Messages in this thread | | | Date | Fri, 14 May 2021 16:44:14 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH V7 2/2] perf/x86: Reset the dirty counter to prevent the leak for an RDPMC task |
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On Thu, May 13, 2021 at 06:14:08PM -0400, Liang, Kan wrote: > On 5/13/2021 11:02 AM, Peter Zijlstra wrote: > > On Thu, May 13, 2021 at 07:23:02AM -0700, kan.liang@linux.intel.com wrote: > > > > > + if (x86_pmu.sched_task && event->hw.target) { > > > + atomic_inc(&event->pmu->sched_cb_usage); > > > + local_irq_save(flags); > > > + x86_pmu_clear_dirty_counters(); > > > + local_irq_restore(flags); > > > + } > > > > So what happens if our mmap() happens after we've already created two > > (or more) threads in the process, all of who already have a counter (or > > more) on? > > > > Shouldn't this be something like? > > That's not enough. > > I implemented a test case as below: > - The main thread A creates a new thread B. > - Bind the thread A to CPU 0. Then the thread A opens a event, mmap, enable > the event, and sleep. > - Bind the thread B to CPU 1. Wait until the event in the thread A is > enabled. Then RDPMC can read the counters on CPU 1.
This?
A B
clone(CLONE_THREAD) ---> set_affine(0) set_affine(1) while (!event-enabled) ; event = perf_event_open() mmap(event) ioctl(event, IOC_ENABLE); ---> RDPMC
sleep(n) schedule(INTERRUPTIBLE) /* idle */
> In the x86_pmu_event_mapped(), we do on_each_cpu_mask(mm_cpumask(mm), > cr4_update_pce, NULL, 1); > The RDPMC from thread B on CPU 1 is not forbidden. > Since the counter is not created in thread B, the sched_task() never gets a > chance to be invoked. The dirty counter is not cleared.
Per-task counters from CPU1 that ran before B ran?
> To fix it, I think we have to move the cr4_update_pce() to the context > switch, and update it only when the RDPMC task is scheduled. But it probably > brings some overhead.
We have CR4:PCE updates in the context switch path, see switch_mm_irqs_off() -> cr4_update_pce_mm().
Doing the clear there might actually make sense and avoids this frobbing of ->sched_task(). When we call cr4_update_pce_mm(), and @mm has rdpmc on, clear dirty or something like that.
Worth a try.
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