linux-kernel - Re: RFC: on adding new CLONE_* flags [WAS Re: [PATCH 0/4] clone: add CLONE

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Date:   Tue, 16 Apr 2019 14:31:44 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Christian Brauner <christian@...uner.io>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jann Horn <jannh@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Daniel Colascione <dancol@...gle.com>
Subject: Re: RFC: on adding new CLONE_* flags [WAS Re: [PATCH 0/4] clone: add CLONE_PIDFD]

On Tue, Apr 16, 2019 at 11:46 AM Enrico Weigelt, metux IT consult
<lkml@...ux.net> wrote:
>
> On 15.04.19 22:29, Andy Lutomirski wrote:
>
> <snip>
>
> > I would personally *love* it if distros started setting no_new_privs> for basically all processes.
>
> Maybe a pam module for that would be fine.
> But this should be configurable per-user, as so many things still rely
> on suid.
>
> Actually, I'd like to move all authentication / privilege switching
> to factotum (login(1), sshd, etc then also could run as unprivileged
> users).
>
> > And pidfd actually gets us part of the> way toward a straightforward way to make sudo and su still work in a>
> no_new_privs world: su could call into a daemon that would spawn the>
> privileged task, and su would get a (read-only!) pidfd back and then>
> wait for the fd and exit.
>
> How exactly would the pidfd improve this scenario ?
> IMHO, would just need to pass the inherited fd's to that daemon (eg.
> via unix socket) which then sets them up in the new child process.
>

It makes it easier to wait until the privileged program exits.
Without pidfd, you can't just wait(2) because the program that gets
spawned isn't a child.  With pidfd, the daemon can pass the pidfd
back.  Without pidfd, of course, you can wait by asking the daemon to
tell you when the program exits, but that's a uglier IMO.

> > I suppose that, done naively, this might> cause some odd effects with respect to tty handling, but I bet it's>
> solveable.
>
> Yes, signals and process groups would be a bit tricky. Some signals
> could be transmitted in a similar way as ssh does.
>
> But: how can we handle things like cgroups ?

Find a secure way to tell the daemon what cgroups to use?


--Andy