linux-kernel - Re: [PATCH v3 1/4] security, lsm: Introduce security_create_user

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Message-ID: <20220722082159.jgvw7jgds3qwfyqk@wittgenstein>
Date:   Fri, 22 Jul 2022 10:21:59 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Frederick Lawler <fred@...udflare.com>
Cc:     kpsingh@...nel.org, revest@...omium.org, jackmanb@...omium.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, jmorris@...ei.org, serge@...lyn.com,
        paul@...l-moore.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org, shuah@...nel.org, casey@...aufler-ca.com,
        ebiederm@...ssion.com, bpf@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, kernel-team@...udflare.com,
        cgzones@...glemail.com, karl@...badwolfsecurity.com
Subject: Re: [PATCH v3 1/4] security, lsm: Introduce security_create_user_ns()

On Thu, Jul 21, 2022 at 12:28:05PM -0500, Frederick Lawler wrote:
> Preventing user namespace (privileged or otherwise) creation comes in a
> few of forms in order of granularity:
> 
>         1. /proc/sys/user/max_user_namespaces sysctl
>         2. OS specific patch(es)
>         3. CONFIG_USER_NS
> 
> To block a task based on its attributes, the LSM hook cred_prepare is a
> good candidate for use because it provides more granular control, and
> it is called before create_user_ns():
> 
>         cred = prepare_creds()
>                 security_prepare_creds()
>                         call_int_hook(cred_prepare, ...
>         if (cred)
>                 create_user_ns(cred)
> 
> Since security_prepare_creds() is meant for LSMs to copy and prepare
> credentials, access control is an unintended use of the hook. Therefore
> introduce a new function security_create_user_ns() with an accompanying
> userns_create LSM hook.
> 
> This hook takes the prepared creds for LSM authors to write policy
> against. On success, the new namespace is applied to credentials,
> otherwise an error is returned.
> 
> Signed-off-by: Frederick Lawler <fred@...udflare.com>
> 
> ---

Nice and straightforward,
Reviewed-by: Christian Brauner (Microsoft) <brauner@...nel.org>