, int dev,
+			    struct snd_seq_event *ev);
 int snd_seq_oss_readq_put_event(struct seq_oss_readq *readq, union evrec *ev);
 int snd_seq_oss_readq_put_timestamp(struct seq_oss_readq *readq, unsigned long curt, int seq_mode);
 int snd_seq_oss_readq_pick(struct seq_oss_readq *q, union evrec *rec);