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

[Stephane Eranian]

On Fri, Aug 21, 2015 at 1:31 PM, Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
>
> On 05/21/2015 07:17 AM, Peter Zijlstra wrote:
> > --- a/arch/x86/kernel/cpu/perf_event_intel.c
> > +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> > @@ -2106,7 +2106,7 @@ static struct event_constraint *
> >  intel_get_event_constraints(struct cpu_hw_events *cpuc, int idx,
> >                           struct perf_event *event)
> >  {
> > -     struct event_constraint *c1 = event->hw.constraint;
> > +     struct event_constraint *c1 = cpuc->event_constraint[idx];
> >       struct event_constraint *c2;
>
> Hey Peter,
>
> I was chasing a memory corruption in this area and I think I found
> a possible culprit:
>
> After this patch, In the code above, we'd access "cpuc->event_constraint[idx]"
> and read/change memory.
>
> The problem is that a valid value for idx is also -1, which isn't checked
> here, so we end up accessing and possibly corrupting memory that isn't ours.
>
>
I believe your analysis is correct, the following path will create the problem:

   validate_group()
         validate_event()
             x86_pmu.get_event_constraints(fake_cpuc, -1, event)
             intel_get_event_constraints(cpuc, idx, event)
                 struct event_constraints *c1  = cpuc->event_constraints[idx];

here idx = -1, and the kernel is accessing an invalid memory location.

If think the code could be changed to:

   struct event_constraint *c1 = NULL;
   if (idx > -1)
       c1 = cpuc->event_constraints[idx];

idx is not used in the __intel_get_event_constraints() path if I read
the code correctly.

Peter?



>
> Thanks,
> Sasha
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