rtc: check if __rtc_read_time was successful in rtc_timer_do_work()

If the __rtc_read_time call fails,, the struct rtc_time tm; may contain
uninitialized data, or an illegal date/time read from the RTC hardware.

When calling rtc_tm_to_ktime later, the result may be a very large value
(possibly KTIME_MAX). If there are periodic timers in rtc->timerqueue,
they will continually expire, may causing kernel softlockup.

Fixes: 6610e08 ("RTC: Rework RTC code to use timerqueue for events")
Signed-off-by: Yongliang Gao <[email protected]>
Acked-by: Jingqun Li <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexandre Belloni <[email protected]>

Author: Yongliang Gao

drivers/rtc/interface.c

@@ -904,13 +904,18 @@ void rtc_timer_do_work(struct work_struct *work)
 	struct timerqueue_node *next;
 	ktime_t now;
 	struct rtc_time tm;
+	int err;
 
 	struct rtc_device *rtc =
 		container_of(work, struct rtc_device, irqwork);
 
 	mutex_lock(&rtc->ops_lock);
 again:
-	__rtc_read_time(rtc, &tm);
+	err = __rtc_read_time(rtc, &tm);
+	if (err) {
+		mutex_unlock(&rtc->ops_lock);
+		return;
+	}
 	now = rtc_tm_to_ktime(tm);
 	while ((next = timerqueue_getnext(&rtc->timerqueue))) {
 		if (next->expires > now)