From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754212AbbLKIOk (ORCPT <rfc822;w@1wt.eu>);
	Fri, 11 Dec 2015 03:14:40 -0500
Received: from mail-wm0-f42.google.com ([74.125.82.42]:36720 "EHLO
	mail-wm0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753215AbbLKIOi (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 11 Dec 2015 03:14:38 -0500
Date: Fri, 11 Dec 2015 09:14:34 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Dmitry Vyukov <dvyukov@google.com>,
        syzkaller <syzkaller@googlegroups.com>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        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: <20151211081433.GB21600@gmail.com>
References: <CACT4Y+ZsKVN+FXjyRNYgtWxMjjYfB9uz8f0QAW7iGjLE5GCQBA@mail.gmail.com>
 <20151208175433.GA75053@ast-mbp.thefacebook.com>
 <CACT4Y+Y_w4oteNLKGE4gVqOpoyrmm+3VQDemGGScbepnXhL4VA@mail.gmail.com>
 <20151208180528.GA77166@ast-mbp.thefacebook.com>
 <CACT4Y+Y-wBH+xfei_q-6FcEoShM8sX4eX3bQyfEppWy522RiYw@mail.gmail.com>
 <20151208195623.GA92501@ast-mbp.thefacebook.com>
 <CACT4Y+bKU96_CPqC6CxJ-M7rnbiBa4sY2BesDMTXf+uLTjjv=Q@mail.gmail.com>
 <20151210035434.GA11217@ast-mbp.thefacebook.com>
 <20151210090251.GV6356@twins.programming.kicks-ass.net>
 <20151210170305.GA28459@ast-mbp.thefacebook.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151210170305.GA28459@ast-mbp.thefacebook.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


* Alexei Starovoitov <alexei.starovoitov@gmail.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