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From:   Kees Cook <keescook@chromium.org>
Date:   Mon, 1 Oct 2018 15:27:40 -0700
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Subject: Re: [PATCH security-next v3 18/29] LSM: Introduce lsm.enable= and lsm.disable=
To:     John Johansen <john.johansen@canonical.com>,
        Paul Moore <paul@paul-moore.com>
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        Casey Schaufler <casey@schaufler-ca.com>,
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        Stephen Smalley <sds@tycho.nsa.gov>,
        "Schaufler, Casey" <casey.schaufler@intel.com>,
        LSM <linux-security-module@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
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On Mon, Oct 1, 2018 at 2:46 PM, John Johansen
<john.johansen@canonical.com> wrote:
> On 09/24/2018 05:18 PM, Kees Cook wrote:
>> This introduces the "lsm.enable=..." and "lsm.disable=..." boot parameters
>> which each can contain a comma-separated list of LSMs to enable or
>> disable, respectively. The string "all" matches all LSMs.
>>
>> This has very similar functionality to the existing per-LSM enable
>> handling ("apparmor.enabled=...", etc), but provides a centralized
>> place to perform the changes. These parameters take precedent over any
>> LSM-specific boot parameters.
>>
>> Disabling an LSM means it will not be considered when performing
>> initializations. Enabling an LSM means either undoing a previous
>> LSM-specific boot parameter disabling or a undoing a default-disabled
>> CONFIG setting.
>>
>> For example: "lsm.disable=apparmor apparmor.enabled=1" will result in
>> AppArmor being disabled. "selinux.enabled=0 lsm.enable=selinux" will
>> result in SELinux being enabled.
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>
> I don't like this. It brings about conflicting kernel params that are
> bound to confuse users. Its pretty easy for a user to understand that
> when they specify a parameter manually at boot, that  it overrides the
> build time default. But conflicting kernel parameters are a lot harder
> to deal with.
>
> I prefer a plain enabled= list being an override of the default build
> time value. Where conflicts with LSM-specific configs always result in
> the LSM being disabled with a complaint about the conflict.
>
> Though I have yet to be convinced its worth the cost, I do recognize
> it is sometimes convenient to disable a single LSM, instead of typing
> in a whole list of what to enable. If we have to have conflicting
> kernel parameters I would prefer that the conflict throw up a warning
> and leaving the LSM with the conflicting config disabled.

Alright, let's drill down a bit more. I thought I had all the
requirements sorted out here. :)

AppArmor and SELinux are "special" here in that they have both:

- CONFIG for enable-ness
- boot param for enable-ness

Now, the way this worked in the past was that combined with
CONFIG_DEFAULT_SECURITY and the link-time ordering, this resulted in a
way to get the LSM enabled, skipped, etc. But it was highly CONFIG
dependent.

SELinux does:

#ifdef CONFIG_SECURITY_SELINUX_BOOTPARAM
int selinux_enabled = CONFIG_SECURITY_SELINUX_BOOTPARAM_VALUE;

static int __init selinux_enabled_setup(char *str)
{
        unsigned long enabled;
        if (!kstrtoul(str, 0, &enabled))
                selinux_enabled = enabled ? 1 : 0;
        return 1;
}
__setup("selinux=", selinux_enabled_setup);
#else
int selinux_enabled = 1;
#endif
...
        if (!security_module_enable("selinux")) {
                selinux_enabled = 0;
                return 0;
        }

        if (!selinux_enabled) {
                pr_info("SELinux:  Disabled at boot.\n");
                return 0;
        }


AppArmor does:

/* Boot time disable flag */
static bool apparmor_enabled = CONFIG_SECURITY_APPARMOR_BOOTPARAM_VALUE;
module_param_named(enabled, apparmor_enabled, bool, S_IRUGO);

static int __init apparmor_enabled_setup(char *str)
{
        unsigned long enabled;
        int error = kstrtoul(str, 0, &enabled);
        if (!error)
                apparmor_enabled = enabled ? 1 : 0;
        return 1;
}

__setup("apparmor=", apparmor_enabled_setup);
...
        if (!apparmor_enabled || !security_module_enable("apparmor")) {
                aa_info_message("AppArmor disabled by boot time parameter");
                apparmor_enabled = false;
                return 0;
        }


Smack and TOMOYO each do:

        if (!security_module_enable("smack"))
                return 0;

        if (!security_module_enable("tomoyo"))
                return 0;


Capability, Integrity, Yama, and LoadPin always run init. (This series
fixes LoadPin to separate enable vs enforce, so we can ignore its
"enable" setting, which isn't an "am I active?" boolean -- its init
was always run.) With the enable logic is lifted out of the LSMs, we
want to have "implicit enable" for 6 of 8 of the LSMs. (Which is why I
had originally suggested CONFIG_LSM_DISABLE, since the normal state is
enabled.) But given your feedback, I made this "implicit disable" and
added CONFIG_LSM_ENABLE instead. (For which "CONFIG_LSM_ENABLE=all"
gets the same results.)


I think, then, the first question (mainly for you and Paul) is:

Should we remove CONFIG_SECURITY_SELINUX_BOOTPARAM_VALUE and
CONFIG_SECURITY_APPARMOR_BOOTPARAM_VALUE in favor of only
CONFIG_LSM_ENABLE?

The answer will affect the next question: what should be done with the
boot parameters? AppArmor has two ways to change enablement:
apparmor=0/1 and apparmor.enabled=0/1. SELinux just has selinux=0/1.
Should those be removed in favor of "lsm.enable=..."? (And if they're
not removed, how do people imagine they should interact?)

Thanks!

-Kees

-- 
Kees Cook
Pixel Security