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timers: Move clearing of base::timer_running under base:: Lock

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syzbot reported KCSAN data races vs. timer_base::timer_running being set to
NULL without holding base::lock in expire_timers().

This looks innocent and most reads are clearly not problematic, but
Frederic identified an issue which is:

 int data = 0;

 void timer_func(struct timer_list *t)
 {
    data = 1;
 }

 CPU 0                                            CPU 1
 ------------------------------                   --------------------------
 base = lock_timer_base(timer, &flags);           raw_spin_unlock(&base->lock);
 if (base->running_timer != timer)                call_timer_fn(timer, fn, baseclk);
   ret = detach_if_pending(timer, base, true);    base->running_timer = NULL;
 raw_spin_unlock_irqrestore(&base->lock, flags);  raw_spin_lock(&base->lock);

 x = data;

If the timer has previously executed on CPU 1 and then CPU 0 can observe
base->running_timer == NULL and returns, assuming the timer has completed,
but it's not guaranteed on all architectures. The comment for
del_timer_sync() makes that guarantee. Moving the assignment under
base->lock prevents this.

For non-RT kernel it's performance wise completely irrelevant whether the
store happens before or after taking the lock. For an RT kernel moving the
store under the lock requires an extra unlock/lock pair in the case that
there is a waiter for the timer, but that's not the end of the world.

Reported-by: syzbot+aa7c2385d46c5eba0b89@syzkaller.appspotmail.com
Reported-by: syzbot+abea4558531bae1ba9fe@syzkaller.appspotmail.com
Fixes: 030dcdd197 ("timers: Prepare support for PREEMPT_RT")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/87lfea7gw8.fsf@nanos.tec.linutronix.de
Cc: stable@vger.kernel.org
This commit is contained in:
Thomas Gleixner 2020-12-06 22:40:07 +01:00
parent ff1176468d
commit bb7262b295
1 changed files with 4 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
kernel/time/timer.c
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@ -1265,8 +1265,10 @@ static inline void timer_base_unlock_expiry(struct timer_base *base)
static void timer_sync_wait_running(struct timer_base *base)
{
if (atomic_read(&base->timer_waiters)) {
raw_spin_unlock_irq(&base->lock);
spin_unlock(&base->expiry_lock);
spin_lock(&base->expiry_lock);
raw_spin_lock_irq(&base->lock);
}
}
@ -1457,14 +1459,14 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
if (timer->flags & TIMER_IRQSAFE) {
raw_spin_unlock(&base->lock);
call_timer_fn(timer, fn, baseclk);
base->running_timer = NULL;
raw_spin_lock(&base->lock);
base->running_timer = NULL;
} else {
raw_spin_unlock_irq(&base->lock);
call_timer_fn(timer, fn, baseclk);
raw_spin_lock_irq(&base->lock);
base->running_timer = NULL;
timer_sync_wait_running(base);
raw_spin_lock_irq(&base->lock);
}
}
}