[PATCH 5.16 0034/1039] rtc: cmos: take rtc_lock while reading from CMOS

[Greg Kroah-Hartman]

From: Mateusz Jończyk <mat.jonczyk@xxxxx>

commit 454f47ff464325223129b9b5b8d0b61946ec704d upstream.

Reading from the CMOS involves writing to the index register and then
reading from the data register. Therefore access to the CMOS has to be
serialized with rtc_lock. This invocation of CMOS_READ was not
serialized, which could cause trouble when other code is accessing CMOS
at the same time.

Use spin_lock_irq() like the rest of the function.

Nothing in kernel modifies the RTC_DM_BINARY bit, so there could be a
separate pair of spin_lock_irq() / spin_unlock_irq() before doing the
math.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@xxxxx>
Reviewed-by: Nobuhiro Iwamatsu <iwamatsu@xxxxxxxxxxx>
Cc: Alessandro Zummo <a.zummo@xxxxxxxxxxxx>
Cc: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/20211210200131.153887-2-mat.jonczyk@xxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/rtc/rtc-cmos.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -457,7 +457,10 @@ static int cmos_set_alarm(struct device
 	min = t->time.tm_min;
 	sec = t->time.tm_sec;
 
+	spin_lock_irq(&rtc_lock);
 	rtc_control = CMOS_READ(RTC_CONTROL);
+	spin_unlock_irq(&rtc_lock);
+
 	if (!(rtc_control & RTC_DM_BINARY) || RTC_ALWAYS_BCD) {
 		/* Writing 0xff means "don't care" or "match all".  */
 		mon = (mon <= 12) ? bin2bcd(mon) : 0xff;