[PATCH v1] landlock: Explain how to support Landlock

[Paul Moore]

On Thu, May 12, 2022 at 8:08 AM Mickaël Salaün <mic at digikod.net> wrote:
>
> Let's help users by documenting how to enable and check for Landlock in
> the kernel and the running system.  The userspace-api section may not be
> the best place for this but it still makes sense to put all the user
> documentation at the same place.
>
> Cc: Paul Moore <paul at paul-moore.com>
> Signed-off-by: Mickaël Salaün <mic at digikod.net>
> Link: https://lore.kernel.org/r/20220512120847.124822-1-mic@digikod.net
> ---
>  Documentation/userspace-api/landlock.rst | 26 ++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> index 7b4fe6218132..e69dbddcc191 100644
> --- a/Documentation/userspace-api/landlock.rst
> +++ b/Documentation/userspace-api/landlock.rst
> @@ -388,6 +388,32 @@ Starting with the Landlock ABI version 2, it is now possible to securely
>  control renaming and linking thanks to the new `LANDLOCK_ACCESS_FS_REFER`
>  access right.
>
> +Landlock support
> +================
> +
> +Kernel configuration
> +--------------------

I would suggest changing the section name to "Kernel build
configuration" to make it clear the instructions below need to be done
at build time.

> +Landlock can be supported since Linux 5.13 but it must be configured in the

I would suggest rephrasing the first part of this sentence slightly:

"Landlock was first introduced in Linux v5.13 but it must be ..."

> +kernel with `CONFIG_SECURITY_LANDLOCK=y`, and it should be enabled at boot time

CONFIG_LSM is a Kconfig build time variable not a runtime command line
variable, yes?

> +with `CONFIG_LSM=landlock,[...]`.  The `[...]` must be replaced with the list
> +of LSMs that may be useful for the running system (see the `CONFIG_LSM` help).
> +
> +Running system configuration
> +----------------------------

This is nit-picky, but I would suggest a section name of "Kernel
command line configuration".

> +If the running kernel doesn't have `landlock` in `CONFIG_LSM`, then we can
> +still enable it by adding `lsm=landlock,[...]` to
> +Documentation/admin-guide/kernel-parameters.rst. The `[...]` must be replaced
> +with the list of LSMs that may be useful for the running system (see the
> +`CONFIG_LSM` help).
> +
> +Running system check
> +--------------------
> +
> +We can check that Landlock is enabled by looking for "landlock: Up and running"
> +in kernel logs: `dmesg | grep landlock`

I would suggest moving the "Running system check" section up closer to
the top of the documentation, perhaps right before the "Landlock
rules" section.  My thinking is that it is better to mention this as
soon as possible in the doc so that users can verify Landlock is
present and functioning before trying any of the code examples and
failing.  I think it is okay to leave the "how to enable Landlock"
sections at the bottom and simply refer to them in the "system check"
section.

-- 
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