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