[PATCH] hrtimer: prevent overrun DoS in hrtimer_forward()

hrtimer_forward() does not check for the possible overflow of
timer->expires.  This can happen on 64 bit machines with large interval
values and results currently in an endless loop in the softirq because the
expiry value becomes negative and therefor the timer is expired all the
time.

Check for this condition and set the expiry value to the max.  expiry time
in the future.  The fix should be applied to stable kernel series as well.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/hrtimer.c

@@ -644,6 +644,12 @@ hrtimer_forward(struct hrtimer *timer, ktime_t now, ktime_t interval)
 		orun++;
 	}
 	timer->expires = ktime_add(timer->expires, interval);
+	/*
+	 * Make sure, that the result did not wrap with a very large
+	 * interval.
+	 */
+	if (timer->expires.tv64 < 0)
+		timer->expires = ktime_set(KTIME_SEC_MAX, 0);
 
 	return orun;
 }