Re: [PATCH v22 01/12] landlock: Add object management

[Jann Horn <jannh@google.com>]

On Tue, Oct 27, 2020 at 9:04 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: Jann Horn <jannh@google.com>
> 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>

except for some minor nits:

[...]
> diff --git a/security/landlock/object.c b/security/landlock/object.c
[...]
> +void landlock_put_object(struct landlock_object *const object)
> +{
> +       /*
> +        * The call to @object->underops->release(object) might sleep e.g.,

s/ e.g.,/, e.g./

> +        * because of iput().
> +        */
> +       might_sleep();
> +       if (!object)
> +               return;
[...]
> +}
> diff --git a/security/landlock/object.h b/security/landlock/object.h
[...]
> +struct landlock_object {
> +       /**
> +        * @usage: This counter is used to tie an object to the rules matching
> +        * it or to keep it alive while adding a new rule.  If this counter
> +        * reaches zero, this struct must not be modified, but this counter can
> +        * still be read from within an RCU read-side critical section.  When
> +        * adding a new rule to an object with a usage counter of zero, we must
> +        * wait until the pointer to this object is set to NULL (or recycled).
> +        */
> +       refcount_t usage;
> +       /**
> +        * @lock: Guards against concurrent modifications.  This lock must be

s/must be/must 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;
[...]
> +};
> +
> +struct landlock_object *landlock_create_object(
> +               const struct landlock_object_underops *const underops,
> +               void *const underojb);

nit: "underobj"