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Date:   Fri, 4 Feb 2022 10:45:15 +0100
From:   Christian Brauner <brauner@kernel.org>
To:     "Anton V. Boyarshinov" <boyarsh@altlinux.org>
Cc:     viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
        ebiederm@xmission.com, legion@kernel.org, ldv@altlinux.org,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com,
        Christoph Hellwig <hch@lst.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] Add ability to disallow idmapped mounts
Message-ID: <20220204094515.6vvxhzcyemvrb2yy@wittgenstein>
References: <20220204065338.251469-1-boyarsh@altlinux.org>
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On Fri, Feb 04, 2022 at 09:53:38AM +0300, Anton V. Boyarshinov wrote:
> Idmapped mounts may have security implications [1] and have
> no knobs to be disallowed at runtime or compile time.
> 
> This patch adds a sysctl and a config option to set its default value.
> 
> [1] https://lore.kernel.org/all/m18s7481xc.fsf@fess.ebiederm.org/
> 
> Based on work from Alexey Gladkov <legion@kernel.org>.
> 
> Signed-off-by: Anton V. Boyarshinov <boyarsh@altlinux.org>
> ---

Thank your for the general idea, Anton.

If you want to turn off idmapped mounts you can already do so today via:
echo 0 > /proc/sys/user/max_user_namespaces

Aside from that, idmapped mounts can only be created by fully privileged
users on the host for a selected number of filesystems. They can neither
be created as an unprivileged user nor can they be created inside user
namespaces.

I appreciate the worry. Any new feature may have security implications
and bugs. In addition, we did address these allegations multiple times
already (see [1], [2], [3], [4], [5]).

As the author/maintainer of this feature,
Nacked-by: Christian Brauner <brauner@kernel.org>

[1]: https://lore.kernel.org/lkml/20210213130042.828076-1-christian.brauner@ubuntu.com/T/#m3a9df31aa183e8797c70bc193040adfd601399ad
[2]: https://lore.kernel.org/lkml/m1r1ifzf8x.fsf@fess.ebiederm.org
[3]: https://lore.kernel.org/lkml/20210213130042.828076-1-christian.brauner@ubuntu.com/T/#m59cdad9630d5a279aeecd0c1f117115144bc15eb
[4]: https://lore.kernel.org/lkml/20210510125147.tkgeurcindldiwxg@wittgenstein
[5]: https://lore.kernel.org/linux-fsdevel/CAHrFyr4AYi_gad7LQ-cJ9Peg=Gt73Sded8k_ZHeRZz=faGzpQA@mail.gmail.com