From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AA010C04EB9 for ; Sat, 1 Dec 2018 12:28:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7369620867 for ; Sat, 1 Dec 2018 12:28:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7369620867 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726727AbeLAXlT convert rfc822-to-8bit (ORCPT ); Sat, 1 Dec 2018 18:41:19 -0500 Received: from mx1.redhat.com ([209.132.183.28]:50114 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726577AbeLAXlT (ORCPT ); Sat, 1 Dec 2018 18:41:19 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 74C9A300208D; Sat, 1 Dec 2018 12:28:53 +0000 (UTC) Received: from oldenburg.str.redhat.com (ovpn-116-35.ams2.redhat.com [10.36.116.35]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D04C47C858; Sat, 1 Dec 2018 12:28:47 +0000 (UTC) From: Florian Weimer To: =?utf-8?Q?J=C3=BCrg?= Billeter Cc: Andrew Morton , Oleg Nesterov , Thomas Gleixner , Eric Biederman , Kees Cook , Andy Lutomirski , linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/1] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON_EXIT References: <20181127225408.7553-2-j@bitron.ch> <20181130080004.23635-1-j@bitron.ch> <20181130080004.23635-2-j@bitron.ch> <87bm66u1j5.fsf@oldenburg.str.redhat.com> Date: Sat, 01 Dec 2018 13:28:37 +0100 In-Reply-To: (=?utf-8?Q?=22J=C3=BCrg?= Billeter"'s message of "Sat, 01 Dec 2018 10:39:28 +0000") Message-ID: <878t19o2h6.fsf@oldenburg.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.47]); Sat, 01 Dec 2018 12:28:54 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Jürg Billeter: > On Fri, 2018-11-30 at 14:40 +0100, Florian Weimer wrote: >> * Jürg Billeter: >> >> > This introduces a new thread group flag that can be set by calling >> > >> > prctl(PR_SET_KILL_DESCENDANTS_ON_EXIT, 1, 0, 0, 0) >> > >> > When a thread group exits with this flag set, it will send SIGKILL to >> > all descendant processes. This can be used to prevent stray child >> > processes. >> > >> > This flag is cleared on privilege gaining execve(2) to ensure an >> > unprivileged process cannot get a privileged process to send SIGKILL. >> >> So this is inherited across regular execve? I'm not sure that's a good >> idea. > > Yes, this matches PR_SET_CHILD_SUBREAPER (and other process > attributes). Besides consistency and allowing a parent to configure the > flag for a spawned process, this is also needed to prevent a process > from clearing the flag (in combination with a seccomp filter). I think the semantics of PR_SET_CHILD_SUBREAPER are different, and the behavior makes more sense there. >> > Descendants that are orphaned and reparented to an ancestor of the >> > current process before the current process exits, will not be killed. >> > PR_SET_CHILD_SUBREAPER can be used to contain orphaned processes. >> >> For double- or triple-forking daemons, the reparenting will be racy, if >> I understand things correctly. > > Can you please elaborate, if you're concerned about a particular race? > As the commit message mentions, for containment this flag can be > combined with PR_SET_CHILD_SUBREAPER (and PR_SET_NO_NEW_PRIVS). Without PR_SET_CHILD_SUBREAPER, if a newly execve'ed daemon performs double/triple forking to disentangle itself from the parent process session, and the parent process which set PR_SET_KILL_DESCENDANTS_ON_EXIT terminates, behavior depends on when exactly the parent process terminates. The daemon process will leak if it has completed its reparenting. I think this could be sufficiently common that solution is needed here. Thanks, Florian