Patch "alarmtimer: Rate limit periodic intervals" has been added to the 3.18-stable tree

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

alarmtimer: Rate limit periodic intervals

to the 3.18-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:
alarmtimer-rate-limit-periodic-intervals.patch
and it can be found in the queue-3.18 subdirectory.

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

From: Thomas Gleixner <[email protected]>
Date: Tue, 30 May 2017 23:15:35 +0200
Subject: alarmtimer: Rate limit periodic intervals

From: Thomas Gleixner <[email protected]>

commit ff86bf0c65f14346bf2440534f9ba5ac232c39a0 upstream.

The alarmtimer code has another source of potentially rearming itself too fast. Interval timers with a very samll interval have a similar CPU hog effect as the previously fixed overflow issue.

The reason is that alarmtimers do not implement the normal protection against this kind of problem which the other posix timer use:

timer expires -> queue signal -> deliver signal -> rearm timer

This scheme brings the rearming under scheduler control and prevents permanently firing timers which hog the CPU.

Bringing this scheme to the alarm timer code is a major overhaul because it lacks all the necessary mechanisms completely.

So for a quick fix limit the interval to one jiffie. This is not problematic in practice as alarmtimers are usually backed by an RTC for suspend which have 1 second resolution. It could be therefor argued that the resolution of this clock should be set to 1 second in general, but that's outside the scope of this fix.

---
 kernel/time/alarmtimer.c |    8 ++++++++
 1 file changed, 8 insertions(+)

@@ -614,6 +614,14 @@ static int alarm_timer_set(struct k_itim
 
 	/* start the timer */
 	timr->it.alarm.interval = timespec_to_ktime(new_setting->it_interval);
+
+	/*
+	 * Rate limit to the tick as a hot fix to prevent DOS. Will be
+	 * mopped up later.
+	 */
+	if (timr->it.alarm.interval < TICK_NSEC)
+		timr->it.alarm.interval = TICK_NSEC;
+
 	exp = timespec_to_ktime(new_setting->it_value);
 	/* Convert (if necessary) to absolute time */
 	if (flags != TIMER_ABSTIME) {

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

queue-3.18/genirq-release-resources-in-__setup_irq-error-path.patch queue-3.18/alarmtimer-rate-limit-periodic-intervals.patch queue-3.18/x86-mm-32-set-the-__vmalloc_start_set-flag-in-initmem_init.patch