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From:   Florian Weimer <fweimer@redhat.com>
To:     =?utf-8?Q?J=C3=BCrg?= Billeter <j@bitron.ch>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@kernel.org>, 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>
        <e92ab3c06fe42fc9872261999623be19e2b0a294.camel@bitron.ch>
Date:   Sat, 01 Dec 2018 13:28:37 +0100
In-Reply-To: <e92ab3c06fe42fc9872261999623be19e2b0a294.camel@bitron.ch>
 (=?utf-8?Q?=22J=C3=BCrg?=
        Billeter"'s message of "Sat, 01 Dec 2018 10:39:28 +0000")
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* 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