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[PATCH 3.18 31/32] alarmtimer: Rate limit periodic intervals

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3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: 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: Peter Zijlstra <peterz@infradead.org>
Cc: Kostya Serebryany <kcc@google.com>
Cc: syzkaller <syzkaller@googlegroups.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Link: http://lkml.kernel.org/r/20170530211655.896767100@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


---
 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(0, TICK_NSEC);
+
 	exp = timespec_to_ktime(new_setting->it_value);
 	/* Convert (if necessary) to absolute time */
 	if (flags != TIMER_ABSTIME) {

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[PATCH 3.18 00/32] 3.18.58-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:40 +0200
  [PATCH 3.18 23/32] USB: gadget: dummy_hcd: fix hub-descriptor removable fields Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:40 +0200
  [PATCH 3.18 27/32] usb: xhci: ASMedia ASM1042A chipset need shorts TX quirk Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:40 +0200
  [PATCH 3.18 19/32] iio: proximity: as3935: recalibrate RCO after resume Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:40 +0200
  [PATCH 3.18 13/32] [media] vb2: Fix an off by one error in vb2_plane_vaddr Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:40 +0200
  [PATCH 3.18 32/32] mm: larger stack guard gap, between vmas Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:40 +0200
  [PATCH 3.18 21/32] usb: core: fix potential memory leak in error path during hcd creation Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:40 +0200
  [PATCH 3.18 06/32] ipv6: Handle IPv4-mapped src to in6addr_any dst. Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:40 +0200
  [PATCH 3.18 11/32] can: gs_usb: fix memory leak in gs_cmd_reset() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:50 +0200
  [PATCH 3.18 31/32] alarmtimer: Rate limit periodic intervals Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:50 +0200
  [PATCH 3.18 16/32] x86/mm/32: Set the __vmalloc_start_set flag in initmem_init() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:50 +0200
  [PATCH 3.18 24/32] usb: r8a66597-hcd: select a different endpoint on timeout Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:50 +0200
  [PATCH 3.18 20/32] USB: hub: fix SS max number of ports Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:50 +0200
  [PATCH 3.18 01/32] s390/vmem: fix identity mapping Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:50 +0200
  [PATCH 3.18 30/32] genirq: Release resources in __setup_irq() error path Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-19 17:50 +0200

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