3.16.48-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Kostya Serebryany <kcc@xxxxxxxxxx> Cc: syzkaller <syzkaller@xxxxxxxxxxxxxxxx> Cc: John Stultz <john.stultz@xxxxxxxxxx> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Link: http://lkml.kernel.org/r/20170530211655.896767100@xxxxxxxxxxxxx [bwh: Backported to 3.16: - Use ktime_to_ns()/ktime_set() as ktime_t is not scalar - Adjust context] Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> --- kernel/time/alarmtimer.c | 8 ++++++++ 1 file changed, 8 insertions(+) --- a/kernel/time/alarmtimer.c +++ b/kernel/time/alarmtimer.c @@ -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 (ktime_to_ns(timr->it.alarm.interval) < TICK_NSEC) + timr->it.alarm.interval = ktime_set(1, 0); + exp = timespec_to_ktime(new_setting->it_value); /* Convert (if necessary) to absolute time */ if (flags != TIMER_ABSTIME) {