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 S1751538AbcGMRBg (ORCPT <rfc822;w@1wt.eu>);
	Wed, 13 Jul 2016 13:01:36 -0400
Received: from mx1.redhat.com ([209.132.183.28]:58849 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1750913AbcGMRBf (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Wed, 13 Jul 2016 13:01:35 -0400
Date: Wed, 13 Jul 2016 19:00:49 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Hector Marco-Gisbert <hecmargi@upv.es>,
        Ismael Ripoll Ripoll <iripoll@upv.es>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Chen Gang <gang.chen.5i5j@gmail.com>, Michal Hocko <mhocko@suse.com>,
        Konstantin Khlebnikov <koct9i@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] mm: refuse wrapped vm_brk requests
Message-ID: <20160713170048.GA24553@redhat.com>
References: <1468014494-25291-1-git-send-email-keescook@chromium.org> <1468014494-25291-3-git-send-email-keescook@chromium.org> <20160711122826.GA969@redhat.com> <CAGXu5j+efUrhOTikpuYK0V8Eqv58f5rQBMOYDqiVM-JWrqRbLA@mail.gmail.com> <20160712133942.GA28837@redhat.com> <CAGXu5j+oZ49K0omm-7yMsR_kFYD-DQcYG8f+urS+TumzFYXR_w@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAGXu5j+oZ49K0omm-7yMsR_kFYD-DQcYG8f+urS+TumzFYXR_w@mail.gmail.com>
User-Agent: Mutt/1.5.18 (2008-05-17)
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Wed, 13 Jul 2016 17:00:31 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 07/12, Kees Cook wrote:
>
> On Tue, Jul 12, 2016 at 9:39 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > I tried to say that, with or without this change, sys_brk() should check
> > for overflow too, otherwise it looks buggy.
>
> Hmm, it's not clear to me the right way to fix sys_brk(), but it looks
> like my change to do_brk() would catch the problem?

How?

Once again, afaics nothing bad can happen, sys_brk() will silently fail,
just the code looks wrong anyway.

Suppose that newbrk == 0 due to overflow, then both

	if (find_vma_intersection(mm, oldbrk, newbrk+PAGE_SIZE))
		goto out;

and
	if (do_brk(oldbrk, newbrk-oldbrk) < 0)
		goto out;

look buggy.

find_vma_intersection(start_addr, end_addr) expects that start_addr < end_addr.
Again, we do not really care if it returns NULL or not, and newbrk == 0 just
means it will certainly return NULL if there is something above oldbrk. Just
looks buggy/confusing.

do_brk(0 - oldbrk) will fail and this is what we want. But not because
your change will catch the problem, PAGE_ALIGNE(-oldbrk) won't necessarily
overflow. However, -oldbrk > TASK_SIZE so get_unmapped_area() should fail.

Nevermind, this is almost off-topic, so let me repeat just in case that
both patches look good to me.

Oleg.