Patch "ALSA: seq: Fix link corruption by event error handling" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

ALSA: seq: Fix link corruption by event error handling

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
alsa-seq-fix-link-corruption-by-event-error-handling.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Takashi Iwai <[email protected]>
Date: Tue, 28 Feb 2017 22:15:51 +0100
Subject: ALSA: seq: Fix link corruption by event error handling

From: Takashi Iwai <[email protected]>

commit f3ac9f737603da80c2da3e84b89e74429836bb6d upstream.

The sequencer FIFO management has a bug that may lead to a corruption (shortage) of the cell linked list. When a sequencer client faces an error at the event delivery, it tries to put back the dequeued cell. When the first queue was put back, this forgot the tail pointer tracking, and the link will be screwed up.

Although there is no memory corruption, the sequencer client may stall forever at exit while flushing the pending FIFO cells in snd_seq_pool_done(), as spotted by syzkaller.

This patch addresses the missing tail pointer tracking at snd_seq_fifo_cell_putback(). Also the patch makes sure to clear the cell->enxt pointer at snd_seq_fifo_event_in() for avoiding a similar mess-up of the FIFO linked list.

---
 sound/core/seq/seq_fifo.c |    3 +++
 1 file changed, 3 insertions(+)

@@ -135,6 +135,7 @@ int snd_seq_fifo_event_in(struct snd_seq
 	f->tail = cell;
 	if (f->head == NULL)
 		f->head = cell;
+	cell->next = NULL;
 	f->cells++;
 	spin_unlock_irqrestore(&f->lock, flags);
 

@@ -214,6 +215,8 @@ void snd_seq_fifo_cell_putback(struct sn
 		spin_lock_irqsave(&f->lock, flags);
 		cell->next = f->head;
 		f->head = cell;
+		if (!f->tail)
+			f->tail = cell;
 		f->cells++;
 		spin_unlock_irqrestore(&f->lock, flags);
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/alsa-hda-add-subwoofer-support-for-dell-inspiron-17-7000-gaming.patch queue-4.4/alsa-seq-fix-link-corruption-by-event-error-handling.patch queue-4.4/alsa-hda-fix-micmute-hotkey-problem-for-a-lenovo-aio-machine.patch queue-4.4/alsa-ctxfi-fallback-dma-mask-to-32bit.patch queue-4.4/alsa-timer-reject-user-params-with-too-small-ticks.patch queue-4.4/alsa-hda-fix-lewisburg-audio-issue.patch queue-4.4/alsa-hda-realtek-cannot-adjust-speaker-s-volume-on-a-dell-aio.patch