clockevents: Sanitize min_delta_ns adjustment and prevent overflows

The current logic which handles clock events programming failures can
increase min_delta_ns unlimited and even can cause overflows.

Sanitize it by:
 - prevent zero increase when min_delta_ns == 1
 - limiting min_delta_ns to a jiffie
 - bail out if the jiffie limit is hit
 - add retries stats for /proc/timer_list so we can gather data

Reported-by: Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 42 insertions, 13 deletions

diff --git a/kernel/time/tick-oneshot.c b/kernel/time/tick-oneshot.c
index 0a8a213016f0..aada0e52680a 100644
--- a/kernel/time/tick-oneshot.c
+++ b/kernel/time/tick-oneshot.c
@@ -22,6 +22,29 @@
 
 #include "tick-internal.h"
 
+/* Limit min_delta to a jiffie */
+#define MIN_DELTA_LIMIT		(NSEC_PER_SEC / HZ)
+
+static int tick_increase_min_delta(struct clock_event_device *dev)
+{
+	/* Nothing to do if we already reached the limit */
+	if (dev->min_delta_ns >= MIN_DELTA_LIMIT)
+		return -ETIME;
+
+	if (dev->min_delta_ns < 5000)
+		dev->min_delta_ns = 5000;
+	else
+		dev->min_delta_ns += dev->min_delta_ns >> 1;
+
+	if (dev->min_delta_ns > MIN_DELTA_LIMIT)
+		dev->min_delta_ns = MIN_DELTA_LIMIT;
+
+	printk(KERN_WARNING "CE: %s increased min_delta_ns to %llu nsec\n",
+	       dev->name ? dev->name : "?",
+	       (unsigned long long) dev->min_delta_ns);
+	return 0;
+}
+
 /**
  * tick_program_event internal worker function
  */
@@ -37,23 +60,28 @@ int tick_dev_program_event(struct clock_event_device *dev, ktime_t expires,
 		if (!ret || !force)
 			return ret;
 
+		dev->retries++;
 		/*
-		 * We tried 2 times to program the device with the given
-		 * min_delta_ns. If that's not working then we double it
+		 * We tried 3 times to program the device with the given
+		 * min_delta_ns. If that's not working then we increase it
 		 * and emit a warning.
 		 */
 		if (++i > 2) {
 			/* Increase the min. delta and try again */
-			if (!dev->min_delta_ns)
-				dev->min_delta_ns = 5000;
-			else
-				dev->min_delta_ns += dev->min_delta_ns >> 1;
-
-			printk(KERN_WARNING
-			       "CE: %s increasing min_delta_ns to %llu nsec\n",
-			       dev->name ? dev->name : "?",
-			       (unsigned long long) dev->min_delta_ns << 1);
-
+			if (tick_increase_min_delta(dev)) {
+				/*
+				 * Get out of the loop if min_delta_ns
+				 * hit the limit already. That's
+				 * better than staying here forever.
+				 *
+				 * We clear next_event so we have a
+				 * chance that the box survives.
+				 */
+				printk(KERN_WARNING
+				       "CE: Reprogramming failure. Giving up\n");
+				dev->next_event.tv64 = KTIME_MAX;
+				return -ETIME;
+			}
 			i = 0;
 		}
 
diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
index bdfb8dd1050c..1a4a7dd78777 100644
--- a/kernel/time/timer_list.c
+++ b/kernel/time/timer_list.c
@@ -228,6 +228,7 @@ print_tickdevice(struct seq_file *m, struct tick_device *td, int cpu)
 	SEQ_printf(m, " event_handler:  ");
 	print_name_offset(m, dev->event_handler);
 	SEQ_printf(m, "\n");
+	SEQ_printf(m, " retries:        %lu\n", dev->retries);
 }
 
 static void timer_list_show_tickdevices(struct seq_file *m)
@@ -257,7 +258,7 @@ static int timer_list_show(struct seq_file *m, void *v)
 	u64 now = ktime_to_ns(ktime_get());
 	int cpu;
 
-	SEQ_printf(m, "Timer List Version: v0.5\n");
+	SEQ_printf(m, "Timer List Version: v0.6\n");
 	SEQ_printf(m, "HRTIMER_MAX_CLOCK_BASES: %d\n", HRTIMER_MAX_CLOCK_BASES);
 	SEQ_printf(m, "now at %Ld nsecs\n", (unsigned long long)now);