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Date: Tue, 14 Mar 2017 16:46:34 +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 04:37:05PM +0100, Oleg Nesterov wrote:
> On 03/14, Peter Zijlstra wrote:
> >
> > Do we want a WARN_ON_ONCE(atomic_read(&tsk->usage)); in free_task()?
> > Because in the above scenario we're freeing it with references on.
> 
> Not sure, in this case copy_process() should decrement tsk->usage
> before free_task(), note the atomic_set(&tsk->usage, 2) in
> dup_task_struct().
> 
> Perhaps we should just add WARN_ON(tsk->usage != 2) into copy_process()
> right before free_task() ?

Sure; that works. I'll try that once I'm back home again, to see if
there's unexpected fail because other things increment it.