Patch "ALSA: timer: Limit max instances per timer" has been added to the 4.13-stable tree

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

ALSA: timer: Limit max instances per timer

to the 4.13-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-timer-limit-max-instances-per-timer.patch
and it can be found in the queue-4.13 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: Sun, 5 Nov 2017 10:07:43 +0100
Subject: ALSA: timer: Limit max instances per timer
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

From: Takashi Iwai <[email protected]>

commit 9b7d869ee5a77ed4a462372bb89af622e705bfb8 upstream.

Currently we allow unlimited number of timer instances, and it may bring the system hogging way too much CPU when too many timer instances are opened and processed concurrently. This may end up with a soft-lockup report as triggered by syzkaller, especially when hrtimer backend is deployed.

Since such insane number of instances aren't demanded by the normal use case of ALSA sequencer and it merely opens a risk only for abuse, this patch introduces the upper limit for the number of instances per timer backend. As default, it's set to 1000, but for the fine-grained timer like hrtimer, it's set to 100.

---
 include/sound/timer.h |    2 +
 sound/core/hrtimer.c  |    1 
 sound/core/timer.c    |   67 ++++++++++++++++++++++++++++++++++++++++----------
 3 files changed, 57 insertions(+), 13 deletions(-)

@@ -90,6 +90,8 @@ struct snd_timer {
 	struct list_head ack_list_head;
 	struct list_head sack_list_head; /* slow ack list head */
 	struct tasklet_struct task_queue;
+	int max_instances;	/* upper limit of timer instances */
+	int num_instances;	/* current number of timer instances */
 };
 
 struct snd_timer_instance {

@@ -159,6 +159,7 @@ static int __init snd_hrtimer_init(void)
 	timer->hw = hrtimer_hw;
 	timer->hw.resolution = resolution;
 	timer->hw.ticks = NANO_SEC / resolution;
+	timer->max_instances = 100; /* lower the limit */
 
 	err = snd_timer_global_register(timer);
 	if (err < 0) {

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

queue-4.13/alsa-seq-avoid-invalid-lockdep-class-warning.patch queue-4.13/alsa-hda-fix-headset-mic-problem-for-dell-machines-with-alc274.patch queue-4.13/alsa-timer-limit-max-instances-per-timer.patch queue-4.13/alsa-seq-fix-oss-sysex-delivery-in-oss-emulation.patch queue-4.13/alsa-usb-audio-support-new-amanero-combo384-firmware-version.patch