ALSA: seq: Fix possible UAF in snd_seq_check_queue()

Although we've covered the races between concurrent write() and
ioctl() in the previous patch series, there is still a possible UAF in
the following scenario:

A: user client closed		B: timer irq
  -> snd_seq_release()		  -> snd_seq_timer_interrupt()
    -> snd_seq_free_client()	    -> snd_seq_check_queue()
				      -> cell = snd_seq_prioq_cell_peek()
      -> snd_seq_prioq_leave()
         .... removing all cells
      -> snd_seq_pool_done()
         .... vfree()
				      -> snd_seq_compare_tick_time(cell)
				         ... Oops

So the problem is that a cell is peeked and accessed without any
protection until it's retrieved from the queue again via
snd_seq_prioq_cell_out().

This patch tries to address it, also cleans up the code by a slight
refactoring.  snd_seq_prioq_cell_out() now receives an extra pointer
argument.  When it's non-NULL, the function checks the event timestamp
with the given pointer.  The caller needs to pass the right reference
either to snd_seq_tick or snd_seq_realtime depending on the event
timestamp type.

A good news is that the above change allows us to remove the
snd_seq_prioq_cell_peek(), too, thus the patch actually reduces the
code size.

Reviewed-by: Nicolai Stange <nstange@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 9 insertions, 19 deletions

diff --git a/sound/core/seq/seq_queue.c b/sound/core/seq/seq_queue.c
index 0428e9061b47..b377f5048352 100644
--- a/sound/core/seq/seq_queue.c
+++ b/sound/core/seq/seq_queue.c
@@ -277,30 +277,20 @@ void snd_seq_check_queue(struct snd_seq_queue *q, int atomic, int hop)
 
       __again:
 	/* Process tick queue... */
-	while ((cell = snd_seq_prioq_cell_peek(q->tickq)) != NULL) {
-		if (snd_seq_compare_tick_time(&q->timer->tick.cur_tick,
-					      &cell->event.time.tick)) {
-			cell = snd_seq_prioq_cell_out(q->tickq);
-			if (cell)
-				snd_seq_dispatch_event(cell, atomic, hop);
-		} else {
-			/* event remains in the queue */
+	for (;;) {
+		cell = snd_seq_prioq_cell_out(q->tickq,
+					      &q->timer->tick.cur_tick);
+		if (!cell)
 			break;
-		}
+		snd_seq_dispatch_event(cell, atomic, hop);
 	}
 
-
 	/* Process time queue... */
-	while ((cell = snd_seq_prioq_cell_peek(q->timeq)) != NULL) {
-		if (snd_seq_compare_real_time(&q->timer->cur_time,
-					      &cell->event.time.time)) {
-			cell = snd_seq_prioq_cell_out(q->timeq);
-			if (cell)
-				snd_seq_dispatch_event(cell, atomic, hop);
-		} else {
-			/* event remains in the queue */
+	for (;;) {
+		cell = snd_seq_prioq_cell_out(q->timeq, &q->timer->cur_time);
+		if (!cell)
 			break;
-		}
+		snd_seq_dispatch_event(cell, atomic, hop);
 	}
 
 	/* free lock */