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

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Date:   Thu, 29 Oct 2020 02:05:47 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...capital.net>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Arnd Bergmann <arnd@...db.de>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Jeff Dike <jdike@...toit.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Richard Weinberger <richard@....at>,
        Shuah Khan <shuah@...nel.org>,
        Vincent Dagonneau <vincent.dagonneau@....gouv.fr>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v22 01/12] landlock: Add object management

On Tue, Oct 27, 2020 at 9:04 PM Mickaël Salaün <mic@...ikod.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@...ei.org>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Serge E. Hallyn <serge@...lyn.com>
> Signed-off-by: Mickaël Salaün <mic@...ux.microsoft.com>

Reviewed-by: Jann Horn <jannh@...gle.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"