linux-kernel - Re: WARNING in event_function

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Date:   Wed, 13 Feb 2019 10:51:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+352bd10e338d9a90e5e0@...kaller.appspotmail.com>,
        Abderrahmane Benbachir <abderrahmane.benbachir@...ymtl.ca>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: WARNING in event_function_local

On Tue, Feb 12, 2019 at 07:40:12PM -0800, Kees Cook wrote:

> > > Is this maybe just an unlucky condition with the event loop running in
> > > an IRQ? Should the WARN be expected, or is running under an IRQ
> > > unexpected?
> 
> Is perf expected to fire during an IRQ? The task == current test seems
> suspicious if so...

So the only possible callchain here is:

  <PMI>
    ...
      perf_event_disable_inatomic()
        irq_work_queue()

  <irq_work-IPI>
    perf_pending_event()
      perf_event_disable_local()
        event_function_local()


The assertion states that:

  if the event is a task event; and the context is active, it _must_ be
  the same task.

Because: if the PMI happens during ctxsw (which has IRQs disabled), the
IPI will not happen until after the ctxsw, at which point we'll also
have switched out the perf context of that task -- IOW the context
should be inactive.


Anyway, it looks like a virt issue; I'll start caring once you can
reproduce on real hardware.