posix-timer: Properly check sigevent->sigev_notify

commit cef31d9a upstream.

timer_create() specifies via sigevent->sigev_notify the signal delivery for
the new timer. The valid modes are SIGEV_NONE, SIGEV_SIGNAL, SIGEV_THREAD
and (SIGEV_SIGNAL | SIGEV_THREAD_ID).

The sanity check in good_sigevent() is only checking the valid combination
for the SIGEV_THREAD_ID bit, i.e. SIGEV_SIGNAL, but if SIGEV_THREAD_ID is
not set it accepts any random value.

This has no real effects on the posix timer and signal delivery code, but
it affects show_timer() which handles the output of /proc/$PID/timers. That
function uses a string array to pretty print sigev_notify. The access to
that array has no bound checks, so random sigev_notify cause access beyond
the array bounds.

Add proper checks for the valid notify modes and remove the SIGEV_THREAD_ID
masking from various code pathes as SIGEV_NONE can never be set in
combination with SIGEV_THREAD_ID.
Reported-by: Eric Biggers <ebiggers3@gmail.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/time/posix-timers.c

@@ -507,17 +507,22 @@ static struct pid *good_sigevent(sigevent_t * event)
 {
 	struct task_struct *rtn = current->group_leader;
 
-	if ((event->sigev_notify & SIGEV_THREAD_ID ) &&
-		(!(rtn = find_task_by_vpid(event->sigev_notify_thread_id)) ||
-		 !same_thread_group(rtn, current) ||
-		 (event->sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_SIGNAL))
+	switch (event->sigev_notify) {
+	case SIGEV_SIGNAL | SIGEV_THREAD_ID:
+		rtn = find_task_by_vpid(event->sigev_notify_thread_id);
+		if (!rtn || !same_thread_group(rtn, current))
 			return NULL;
-
-	if (((event->sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_NONE) &&
-	    ((event->sigev_signo <= 0) || (event->sigev_signo > SIGRTMAX)))
+		/* FALLTHRU */
+	case SIGEV_SIGNAL:
+	case SIGEV_THREAD:
+		if (event->sigev_signo <= 0 || event->sigev_signo > SIGRTMAX)
 			return NULL;
-
+		/* FALLTHRU */
+	case SIGEV_NONE:
 		return task_pid(rtn);
+	default:
+		return NULL;
+	}
 }
 
 void posix_timers_register_clock(const clockid_t clock_id,
@@ -745,8 +750,7 @@ common_timer_get(struct k_itimer *timr, struct itimerspec *cur_setting)
 	/* interval timer ? */
 	if (iv.tv64)
 		cur_setting->it_interval = ktime_to_timespec(iv);
-	else if (!hrtimer_active(timer) &&
-		 (timr->it_sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_NONE)
+	else if (!hrtimer_active(timer) && timr->it_sigev_notify != SIGEV_NONE)
 		return;
 
 	now = timer->base->get_time();
@@ -757,7 +761,7 @@ common_timer_get(struct k_itimer *timr, struct itimerspec *cur_setting)
 	 * expiry is > now.
 	 */
 	if (iv.tv64 && (timr->it_requeue_pending & REQUEUE_PENDING ||
-	    (timr->it_sigev_notify & ~SIGEV_THREAD_ID) == SIGEV_NONE))
+			timr->it_sigev_notify == SIGEV_NONE))
 		timr->it_overrun += (unsigned int) hrtimer_forward(timer, now, iv);
 
 	remaining = __hrtimer_expires_remaining_adjusted(timer, now);
@@ -767,7 +771,7 @@ common_timer_get(struct k_itimer *timr, struct itimerspec *cur_setting)
 		 * A single shot SIGEV_NONE timer must return 0, when
 		 * it is expired !
 		 */
-		if ((timr->it_sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_NONE)
+		if (timr->it_sigev_notify != SIGEV_NONE)
 			cur_setting->it_value.tv_nsec = 1;
 	} else
 		cur_setting->it_value = ktime_to_timespec(remaining);
@@ -865,7 +869,7 @@ common_timer_set(struct k_itimer *timr, int flags,
 	timr->it.real.interval = timespec_to_ktime(new_setting->it_interval);
 
 	/* SIGEV_NONE timers are not queued ! See common_timer_get */
-	if (((timr->it_sigev_notify & ~SIGEV_THREAD_ID) == SIGEV_NONE)) {
+	if (timr->it_sigev_notify == SIGEV_NONE) {
 		/* Setup correct expiry time for relative timers */
 		if (mode == HRTIMER_MODE_REL) {
 			hrtimer_add_expires(timer, timer->base->get_time());