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

Mickaël Salaün
2020-10-29T09:30:18Z

On 29/10/2020 02:05, Jann Horn wrote:
> 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>

Thanks for the review.

>
> 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./

I indeed prefer the comma preceding the "e.g.", but it seems that there
is a difference between UK english and US english:
https://english.stackexchange.com/questions/16172/should-i-always-use-a-comma-after-e-g-or-i-e
Looking at the kernel documentation makes it clear:
$ git grep -F 'e.g. ' | wc -l
1179
$ git grep -F 'e.g., ' | wc -l
160

I'll apply your fix in the whole patch series.

>
>> + * 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/ ?

Right.

>
>> + * 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"
>

Good catch!