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Date: Tue, 14 Mar 2017 15:07:54 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
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        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        syzkaller <syzkaller@googlegroups.com>
Subject: Re: perf: use-after-free in perf_release
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On Tue, Mar 14, 2017 at 03:03:02PM +0100, Oleg Nesterov wrote:
> On 03/14, Peter Zijlstra wrote:
> >
> > Yes, this looks buggy. But I cannot explain how that would result in the
> > observed use-after-free.
> 
> Yes...
> 
> Suppose that copy_process() fails after perf_event_init_task(). In this
> case perf_event_free_task() does put_ctx(), but if this ctx has another
> reference (ctx->refcount > 1) then ctx->task will point to the already
> freed task, copy_process() does free_task() at the end of error path.
> And we can't replace it with put_task_struct().
> 
> I am looking at TASK_TOMBSTONE, perhaps perf_event_free_task() should
> use it too?

The idea was that the task isn't visible when we use
perf_event_free_task(). But I'll have a look.