timerfd: Reject ALARM timerfds without CAP_WAKE_ALARM

timerfd gives processes a way to set wake alarms, but unlike timers made using
timer_create, timerfds don't check whether the process has CAP_WAKE_ALARM
before setting alarm-time timers. CAP_WAKE_ALARM is supposed to gate this
behavior and so it makes sense that we should deny permission to create such
timerfds if the process doesn't have this capability.

Signed-off-by: Eric Caruso <ejcaruso@google.com>
Cc: Todd Poynor <toddpoynor@google.com>
Link: http://lkml.kernel.org/r/1465427339-96209-1-git-send-email-ejcaruso@chromium.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

fs/timerfd.c

@@ -390,6 +390,11 @@ SYSCALL_DEFINE2(timerfd_create, int, clockid, int, flags)
 	     clockid != CLOCK_BOOTTIME_ALARM))
 		return -EINVAL;
 
+	if (!capable(CAP_WAKE_ALARM) &&
+	    (clockid == CLOCK_REALTIME_ALARM ||
+	     clockid == CLOCK_BOOTTIME_ALARM))
+		return -EPERM;
+
 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
 	if (!ctx)
 		return -ENOMEM;
@@ -433,6 +438,11 @@ static int do_timerfd_settime(int ufd, int flags,
 		return ret;
 	ctx = f.file->private_data;
 
+	if (!capable(CAP_WAKE_ALARM) && isalarm(ctx)) {
+		fdput(f);
+		return -EPERM;
+	}
+
 	timerfd_setup_cancel(ctx, flags);
 
 	/*