linux-kernel - Re: [PATCH v2 1/1] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON

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Date:   Sat, 01 Dec 2018 10:39:28 +0000
From:   Jürg Billeter <j@...ron.ch>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON_EXIT

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).

> 
> > 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).

Jürg