By the way, misuse of ectool can crash your system; I think that's expected. But it sounds like it uncovered a bug, so that's good. 

Yeah, the bug's not really about ectool. This interface is exposed via the kernel RTC interface. We don't expect timers to be able to kill the system.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/ec/+/e2a9ede352111e0943377a275accf4a13c71d777

commit e2a9ede352111e0943377a275accf4a13c71d777
Author: Philip Chen <philipchen@google.com>
Date: Thu Dec 07 01:30:47 2017

chip/stm32/clock: Handle illegal alarm timeout gracefully

Even if we set the rtc alarm timeout for more than 86400 secs,
we should not crash the system.

BUG= chromium:768042 
BRANCH=none
TEST=on AP console, do 'ectool rtcsetalarm 99999' and
then see 'EC result 3 (INVALID_PARAM)' without crash.

Change-Id: Ic0fa92ff101bce1f4791221c4e1eadaf7a005355
Signed-off-by: Philip Chen <philipchen@google.com>
Reviewed-on: https://chromium-review.googlesource.com/807624
Commit-Ready: Philip Chen <philipchen@chromium.org>
Tested-by: Philip Chen <philipchen@chromium.org>
Reviewed-by: Shawn N <shawnn@chromium.org>

[modify] https://crrev.com/e2a9ede352111e0943377a275accf4a13c71d777/chip/stm32/clock-f.c