From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751286AbdE3VmD (ORCPT ); Tue, 30 May 2017 17:42:03 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:55704 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751120AbdE3VmA (ORCPT ); Tue, 30 May 2017 17:42:00 -0400 Message-Id: <20170530211655.896767100@linutronix.de> User-Agent: quilt/0.63-1 Date: Tue, 30 May 2017 23:15:35 +0200 From: Thomas Gleixner To: LKML Cc: John Stultz , Peter Zijlstra , Ingo Molnar , Dmitry Vyukov , Kostya Serebryany , syzkaller Subject: [patch 02/26] alarmtimer: Rate limit periodic intervals References: <20170530211533.216608851@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Disposition: inline; filename=alarmtimer--Rate-limit-intervals.patch Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Cc: Dmitry Vyukov Cc: Kostya Serebryany Cc: syzkaller --- kernel/time/alarmtimer.c | 8 ++++++++ 1 file changed, 8 insertions(+) --- a/kernel/time/alarmtimer.c +++ b/kernel/time/alarmtimer.c @@ -662,6 +662,14 @@ static int alarm_timer_set(struct k_itim /* start the timer */ timr->it.alarm.interval = timespec64_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 = timespec64_to_ktime(new_setting->it_value); /* Convert (if necessary) to absolute time */ if (flags != TIMER_ABSTIME) {