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

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Message-ID: <afa8e978-d22c-f06a-d57b-e0d1a9918062@digikod.net>
Date:   Thu, 29 Oct 2020 10:30:04 +0100
From:   Mickaël Salaün <mic@...ikod.net>
To:     Jann Horn <jannh@...gle.com>
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 29/10/2020 02:05, Jann Horn wrote:
> 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>

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!