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

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Message-ID: <b684a6b7-93d5-3040-32ca-caf4d244cd6f@digikod.net>
Date:   Sat, 21 Nov 2020 11:11:27 +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 v24 01/12] landlock: Add object management


On 21/11/2020 08:00, Jann Horn wrote:
> On Thu, Nov 12, 2020 at 9:51 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: 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>
> 
> 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.