linux-kernel - Re: [PATCH 01/10] perf,x86: Fix event/group validation

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Date:	Thu, 21 May 2015 06:18:15 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Jiri Olsa <jolsa@...hat.com>,
	"Liang, Kan" <kan.liang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Hunter <ahh@...gle.com>,
	Maria Dimakopoulou <maria.n.dimakopoulou@...il.com>
Subject: Re: [PATCH 01/10] perf,x86: Fix event/group validation

On Thu, May 21, 2015 at 6:09 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, May 21, 2015 at 06:07:20AM -0700, Stephane Eranian wrote:
>> On Thu, May 21, 2015 at 5:56 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Thu, May 21, 2015 at 05:35:02AM -0700, Stephane Eranian wrote:
>> >> > Commit e979121b1b15 ("perf/x86/intel: Implement cross-HT corruption
>> >> > bug workaround") made the situation much worse by actually setting the
>> >> > event->hw.constraint value to NULL, so when validation and actual
>> >> > scheduling interact we get NULL ptr derefs.
>> >> >
>> >>
>> >> But  x86_schedule_events() does reset the hw.constraint for each invocation:
>> >>
>> >>            c = x86_pmu.get_event_constraints(cpuc, i, cpuc->event_list[i]);
>> >>            hwc->constraint = c;
>> >
>> > Yes, so if you have:
>> >
>> >         validate_group()
>> >
>> >                 hwc->constraint = c;
>> >
>> Ok, you get that because validate_group() invokes x6_schedule_events() but
>> on the fake_cpuc. This on fake_cpuc->event_list[]->hwc.
>>
>> >         <context switch>
>> >
>> >                 c = hwc->constraint;
>> >
>> > The second c might not be the first.
>> And where does this assignment come from?
>
> That's a read. The <context switch> can include a call to
> x86_schedule_events().
Yes, but x86_schedule_events() never reads the constraint
without setting it again before.

>
>> For actual scheduling, we are using the actual cpuc, not fake_cpuc.
>> Validate_group() does not modify global cpuc state. Or am I missing
>> something?
>
> No, but x86_schedule_event() can modify event state, which is the fail.
>
Yes, it does modify the cpuc->event_list[]->hwc, because it is used as a
cache for *EACH* invocation of the function. It is irrelevant outside the
function.

>
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