Re: [PATCH v24 01/12] landlock: Add object management

[Jann Horn <jannh@google.com>]

On Thu, Nov 12, 2020 at 9:51 PM Mickaël Salaün <mic@digikod.net> wrote:
> A Landlock object enables to identify a kernel object (e.g. an inode).
> A Landlock rule is a set of access rights allowed on an object.  Rules
> are grouped in rulesets that may be tied to a set of processes (i.e.
> subjects) to enforce a scoped access-control (i.e. a domain).
>
> Because Landlock's goal is to empower any process (especially
> unprivileged ones) to sandbox themselves, we cannot rely on a
> system-wide object identification such as file extended attributes.
> Indeed, we need innocuous, composable and modular access-controls.
>
> The main challenge with these constraints is to identify kernel objects
> while this identification is useful (i.e. when a security policy makes
> use of this object).  But this identification data should be freed once
> no policy is using it.  This ephemeral tagging should not and may not be
> written in the filesystem.  We then need to manage the lifetime of a
> rule according to the lifetime of its objects.  To avoid a global lock,
> this implementation make use of RCU and counters to safely reference
> objects.
>
> A following commit uses this generic object management for inodes.
>
> Cc: James Morris <jmorris@namei.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Serge E. Hallyn <serge@hallyn.com>
> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
> Reviewed-by: Jann Horn <jannh@google.com>

Still looks good, except for one comment:

[...]
> +       /**
> +        * @lock: Guards against concurrent modifications.  This lock might be
> +        * held from the time @usage drops to zero until any weak references
> +        * from @underobj to this object have been cleaned up.
> +        *
> +        * Lock ordering: inode->i_lock nests inside this.
> +        */
> +       spinlock_t lock;

Why did you change this to "might be held" (v22 had "must")? Is the
"might" a typo?