linux-kernel - Re: [PATCH net-next v6 04/11] landlock: Add LSM hooks related to filesystem

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Message-ID: <CAGXu5jJCvJ6-uZ=Kfhh3xD7UvaY+G99e9NXFMzvi=9OQzA6Ecg@mail.gmail.com>
Date:   Wed, 19 Apr 2017 18:48:47 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Drysdale <drysdale@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        James Morris <james.l.morris@...cle.com>,
        Jann Horn <jann@...jh.net>, Jonathan Corbet <corbet@....net>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Paul Moore <paul@...l-moore.com>,
        Sargun Dhillon <sargun@...gun.me>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Shuah Khan <shuah@...nel.org>, Tejun Heo <tj@...nel.org>,
        Thomas Graf <tgraf@...g.ch>, Will Drewry <wad@...omium.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v6 04/11] landlock: Add LSM hooks related to filesystem

On Wed, Apr 19, 2017 at 3:03 PM, Mickaël Salaün <mic@...ikod.net> wrote:
>
> On 19/04/2017 01:40, Kees Cook wrote:
>> On Tue, Apr 18, 2017 at 4:16 PM, Casey Schaufler <casey@...aufler-ca.com> wrote:
>>> On 4/18/2017 3:44 PM, Mickaël Salaün wrote:
>>>> On 19/04/2017 00:17, Kees Cook wrote:
>>>>> On Tue, Mar 28, 2017 at 4:46 PM, Mickaël Salaün <mic@...ikod.net> wrote:
>>>>>> +void __init landlock_add_hooks(void)
>>>>>> +{
>>>>>> +       pr_info("landlock: Version %u", LANDLOCK_VERSION);
>>>>>> +       landlock_add_hooks_fs();
>>>>>> +       security_add_hooks(NULL, 0, "landlock");
>>>>>> +       bpf_register_prog_type(&bpf_landlock_type);
>>>>> I'm confused by the separation of hook registration here. The call to
>>>>> security_add_hooks is with count=0 is especially weird. Why isn't this
>>>>> just a single call with security_add_hooks(landlock_hooks,
>>>>> ARRAY_SIZE(landlock_hooks), "landlock")?
>>>> Yes, this is ugly with the new security_add_hooks() with three arguments
>>>> but I wanted to split the hooks definition in multiple files.
>>>
>>> Why? I'll buy a good argument, but there are dangers in
>>> allowing multiple calls to security_add_hooks().
>
> I prefer to have one file per hook "family" (e.g. filesystem, network,
> ptrace…). This reduce the mess with all the included files (needed for
> LSM hook argument types) and make the files easier to read, understand
> and maintain.
>
>>>
>>>>
>>>> The current security_add_hooks() use lsm_append(lsm, &lsm_names) which
>>>> is not exported. Unfortunately, calling multiple security_add_hooks()
>>>> with the same LSM name would register multiple names for the same LSM…
>>>> Is it OK if I modify this function to not add duplicated entries?
>>>
>>> It may seem absurd, but it's conceivable that a module might
>>> have two hooks it wants called. My example is a module that
>>> counts the number of times SELinux denies a process access to
>>> things (which needs to be called before and after SELinux in
>>> order to detect denials) and takes "appropriate action" if
>>> too many denials occur. It would be weird, wonky and hackish,
>>> but that never stopped anybody before.
>
> Right, but now, with the new lsm_append(), module names are concatenated
> ("%s,%s") in the lsm_names variable. It would be nice to not pollute
> this string with multiple time the same module name.

Perhaps security_add_hooks could be modified to accept a NULL lsm to
skip the lsm_append() call, so it could do:

security_add_hooks(hooks1, count1, NULL);
security_add_hooks(hooks2, count2, NULL);
security_add_hooks(NULL, 0, "landlock");

Or, as Casey suggests, disregard adding the name when it already exists:

security_add_hooks(hooks1, count1, "landlock");
security_add_hooks(hooks2, count2, "landlock");

Yeah, I think I prefer this...

-Kees

>
>>
>> If ends up being sane and clear, I'm fine with allowing multiple calls.
>>
>> -Kees
>>
>



-- 
Kees Cook
Pixel Security