timerfd: Only check CAP_WAKE_ALARM when it is needed

timerfd_create() and do_timerfd_settime() evaluate capable(CAP_WAKE_ALARM)
unconditionally although CAP_WAKE_ALARM is only required for
CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM.

This can cause extraneous audit messages when using a LSM such as SELinux,
incorrectly causes PF_SUPERPRIV to be set even when no privilege was
exercised, and is inefficient.

Flip the order of the tests in both functions so that we only call
capable() if the capability is truly required for the operation.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-security-module@vger.kernel.org
Cc: selinux@tycho.nsa.gov
Link: http://lkml.kernel.org/r/1487344439-22293-1-git-send-email-sds@tycho.nsa.gov
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

fs/timerfd.c

@@ -400,9 +400,9 @@ 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))
+	if ((clockid == CLOCK_REALTIME_ALARM ||
+	     clockid == CLOCK_BOOTTIME_ALARM) &&
+	    !capable(CAP_WAKE_ALARM))
 		return -EPERM;
 
 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
@@ -449,7 +449,7 @@ static int do_timerfd_settime(int ufd, int flags,
 		return ret;
 	ctx = f.file->private_data;
 
-	if (!capable(CAP_WAKE_ALARM) && isalarm(ctx)) {
+	if (isalarm(ctx) && !capable(CAP_WAKE_ALARM)) {
 		fdput(f);
 		return -EPERM;
 	}