Code: static void bump_cpu_timer(struct k_itimer *timer, u64 now)
{
	int i;
	u64 delta, incr;

	if (timer->it.cpu.incr == 0)
		return;

	if (now < timer->it.cpu.expires)
		return;

	incr = timer->it.cpu.incr;
	delta = now + incr - timer->it.cpu.expires;

	/* Don't use (incr*2 < delta), incr*2 might overflow. */
	for (i = 0; incr < delta - incr; i++)
		incr = incr << 1;

	for (; i >= 0; incr >>= 1, i--) {
		if (delta < incr)
 			continue;
 
 		timer->it.cpu.expires += incr;
		timer->it_overrun += 1 << i;
 		delta -= incr;
 	}
 }

Commit Message: posix-timers: Sanitize overrun handling

The posix timer overrun handling is broken because the forwarding functions
can return a huge number of overruns which does not fit in an int. As a
consequence timer_getoverrun(2) and siginfo::si_overrun can turn into
random number generators.

The k_clock::timer_forward() callbacks return a 64 bit value now. Make
k_itimer::ti_overrun[_last] 64bit as well, so the kernel internal
accounting is correct. 3Remove the temporary (int) casts.

Add a helper function which clamps the overrun value returned to user space
via timer_getoverrun(2) or siginfo::si_overrun limited to a positive value
between 0 and INT_MAX. INT_MAX is an indicator for user space that the
overrun value has been clamped.

Reported-by: Team OWL337 <icytxw@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Link: https://lkml.kernel.org/r/20180626132705.018623573@linutronix.de
CWE ID: CWE-190
static void bump_cpu_timer(struct k_itimer *timer, u64 now)
{
	int i;
	u64 delta, incr;

	if (timer->it.cpu.incr == 0)
		return;

	if (now < timer->it.cpu.expires)
		return;

	incr = timer->it.cpu.incr;
	delta = now + incr - timer->it.cpu.expires;

	/* Don't use (incr*2 < delta), incr*2 might overflow. */
	for (i = 0; incr < delta - incr; i++)
		incr = incr << 1;

	for (; i >= 0; incr >>= 1, i--) {
		if (delta < incr)
 			continue;
 
 		timer->it.cpu.expires += incr;
		timer->it_overrun += 1LL << i;
 		delta -= incr;
 	}
 }
