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 S1752552AbbG1WLR (ORCPT <rfc822;w@1wt.eu>);
	Tue, 28 Jul 2015 18:11:17 -0400
Received: from mail-wi0-f180.google.com ([209.85.212.180]:34624 "EHLO
	mail-wi0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751910AbbG1WLQ (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 28 Jul 2015 18:11:16 -0400
Date: Wed, 29 Jul 2015 01:11:11 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>, David Howells <dhowells@redhat.com>,
        linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Rik van Riel <riel@redhat.com>,
        Vladimir Davydov <vdavydov@parallels.com>,
        Ricky Zhou <rickyz@chromium.org>, Julien Tinnes <jln@google.com>
Subject: Re: [PATCH] user_ns: use correct check for single-threadedness
Message-ID: <20150728221111.GA23391@node.dhcp.inet.fi>
References: <20150728171500.GA2871@www.outflux.net>
 <20150728143504.5aa996ba5955522a19c2d5f1@linux-foundation.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150728143504.5aa996ba5955522a19c2d5f1@linux-foundation.org>
User-Agent: Mutt/1.5.23.1 (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

On Tue, Jul 28, 2015 at 02:35:04PM -0700, Andrew Morton wrote:
> On Tue, 28 Jul 2015 10:15:00 -0700 Kees Cook <keescook@chromium.org> wrote:
> 
> > From: Ricky Zhou <rickyz@chromium.org>
> > 
> > Checking mm_users > 1 does not mean a process is multithreaded. For
> > example, reading /proc/PID/maps temporarily increments mm_users, allowing
> > other processes to (accidentally) interfere with unshare() calls.
> > 
> > This fixes observed failures of unshare(CLONE_NEWUSER) incorrectly
> > returning EINVAL if another processes happened to be simultaneously
> > reading the maps file.
> 
> Yikes.  current_is_single_threaded() is expensive.  Are we sure this
> isn't going to kill someone's workload?

It's expensive only if mm_users > 1. We will go to for_each_process() only
if somebody outside of the process grabs mm_users references (like reading
/proc/PID/maps). Or if it called it from multithreaded application.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov