posix-cpu-timers: Make timespec to nsec conversion safe

The expiry time of a posix cpu timer is supplied through sys_timer_set()
via a struct timespec. The timespec is validated for correctness.

In the actual set timer implementation the timespec is converted to a
scalar nanoseconds value. If the tv_sec part of the time spec is large
enough the conversion to nanoseconds (sec * NSEC_PER_SEC) overflows 64bit.

Mitigate that by using the timespec_to_ktime() conversion function, which
checks the tv_sec part for a potential mult overflow and clamps the result
to KTIME_MAX, which is about 292 years. 

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170620154113.588276707@linutronix.de

kernel/time/posix-cpu-timers.c

@@ -580,7 +580,11 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
 
 	WARN_ON_ONCE(p == NULL);
 
-	new_expires = timespec64_to_ns(&new->it_value);
+	/*
+	 * Use the to_ktime conversion because that clamps the maximum
+	 * value to KTIME_MAX and avoid multiplication overflows.
+	 */
+	new_expires = ktime_to_ns(timespec64_to_ktime(new->it_value));
 
 	/*
 	 * Protect against sighand release/switch in exit/exec and p->cpu_timers