[PATCH] drivers/rtc/interface.c: Validate alarm-time before handling rollover

From: Vaibhav Jain
Date: Fri May 19 2017 - 12:51:12 EST

In function __rtc_read_alarm() its possible for an alarm time-stamp to
be invalid even after replacing missing components with current
time-stamp. The condition 'alarm->time.tm_year < 70' will trigger this
case and will cause the call to 'rtc_tm_to_time64(&alarm->time)'
return a negative value for variable t_alm.

While handling alarm rollover this negative t_alm (assumed to seconds
offset from '1970-01-01 00:00:00') is converted back to rtc_time via
rtc_time64_to_tm() which results in this error log with seemingly
garbage values:

"rtc rtc0: invalid alarm value: -2-1--1041528741

This error was generated when the rtc driver (rtc-opal in this case)
returned an alarm time-stamp of '00-00-00 00:00:00' to indicate that
the alarm is disabled. Though I have submitted a separate fix for the
rtc-opal driver, this issue may potentially impact other
existing/future rtc drivers.

To fix this issue the patch validates the alarm time-stamp just after
filling up the missing datetime components and if rtc_valid_tm() still
reports it to be invalid then bails out of the function without
handling the rollover.

Reported-by: Steve Best <sbest@xxxxxxxxxx>
Signed-off-by: Vaibhav Jain <vaibhav@xxxxxxxxxxxxxxxxxx>
drivers/rtc/interface.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index fc0fa75..8cec9a0 100644
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -227,6 +227,13 @@ int __rtc_read_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
missing = year;

+ /* Can't proceed if alarm is still invalid after replacing
+ * missing fields.
+ */
+ err = rtc_valid_tm(&alarm->time);
+ if (err)
+ goto done;
/* with luck, no rollover is needed */
t_now = rtc_tm_to_time64(&now);
t_alm = rtc_tm_to_time64(&alarm->time);
@@ -278,9 +285,9 @@ int __rtc_read_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
dev_warn(&rtc->dev, "alarm rollover not handled\n");

err = rtc_valid_tm(&alarm->time);

if (err) {
dev_warn(&rtc->dev, "invalid alarm value: %d-%d-%d %d:%d:%d\n",
alarm->time.tm_year + 1900, alarm->time.tm_mon + 1,