[PATCH v3 1/4] security, lsm: Introduce security_create_user_ns()

[Christian Brauner]

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 at cloudflare.com>
> 
> ---

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