Re: uprobes/perf: KASAN: use-after-free in uprobe_perf_close

[Prashant Bhole]

On 4/9/2018 4:38 PM, Peter Zijlstra wrote:
> On Tue, Mar 06, 2018 at 06:49:10PM +0900, Prashant Bhole wrote:
> > Sorry for late reply. I tried these changes. It didn't fix the problem. With
>
> He, sorry for completely forgetting about this one :/
>
> > these changes, the use-after-free access of task_struct occurs at
> > _free_event() for the last remaining event.
> >
> > In your changes, I tried keeping get/put_task_struct() in
> > perf_alloc_context()/put_ctx() intact and The problem did not occur. Changes
> > are mentioned below.
>
> Yes, I think you're right in that this is the cleanest solution; it adds
> reference counting to the exact pointer we're using.
>
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index c98cce4ceebd..65889d2b5ae2 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -4109,6 +4109,8 @@ static void _free_event(struct perf_event *event)
> >
> >   	if (event->ctx)
> >   		put_ctx(event->ctx);
> > +	if (event->hw.target)
> > +		put_task_struct(event->hw.target);
> >
> >   	exclusive_event_destroy(event);
> >   	module_put(event->pmu->module);
> > @@ -9593,6 +9595,7 @@ perf_event_alloc(struct perf_event_attr *attr, int
> > cpu,
> >   		 * and we cannot use the ctx information because we need the
> >   		 * pmu before we get a ctx.
> >   		 */
> > +		get_task_struct(task);
> >   		event->hw.target = task;
> >   	}
> >
> > @@ -9708,6 +9711,8 @@ perf_event_alloc(struct perf_event_attr *attr, int
> > cpu,
> >   		perf_detach_cgroup(event);
> >   	if (event->ns)
> >   		put_pid_ns(event->ns);
> > +	if (task)
>
> Should this not too be 'event->hw.target', for consistency and clarity?


Yes, I am sending a patch with this change. Thanks.

-Prashant