linux-kernel - Re: plan9 semantics on Linux

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Message-ID: <20180214203933.55yraxue7hpup65x@gordon>
Date:   Thu, 15 Feb 2018 07:39:33 +1100
From:   Aleksa Sarai <asarai@...e.de>
To:     Enrico Weigelt <lkml@...ux.net>
Cc:     Richard Weinberger <richard@...ma-star.at>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: plan9 semantics on Linux - mount namespaces

On 2018-02-14, Enrico Weigelt <lkml@...ux.net> wrote:
> But still I wonder whether user_ns really solves my problem, as I don't
> want to create sandboxed users, but only private namespaces just like
> on Plan9.

On Linux you need to have CAP_SYS_ADMIN (in the user_ns that owns your
current mnt_ns) in order to mount anything, and to create any namespaces
(in your current user_ns). So, in order to use the functionality of
mnt_ns (the ability to create mounts only a subset of processes can
see) as an unprivileged user, you need to use user_ns.

(Note there is an additional restriction, namely that a mnt_ns that was
set up in the non-root user_ns cannot mount any filesystems that do not
have the FS_USERNS_MOUNT option set. This is also for security, as
exposing the kernel filesystem parser to arbitrary data by unprivileged
users wasn't deemed to be a safe thing to do. The unprivileged FUSE work
that Richard linked to will likely be useful for pushing FS_USERNS_MOUNT
into more filesystems -- like 9p.)

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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