[PATCH 4.11 70/78] alarmtimer: Prevent overflow of relative timers

From: Greg Kroah-Hartman
Date: Mon Jun 19 2017 - 11:28:15 EST

4.11-stable review patch. If anyone has any objections, please let me know.


From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit f4781e76f90df7aec400635d73ea4c35ee1d4765 upstream.

Andrey reported a alartimer related RCU stall while fuzzing the kernel with

The reason for this is an overflow in ktime_add() which brings the
resulting time into negative space and causes immediate expiry of the
timer. The following rearm with a small interval does not bring the timer
back into positive space due to the same issue.

This results in a permanent firing alarmtimer which hogs the CPU.

Use ktime_add_safe() instead which detects the overflow and clamps the
result to KTIME_SEC_MAX.

Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Kostya Serebryany <kcc@xxxxxxxxxx>
Cc: syzkaller <syzkaller@xxxxxxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/20170530211655.802921648@xxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

kernel/time/alarmtimer.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -387,7 +387,7 @@ void alarm_start_relative(struct alarm *
struct alarm_base *base = &alarm_bases[alarm->type];

- start = ktime_add(start, base->gettime());
+ start = ktime_add_safe(start, base->gettime());
alarm_start(alarm, start);
@@ -475,7 +475,7 @@ u64 alarm_forward(struct alarm *alarm, k

- alarm->node.expires = ktime_add(alarm->node.expires, interval);
+ alarm->node.expires = ktime_add_safe(alarm->node.expires, interval);
return overrun;
@@ -666,7 +666,7 @@ static int alarm_timer_set(struct k_itim
ktime_t now;

now = alarm_bases[timr->it.alarm.alarmtimer.type].gettime();
- exp = ktime_add(now, exp);
+ exp = ktime_add_safe(now, exp);

alarm_start(&timr->it.alarm.alarmtimer, exp);