[PATCH backport pre 4.11] alarmtimer: Rate limit periodic intervals

[Thomas Gleixner <tglx@linutronix.de>]

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.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: peterz@infradead.org
Cc: stable@vger.kernel.org
Cc: kcc@google.com
Cc: syzkaller@googlegroups.com
Cc: john.stultz@linaro.org
Cc: dvyukov@google.com
Link: http://lkml.kernel.org/r/20170530211655.896767100@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

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

--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -660,6 +660,14 @@ 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 (ktime_to_ns(timr->it.alarm.interval) < TICK_NSEC)
+		timr->it.alarm.interval = ktime_set(0, TICK_NSEC);
+
 	exp = timespec_to_ktime(new_setting->it_value);
 	/* Convert (if necessary) to absolute time */
 	if (flags != TIMER_ABSTIME) {