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A fix for a long standing issue in the alarmtimer code:

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Posix-timers armed with a short interval with an ignored signal result
  in an unpriviledged DoS. Due to the ignored signal the timer switches
  into self rearm mode. This issue had been "fixed" before but a rework of
  the alarmtimer code 5 years ago lost that workaround.
 
  There is no real good solution for this issue, which is also worked around
  in the core posix-timer code in the same way, but it certainly moved way
  up on the ever growing todo list.
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Merge tag 'timers-urgent-2023-02-19' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull timer fix from Thomas Gleixner:
 "A fix for a long standing issue in the alarmtimer code.

  Posix-timers armed with a short interval with an ignored signal result
  in an unpriviledged DoS. Due to the ignored signal the timer switches
  into self rearm mode. This issue had been "fixed" before but a rework
  of the alarmtimer code 5 years ago lost that workaround.

  There is no real good solution for this issue, which is also worked
  around in the core posix-timer code in the same way, but it certainly
  moved way up on the ever growing todo list"

* tag 'timers-urgent-2023-02-19' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  alarmtimer: Prevent starvation by small intervals and SIG_IGN
This commit is contained in:
Linus Torvalds 2023-02-18 17:46:50 -08:00
commit 0097c18e45
1 changed files with 29 additions and 4 deletions
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33
kernel/time/alarmtimer.c
View File

@ -470,11 +470,35 @@ u64 alarm_forward(struct alarm *alarm, ktime_t now, ktime_t interval)
}
EXPORT_SYMBOL_GPL(alarm_forward);
u64 alarm_forward_now(struct alarm *alarm, ktime_t interval)
static u64 __alarm_forward_now(struct alarm *alarm, ktime_t interval, bool throttle)
{
struct alarm_base *base = &alarm_bases[alarm->type];
ktime_t now = base->get_ktime();
return alarm_forward(alarm, base->get_ktime(), interval);
if (IS_ENABLED(CONFIG_HIGH_RES_TIMERS) && throttle) {
/*
* Same issue as with posix_timer_fn(). Timers which are
* periodic but the signal is ignored can starve the system
* with a very small interval. The real fix which was
* promised in the context of posix_timer_fn() never
* materialized, but someone should really work on it.
*
* To prevent DOS fake @now to be 1 jiffie out which keeps
* the overrun accounting correct but creates an
* inconsistency vs. timer_gettime(2).
*/
ktime_t kj = NSEC_PER_SEC / HZ;
if (interval < kj)
now = ktime_add(now, kj);
}
return alarm_forward(alarm, now, interval);
}
u64 alarm_forward_now(struct alarm *alarm, ktime_t interval)
{
return __alarm_forward_now(alarm, interval, false);
}
EXPORT_SYMBOL_GPL(alarm_forward_now);
@ -551,9 +575,10 @@ static enum alarmtimer_restart alarm_handle_timer(struct alarm *alarm,
if (posix_timer_event(ptr, si_private) && ptr->it_interval) {
/*
* Handle ignored signals and rearm the timer. This will go
* away once we handle ignored signals proper.
* away once we handle ignored signals proper. Ensure that
* small intervals cannot starve the system.
*/
ptr->it_overrun += alarm_forward_now(alarm, ptr->it_interval);
ptr->it_overrun += __alarm_forward_now(alarm, ptr->it_interval, true);
++ptr->it_requeue_pending;
ptr->it_active = 1;
result = ALARMTIMER_RESTART;