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

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Message-ID: <20151211081433.GB21600@gmail.com>
Date:	Fri, 11 Dec 2015 09:14:34 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	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


* 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.

Thanks,

	Ingo
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