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

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Date:   Sat, 01 Dec 2018 13:28:37 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Jürg Billeter <j@...ron.ch>
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

* 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