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 >> > + 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? Thanks, tglx