linux-kernel - Re: uprobes/perf: KASAN: use-after-free in uprobe_perf

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Date:   Mon, 9 Apr 2018 19:00:06 +0900
From:   Prashant Bhole <bhole_prashant_q7@....ntt.co.jp>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: uprobes/perf: KASAN: use-after-free in uprobe_perf_close



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