linux-kernel - Re: [PATCH v2] prctl: add PR_[GS]ET

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Date:   Fri, 3 Aug 2018 14:14:07 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jürg Billeter <j@...ron.ch>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] prctl: add PR_[GS]ET_KILLABLE

On 08/03, Jürg Billeter wrote:
>
> On Wed, 2018-08-01 at 16:19 +0200, Oleg Nesterov wrote:
> > On 07/31, Jürg Billeter wrote:
> > >
> > > > Could you explain your use-case? Why a shell wants to use
> > > > CLONE_NEWPID?
> > >
> > > To guarantee that there won't be any runaway processes, i.e., ensure
> > > that no descendants (background helper daemons or misbehaving
> > > processes) survive when the child process is terminated.
> >
> > We already have PR_SET_CHILD_SUBREAPER.
> >
> > Perhaps we can finally add PR_KILL_MY_DESCENDANTS_ON_EXIT? This was already
> > discussed some time ago, but I can't find the previous discussion... Simple
> > to implement.
>
> This would definitely be an option. You mentioned it last October in
> the PR_SET_PDEATHSIG_PROC discussion¹. However, as PID namespaces
> already exist and appear to be a good fit for the most part,

Sure, if CLONE_NEWPID fits your needs you can use it,

> I think it
> makes sense to just add the missing pieces to PID namespaces instead of
> duplicating part of the PID namespace functionality.

Again, I am not arguing with your change.

PR_KILL_MY_DESCENDANTS_ON_EXIT can make sense just like PR_SET_CHILD_SUBREAPER
even if PID namespace functionality implies both. Simply because CLONE_NEWPID
is not necessarily the best tool, if nothing else you do not necessarily want
the pid isolation.

> Also, based on Eric's comment in that other discussion about
> no_new_privs not being allowed to increase the attack surface,
> PR_KILL_MY_DESCENDANTS_ON_EXIT might require CAP_SYS_ADMIN as well (due
> to setuid children).

No, no, the exiting parent should simply do group_send_sig_info(SIGKILL)
for every descendant and rely on check_kill_permission().

OK, lets forget it for now.

Oleg.