linux-kernel - Re: [RFC] Allow user namespace inside chroot

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Message-ID: <CALCETrU-UWQve_cQm58sMENRW2K=z-Vf94KYoWfXCtz5gc_nFg@mail.gmail.com>
Date:   Mon, 15 Oct 2018 10:27:09 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Jann Horn <jannh@...gle.com>
Cc:     Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Serge Hallyn <serge.hallyn@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Subject: Re: [RFC] Allow user namespace inside chroot

On Mon, Oct 15, 2018 at 10:22 AM Jann Horn <jannh@...gle.com> wrote:
>
> On Mon, Oct 15, 2018 at 7:10 PM <nagarathnam.muthusamy@...cle.com> wrote:
> > Following commit disables the creation of user namespace inside
> > the chroot environment.
> >
> > userns: Don't allow creation if the user is chrooted
> >
> > commit 3151527ee007b73a0ebd296010f1c0454a919c7d
> >
> > Consider a system in which a non-root user creates a combination
> > of user, pid and mount namespaces and confines a process to it.
> > The system will have multiple levels of nested namespaces.
> > The root namespace in the system will have lots of directories
> > which should not be exposed to the child confined to the set of
> > namespaces.
> >
> > Without chroot, we will have to hide all unwanted directories
> > individually using bind mounts and mount namespace.
>
> IMO what you really should be doing is to create a tmpfs, bind-mount
> the directories you want into it, and then pivot_root() into that, not
> the other way around.

Indeed.  Or you can just recursive bind-mount the subtree you want and
then pivot_root() into it.

--Andy