FAILED: patch "[PATCH] ALSA: timer: Limit max instances per timer" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Takashi Iwai <[email protected]>
Date: Sun, 5 Nov 2017 10:07:43 +0100
Subject: [PATCH] ALSA: timer: Limit max instances per timer
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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.

index c4d76ff056c6..7ae226ab6990 100644
@@ -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 {

index 1ac0c423903e..6e47b823bcaa 100644
@@ -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) {