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 S1752010AbbHLRvG (ORCPT <rfc822;w@1wt.eu>);
	Wed, 12 Aug 2015 13:51:06 -0400
Received: from mx1.redhat.com ([209.132.183.28]:50129 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1751084AbbHLRvE (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Wed, 12 Aug 2015 13:51:04 -0400
Date: Wed, 12 Aug 2015 19:48:47 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>, 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 1/2] unshare: Unsharing a thread does not require
	unsharing a vm
Message-ID: <20150812174847.GA6703@redhat.com>
References: <20150728171500.GA2871@www.outflux.net> <20150728143504.5aa996ba5955522a19c2d5f1@linux-foundation.org> <20150728221111.GA23391@node.dhcp.inet.fi> <20150805172356.GA20490@redhat.com> <87wpx9sjhq.fsf@x220.int.ebiederm.org> <87614tr2jd.fsf@x220.int.ebiederm.org> <20150806130629.GA4728@redhat.com> <20150806134426.GA6843@redhat.com> <871tf9cnbi.fsf_-_@x220.int.ebiederm.org> <87vbclb8op.fsf_-_@x220.int.ebiederm.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87vbclb8op.fsf_-_@x220.int.ebiederm.org>
User-Agent: Mutt/1.5.18 (2008-05-17)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 08/11, Eric W. Biederman wrote:
>
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1866,13 +1866,17 @@ static int check_unshare_flags(unsigned long unshare_flags)
>  				CLONE_NEWUSER|CLONE_NEWPID))
>  		return -EINVAL;
>  	/*
> -	 * Not implemented, but pretend it works if there is nothing to
> -	 * unshare. Note that unsharing CLONE_THREAD or CLONE_SIGHAND
> -	 * needs to unshare vm.
> +	 * Not implemented, but pretend it works if there is nothing
> +	 * to unshare.  Note that unsharing the address space or the
> +	 * signal handlers also need to unshare the signal queues (aka
> +	 * CLONE_THREAD).
>  	 */
>  	if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
> -		/* FIXME: get_task_mm() increments ->mm_users */
> -		if (atomic_read(&current->mm->mm_users) > 1)
> +		if (!thread_group_empty(current))
> +			return -EINVAL;
> +	}
> +	if (unshare_flags & CLONE_VM) {
> +		if (!current_is_single_threaded())
>  			return -EINVAL;
>  	}

OK, but then you can remove "| CLONE_VM" from the previous check...


> @@ -1941,16 +1945,16 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
>  	if (unshare_flags & CLONE_NEWUSER)
>  		unshare_flags |= CLONE_THREAD | CLONE_FS;
>  	/*
> -	 * If unsharing a thread from a thread group, must also unshare vm.
> -	 */
> -	if (unshare_flags & CLONE_THREAD)
> -		unshare_flags |= CLONE_VM;

OK,

>  	/*
> +	 * If unsharing a signal handlers, must also unshare the signal queues.
> +	 */
> +	if (unshare_flags & CLONE_SIGHAND)
> +		unshare_flags |= CLONE_THREAD;

This looks unnecessary, check_unshare_flags() checks "THREAD | SIGHAND".
And to me the comment looks misleading although I won't argue.

And in fact this doesn't look exactly right, or I am totally confused.
Shouldn't we do

	if (unshare_flags & CLONE_SIGHAND)
		unshare_flags |= CLONE_VM;

? Or change check_unshare_flags()...

Otherwise suppose that a single threaded process does clone(VM | SIGHAND)
and (say) child does sys_unshare(SIGHAND). This will wrongly succeed afaics.

Oleg.