Re: use-after-free in __perf_install_in_context

From: Dmitry Vyukov <dvyukov@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: 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
Date: Thu, 17 Dec 2015 15:35:32 +0100	[thread overview]
Message-ID: <CACT4Y+ZTM7ine2Sz2H=TpGVuXgTg4An+LwiGPNLUcYcMQHcx1Q@mail.gmail.com> (raw)
In-Reply-To: <20151217142823.GO6373@twins.programming.kicks-ass.net>

On Thu, Dec 17, 2015 at 3:28 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>> > >> I'm not sure I can explain your problem with this, but I figure its
>> > >> worth a try.
>> > >
>> > > Did it make a difference?
>> > >
>> > > Also, does the syzkaller thing do cpu-hotplug during its runs?
>> >
>> > See my email on Dec 15.
>>
>> I have 4 emails from you on that day (and one private), they pertain to
>> getting stuck in perf_{install,remove}_from_context(). None of those
>> emails seem to contain an answer to either of the two above questions.
>>
>>   CACT4Y+Za4L4q6FZLXjyGbnSYRwa7RJZYcG_s562q+CrNy9C75w@mail.gmail.com
>>   CACT4Y+YUkYgCuQFW6eBTtdfRqdF7EEtrB3VaYJk+LUXE_-iGRQ@mail.gmail.com
>>   CACT4Y+bvHWuyDZjoFhLAOiK1rfNQcfuSJCY0oS5xA=u8j04bXQ@mail.gmail.com
>>   CACT4Y+an67KnTsVVzYVvrYKW9yg7PbDuC8WyB38PiWRoSt0UCQ@mail.gmail.com
>>
>> and in private:
>>
>>   CACT4Y+Zb1vU192JbCnyS9qY3cpsvfAUJdpBVUsmosueCtQjSXQ@mail.gmail.com
>>
>> Did I miss an email?
>
> Yes, I seem to have missed this one:
>
>   CACT4Y+Y_YdEd-YGdWq-wQV6S+1M2nKA6atYO2DOFa8-wtQ8QWg@mail.gmail.com
>
> found it, thanks!

Yes, that one.
In short, I did not see use-after-frees but perf_event_open fuzzing
started hanging VMs very frequently, so testing is inconclusive.

And, no, nobody touches anything in /sys/devices.