[PATCH v1 09/11] landlock: Document LANDLOCK_ACCESS_FS_REFER and ABI versioning

[Paul Moore]

On Mon, Feb 21, 2022 at 4:15 PM Mickaël Salaün <mic at digikod.net> wrote:
>
> From: Mickaël Salaün <mic at linux.microsoft.com>
>
> Add LANDLOCK_ACCESS_FS_REFER in the example and properly check to only
> use it if the current kernel support it thanks to the Landlock ABI
> version.
>
> Move the file renaming and linking limitation to a new "Previous
> limitations" section.
>
> Improve documentation about the backward and forward compatibility,
> including the rational for ruleset's handled_access_fs.
>
> Signed-off-by: Mickaël Salaün <mic at linux.microsoft.com>
> Link: https://lore.kernel.org/r/20220221212522.320243-10-mic@digikod.net
> ---
>  Documentation/userspace-api/landlock.rst | 124 +++++++++++++++++++----
>  1 file changed, 104 insertions(+), 20 deletions(-)

Thanks for remembering to update the docs :)  I made a few phrasing
suggestions below, but otherwise it looks good to me.

Reviewed-by: Paul Moore <paul at paul-moore.com>

> diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> index f35552ff19ba..97db09d36a5c 100644
> --- a/Documentation/userspace-api/landlock.rst
> +++ b/Documentation/userspace-api/landlock.rst
> @@ -281,6 +347,24 @@ Memory usage
>  Kernel memory allocated to create rulesets is accounted and can be restricted
>  by the Documentation/admin-guide/cgroup-v1/memory.rst.
>
> +Previous limitations
> +====================
> +
> +File renaming and linking (ABI 1)
> +---------------------------------
> +
> +Because Landlock targets unprivileged access controls, it is needed to properly
                                                          ^^^^^
                                           "... controls, it needs to ..."

> +handle composition of rules.  Such property also implies rules nesting.
> +Properly handling multiple layers of ruleset, each one of them able to restrict
                                        ^^^^^^^
                                      "rulesets,"

> +access to files, also implies to inherit the ruleset restrictions from a parent
                                 ^^^^^^^^^^
                    "... implies inheritance of the ..."

> +to its hierarchy.  Because files are identified and restricted by their
> +hierarchy, moving or linking a file from one directory to another implies to
> +propagate the hierarchy constraints.

"... one directory to another implies propagation of the hierarchy constraints."

> +                                     To protect against privilege escalations

> +through renaming or linking, and for the sake of simplicity, Landlock previously
> +limited linking and renaming to the same directory.  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.

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