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Date: Tue, 8 Dec 2015 19:57:38 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>,
        Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Sasha Levin <sasha.levin@oracle.com>
Subject: Re: use-after-free in __perf_install_in_context
Message-ID: <20151208185737.GB3004@gmail.com>
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 <20151208162227.GB6357@twins.programming.kicks-ass.net>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Fri, Dec 04, 2015 at 09:04:35PM +0100, Dmitry Vyukov wrote:
> > Hello,
> > 
> > While running syzkaller fuzzer I am seeing lots of the following
> > use-after-free reports. Unfortunately all my numerous attempts to
> > reproduce them in a controlled environment failed. They pop up during
> > fuzzing periodically (once in several hours in a single VM), but
> > whenever I try to stress-replay what happened in the VM before the
> > report, the use-after-free does not reproduce. 
> 
> Typical that :/
> 
> > Can somebody knowledgeable in perf subsystem look at the report? Maybe
> > it is possible to figure out what happened based purely on the report.
> > I can pretty reliably test any proposed fixes.
> 
> So I'm still going over the code, but meanwhile I tried reproducing this
> using the perf_fuzzer and some debug code, but no luck with that.
> 
> Since you seem to be able to reproduce, could you do a run with the
> below patch in to see if it tickles something?

Btw., could we add more redundancy / debug code to the refcounting code? It seems 
to be a frequent source of very hard to find/fix races/bugs - so it should be ripe 
for some extra debug infrastructure ...

Thanks,


	Ingo