• Thomas Gleixner's avatar
    timers: Move clearing of base::timer_running under base:: Lock · bb7262b2
    Thomas Gleixner authored
    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: 030dcdd1 ("timers: Prepare support for PREEMPT_RT")
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Tested-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
    Link: https://lore.kernel.org/r/87lfea7gw8.fsf@nanos.tec.linutronix.de
    Cc: stable@vger.kernel.org
    bb7262b2
timer.c 60.1 KB