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Date: Thu, 29 May 2014 18:50:57 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sasha Levin <sasha.levin@oracle.com>
Cc: Ingo Molnar <mingo@kernel.org>, acme@ghostprotocols.net,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Dave Jones <davej@redhat.com>
Subject: Re: perf: use after free in perf_remove_from_context
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On Thu, May 29, 2014 at 12:44:23PM -0400, Sasha Levin wrote:
> On 05/29/2014 11:07 AM, Peter Zijlstra wrote:
> > On Thu, May 29, 2014 at 10:47:09AM -0400, Sasha Levin wrote:
> >> It doesn't work out well because we later lock a mutex in sync_child_event().
> >>
> > 
> > Urgh, right you are. I'll go stare at it more. It shouldn't have
> > mattered, because the mutex we take just before should ensure existence,
> > but.. you know.. :-)
> > 
> 
> So the only caller to sync_child_event() is that loop. According to what you said
> it should be safe to remove that mutex lock, but doing that triggers a list
> corruption:
> 
> [ 1204.341887] WARNING: CPU: 20 PID: 12839 at lib/list_debug.c:62 __list_del_entry+0xa1/0xe0()
> [ 1204.347597] list_del corruption. next->prev should be ffff8806ca68b108, but was ffff88051a67c398
> [...]
> 
> I don't see how that would happen :/

No, what I said is that the mutex in perf_event_exit_task() should be
sufficient to guard the list iteration calling __perf_event_exit_task().

Ading the RCU was a bit of paranoia..