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Subject: Re: [PATCH v24 01/12] landlock: Add object management
To:     Jann Horn <jannh@google.com>
Cc:     James Morris <jmorris@namei.org>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@amacapital.net>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Jeff Dike <jdike@addtoit.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Richard Weinberger <richard@nod.at>,
        Shuah Khan <shuah@kernel.org>,
        Vincent Dagonneau <vincent.dagonneau@ssi.gouv.fr>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
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        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@linux.microsoft.com>
References: <20201112205141.775752-1-mic@digikod.net>
 <20201112205141.775752-2-mic@digikod.net>
 <CAG48ez0GryN4i0xCP22utLTqF5_o5J3nMBs+VC0DpQ+s09Bx6g@mail.gmail.com>
From:   =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@digikod.net>
Message-ID: <b684a6b7-93d5-3040-32ca-caf4d244cd6f@digikod.net>
Date:   Sat, 21 Nov 2020 11:11:27 +0100
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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.