linux-kernel - Re: [PATCH linux 0/3] [PATCH] userns: add sysctl "kernel.userns_group

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Message-ID: <CAHC9VhRk3WhXh-GTDSKFW3PujXiQCDy3xk4Xb9_Lo4szgQ5G6Q@mail.gmail.com>
Date:   Thu, 1 Jun 2023 21:01:55 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     "~akihirosuda" <suda.kyoto@...il.com>,
        linux-kernel@...r.kernel.org, containers@...ts.linux.dev,
        serge@...lyn.com, brauner@...nel.org, akihiro.suda.cz@....ntt.co.jp
Subject: Re: [PATCH linux 0/3] [PATCH] userns: add sysctl "kernel.userns_group_range"

On Thu, Jun 1, 2023 at 8:14 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Paul Moore <paul@...l-moore.com> writes:
> > On Tue, May 30, 2023 at 2:50 PM ~akihirosuda <akihirosuda@....sr.ht> wrote:
> >>
> >> This sysctl limits groups who can create a new userns without
> >> CAP_SYS_ADMIN in the current userns, so as to mitigate potential kernel
> >> vulnerabilities around userns.
> >>
> >> The sysctl value format is same as "net.ipv4.ping_group_range".
> >>
> >> To disable creating new unprivileged userns, set the sysctl value to "1
> >> 0" in the initial userns.
> >>
> >> To allow everyone to create new userns, set the sysctl value to "0
> >> 4294967294". This is the default value.
> >>
> >> This sysctl replaces "kernel.unprivileged_userns_clone" that is found in
> >> Ubuntu [1] and Debian GNU/Linux.
> >>
> >> Link: https://git.launchpad.net/~ubuntu-
> >> kernel/ubuntu/+source/linux/+git/jammy/commit?id=3422764 [1]
> >
> > Given the challenges around adding access controls to userns
> > operations, have you considered using the LSM support that was added
> > upstream last year?  The relevant LSM hook can be found in commit
> > 7cd4c5c2101c ("security, lsm: Introduce security_create_user_ns()"),
>
> Paul how have you handled the real world regression I reported against
> chromium?

I don't track chromium development.

> Paul are you aware that the LSM hook can not be used to achieve the
> objective of this patchset?

/me shrugs

I thought one could look into a cred struct using a BPF LSM, which
would allow one to make access control decisions based on group ID,
but I will be the first to admit I'm not a BPF LSM expert.
Regardless, one could introduce a group ID check into a LSM if they
were so inclined.

I also find it slightly amusing that you are arguing against my reply
that was discussing *not* adding another userns control point; of all
people I thought you would be supportive of this ... /me shrugs again.

> > and although only SELinux currently provides an access control
> > implementation, there is no reason you couldn't add support for your
> > favorite LSM, or even just a simple BPF LSM to enforce the group
> > controls as you've described them here.
>
> Is there a publicly available SELinux policy that uses that LSM hook?

I have no idea, I don't track all of the publicly available SELinux
policies because frankly it doesn't matter; the SELinux feature
exists, and it is my role to support and maintain it.  There are
LSM/SELinux features which are not widely exercised in general purpose
SELinux policies for various reasons, but *are* used in specialized
environments that are not often discussed on public mailing lists.

-- 
paul-moore.com