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Date: Fri, 4 Dec 2015 12:32:14 -0800
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, 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: <20151204203212.GA64517@ast-mbp.thefacebook.com>
References: <CACT4Y+bQqHPsxdh5nhSA154KeCxRirk8QGwRe-5pMUToGE8yAg@mail.gmail.com>
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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. 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.
> All reports look like this one. Then it is usually followed by other
> reports and eventually kernel hangs or dies. What happens in the
> fuzzer is essentially random syscalls with random arguments, tasks
> born and die concurrently and so on. I was able to reproduce it by
> restricting syscalls only to perf_event_open, perf ioctls and bpf
> syscall.

For the sake of trying to narrow it down:
does the error disappear when you stop using bpf syscall in your fuzzing?
If yes, then I could have missed some interaction between perf_event_free,
kprobe free and bpf_prog_free.
There was a race there before.
May be there is still something else.