Re: [PATCH v7 clocksource 3/5] clocksource: Check per-CPU clock synchronization when marked unstable

Paul E. McKenney
2021-04-11T16:46:15Z

On Sun, Apr 11, 2021 at 12:33:44PM +0200, Thomas Gleixner wrote:
> On Sat, Apr 10 2021 at 17:20, Paul E. McKenney wrote:
> > On Sat, Apr 10, 2021 at 11:00:25AM +0200, Thomas Gleixner wrote:
> >> > + if (WARN_ON_ONCE(!cs))
> >> > + return;
> >> > + pr_warn("Checking clocksource %s synchronization from CPU %d.\n",
> >> > + cs->name, smp_processor_id());
> >> > + cpumask_clear(&cpus_ahead);
> >> > + cpumask_clear(&cpus_behind);
> >> > + csnow_begin = cs->read(cs);
> >>
> >> So this is invoked via work and the actual clocksource change is done
> >> via work too. Once the clocksource is not longer actively used for
> >> timekeeping it can go away. What's guaranteeing that this runs prior to
> >> the clocksource change and 'cs' is valid throughout this function?
> >
> > From what I can see, cs->read() doesn't care whether or not the
> > clocksource has been marked unstable. So it should be OK to call
> > cs->read() before, during, or after the call to __clocksource_unstable().
> >
> > Also, this is only done on clocksources marked CLOCK_SOURCE_VERIFY_PERCPU,
> > so any clocksource that did not like cs->read() being invoked during
> > or after the call to __clocksource_unstable() should leave off the
> > CLOCK_SOURCE_VERIFY_PERCPU bit.
> >
> > Or did I take a wrong turn somewhere in the pointers to functions?
>
> Right. cs->read() does not care, but what guarantees that cs is valid
> and not freed yet? It's not an issue with TSC and KVMCLOCK, but
> conceptually the following is possible:
>
> watchdog()
> queue_work(synccheck);
> queue_work(clocksource_change);
>
> work:
> synccheck() clocksource_change()
> preemption ...
> ...
> some_other_code():
> unregister_clocksource(cs)
> free(cs)
> cs->read() <- UAF

Got it, with the ingenic_tcu_init() function being case in point.
It invokes clcoksource_unregister() shortly followed by clk_put(), which,
if I found the correct clk_put(), can kfree() it.

Thank you!

> >> > + queue_work(system_highpri_wq, &clocksource_verify_work);
> >>
> >> This does not guarantee anything. So why does this need an extra work
> >> function which is scheduled seperately?
> >
> > Because I was concerned about doing smp_call_function() while holding
> > watchdog_lock, which is also acquired elsewhere using spin_lock_irqsave().
> > And it still looks like on x86 that spin_lock_irqsave() spins with irqs
> > disabled, which could result in deadlock. The smp_call_function_single()
> > would wait for the target CPU to enable interrupts, which would not
> > happen until after the smp_call_function_single() returned due to its
> > caller holding watchdog_lock.
> >
> > Or is there something that I am missing that prevents this deadlock
> > from occurring?
>
> The unstable mechanism is:
>
> watchdog()
> __clocksource_unstable()
> schedule_work(&watchdog_work);
>
> watchdog_work()
> kthread_run(clocksource_watchdog_thread);
>
> cs_watchdog_thread()
> mutex_lock(&clocksource_mutex);
> if (__clocksource_watchdog_kthread())
> clocksource_select();
> mutex_unlock(&clocksource_mutex);
>
> So what prevents you from doing that right in watchdog_work() or even in
> cs_watchdog_thread() properly ordered against the actual clocksource
> switch?
>
> Hmm?

My own confusion, apparently. :-/

So I need to is inline clocksource_verify_percpu_wq()
into clocksource_verify_percpu() and then move the call to
clocksource_verify_percpu() to __clocksource_watchdog_kthread(), right
before the existing call to list_del_init(). Will do!

Thanx, Paul