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

Mickaël Salaün
2020-11-21T10:11:51Z

On 21/11/2020 08:00, Jann Horn wrote:
> 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?
>

Good catch, a typo indeed.