linux-kernel - Re: [PATCH v1] LSM: Enable multiple calls to security_add

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Message-ID: <aabebe53-f4e6-c91e-01f4-e027bec38cd9@schaufler-ca.com>
Date:   Mon, 8 May 2017 13:07:50 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Mickaël Salaün <mic@...ikod.net>,
        James Morris <jmorris@...ei.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     linux-kernel@...r.kernel.org,
        James Morris <james.l.morris@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v1] LSM: Enable multiple calls to security_add_hooks() for
 the same LSM

On 5/8/2017 12:24 PM, Mickaël Salaün wrote:
> On 01/05/2017 01:28, James Morris wrote:
>> On Sat, 29 Apr 2017, Mickaël Salaün wrote:
>>
>>> Check if the registering LSM already registered hooks just before. This
>>> enable to split hook declarations into multiple files without
>>> registering multiple time the same LSM name, starting from commit
>>> d69dece5f5b6 ("LSM: Add /sys/kernel/security/lsm").
>> Please include a detailed rationale for these patches.  The above tells us 
>> very little about why they are needed.
> Right, what do you think about that?
>
> The commit d69dece5f5b6 ("LSM: Add /sys/kernel/security/lsm") extend
> security_add_hooks() with a new parameter to register the LSM name,
> which may be useful to make the list of currently loaded LSM available
> to userspace. However, there is no clean way for an LSM no split its
> hook declarations into multiple files, which may reduce the mess with
> all the included files (needed for LSM hook argument types) and make the
> source code easier to review and maintain.
>
> This change allows an LSM to register multiple times its hook while
> keeping a consistent list of LSM names as described in
> Documentation/security/LSM.txt . The list reflects the order in which
> checks are made. This patch only check for the last registered LSM,
> which should be the only case. If an LSM register multiple times its
> hooks, interleaved with other LSM registrations, which should not
> happen, its name will still appear in the same order that the hooks are
> called, hence multiple times.
>
>
> Casey, Tetsuo, are you OK with this approach or do you want me to handle
> the case with interleaved hook registration, i.e. no duplicate name nor
> following the current Documentation/security/LSM.txt?
> What about the API with the NULL name (which is much simpler)?

Initially I thought that the module name should
never appear more than once, however I could see
a module that would bracket another or another set,
so

	capability,spiffy,selinux

might be different behaviorally than

	capability,spiffy,selinux,spiffy

and userspace might care. I still don't see any value
in

	capability,selinux,spiffy,spiffy

Passing a NULL name could lead to ambiguity if more
than one module did that, so I can't say I approve.


>
> Regards,
>  Mickaël
>