Re: [PATCH security-next v3 18/29] LSM: Introduce lsm.enable= and lsm.disable=

[John Johansen <john.johansen@canonical.com>]

On 10/01/2018 04:30 PM, Kees Cook wrote:
> On Mon, Oct 1, 2018 at 3:48 PM, John Johansen
> <john.johansen@canonical.com> wrote:
>> On 10/01/2018 03:27 PM, Kees Cook wrote:
>>> 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?
>>>
>>
>> We can remove the Kconfig for the apparmor bootparam value. In fact I
>> will attach that patch below. I can't get rid of the parameter as it
>> is part of the userspace api. There are tools and applications
>> checking /sys/module/apparmor/parameters/enabled
>>
>> but we can certainly default it to enabled and make it work only as a
>> runtime kernel parameter to disable apparmor which is how it has been
>> traditionally been used.
>>
>>> 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?)
>>
>> I am not against removing the apparmor one, it does mean retraining
>> users but it is seldmon used so it may be worth dropping. If we keep
>> it, it should be a disable only flag that where the use of apparmor=0
>> or apparmor.enable=0 (same thing) means apparmor is disabled.
> 
> If we keep it, "apparmor=0 lsm_enable=apparmor" would mean it's
> enabled. Is that okay?
> 
ugh I would rather get rid of apparmor=0 or to emit a warning with apparmor
disabled, but if we have to live with it then yes I can live with last
option wins