linux-kernel - Re: use-after-free in __perf_install_in

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Date:	Tue, 15 Dec 2015 14:11:12 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: use-after-free in __perf_install_in_context

On Fri, Dec 11, 2015 at 9:14 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
>
>> On Thu, Dec 10, 2015 at 10:02:51AM +0100, Peter Zijlstra wrote:
>> > On Wed, Dec 09, 2015 at 07:54:35PM -0800, Alexei Starovoitov wrote:
>> > > Freeing memory is a requirement regardless.
>> > > Even when kernel running with kasan, there must be a way to stop
>> > > stack collection and free that memory.
>> > > You cannot treat kernel as your test program or 'device under test'.
>> >
>> > Relax, that is exactly what lockdep does. It cannot dynamically allocate
>> > things because allocators use lock etc..
>> >
>> > Its fine to build up state for debug bits, esp. if its bounded, like the
>> > number of unique callchains.
>>
>> except the code in question is doing unbounded alloc_pages()
>
> Yes, but the trick is to still have a bound sized debug pool - which runs out of
> entries gracefully.
>
> Which in practice is plenty enough for most types of testing, and is a lot more
> robust than any dynamic scheme.

A hard upper bound on consumed memory would work for us without
introducing any slowdown and without increasing code complexity. So it
sounds good to me.
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