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Date: Mon, 9 Apr 2018 09:38:27 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Cc: Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: uprobes/perf: KASAN: use-after-free in uprobe_perf_close
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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?

> +		put_task_struct(task);
>  	kfree(event);
> 
>  	return ERR_PTR(err);