[PATCH security-next v4 23/32] selinux: Remove boot parameter

[Randy Dunlap]

On 10/3/18 4:55 PM, Kees Cook wrote:
> On Wed, Oct 3, 2018 at 2:34 PM, James Morris <jmorris at namei.org> wrote:
>> On Wed, 3 Oct 2018, Kees Cook wrote:
>>
>>> On Wed, Oct 3, 2018 at 11:28 AM, James Morris <jmorris at namei.org> wrote:
>>>> On Wed, 3 Oct 2018, Kees Cook wrote:
>>>>
>>>>> On Wed, Oct 3, 2018 at 11:17 AM, James Morris <jmorris at namei.org> wrote:
>>>>>> On Tue, 2 Oct 2018, John Johansen wrote:
>>>>>>> To me a list like
>>>>>>>   lsm.enable=X,Y,Z
>>>>>>
>>>>>> What about even simpler:
>>>>>>
>>>>>> lsm=selinux,!apparmor,yama
>>>>>
>>>>> We're going to have lsm.order=, so I'd like to keep it with a dot
>>>>> separator (this makes it more like module parameters, too). You want
>>>>> to mix enable/disable in the same string? That implies you'd want
>>>>> implicit enabling (i.e. it complements the builtin enabling), which is
>>>>> opposite from what John wanted.
>>>>>
>>>>
>>>> Why can't this be the order as well?
>>>
>>> That was covered extensively in the earlier threads. It boils down to
>>> making sure we do not create a pattern of leaving LSMs disabled by
>>> default when they are added to the kernel. The v1 series used
>>> security= like this:
>>>
>>> +       security=       [SECURITY] An ordered comma-separated list of
>>> +                       security modules to attempt to enable at boot. If
>>> +                       this boot parameter is not specified, only the
>>> +                       security modules asking for initialization will be
>>> +                       enabled (see CONFIG_DEFAULT_SECURITY). Duplicate
>>> +                       or invalid security modules will be ignored. The
>>> +                       capability module is always loaded first, without
>>> +                       regard to this parameter.
>>>
>>> This meant booting "security=apparmor" would disable all the other
>>> LSMs, which wasn't friendly at all. So "security=" was left alone (to
>>> leave it to only select the "major" LSM: all major LSMs not matching
>>> "security=" would be disabled). So I proposed "lsm.order=" to specify
>>> the order things were going to be initialized in, but to avoid kernels
>>> booting with newly added LSMs forced-off due to not being listed in
>>> "lsm.order=", it had to have implicit fall-back for unlisted LSMs.
>>> (i.e. anything missing from lsm.order would then follow their order in
>>> CONFIG_LSM_ORDER, and anything missing there would fall back to
>>> link-time ordering.) However, then the objection was raised that this
>>> didn't provide a way to explicitly disable an LSM. So I proposed
>>> lsm.enable/disable, and John argued for CONFIG_LSM_ENABLE over
>>> CONFIG_LSM_DISABLE.
>>
>> Ok, but it may end up being clearer, simpler, and thus more secure to just
>> have a single way to configure LSM.
>>
>> For example:
>>
>>   - All LSMs which are built are NOT enabled by default
>>
>>   - You specify enablement and order via a Kconfig:
>>
>>         CONFIG_LSM="selinux,yama"
>>
>>   - This can be entirely overridden by a boot param:
>>
>>         lsm="apparmor,landlock"
> 
> This doesn't work with how SELinux and AppArmor do their bootparams,
> unfortunately. (And Paul and Stephen have expressed that the
> documented selinux on/off must continue to work.) For example, let's
> say you've built an Ubuntu kernel with:
> 
> CONFIG_SELINUX=y
> ...
> CONFIG_LSM="yama,apparmor"
> 
> (i.e. you want SELinux available, but not enabled, so it's left out of
> CONFIG_LSM)
> 
> Then someone boots the system with:
> 
> selinux=1 security=selinux
> 
> In what order does selinux get initialized relative to yama?
> (apparmor, flagged as a "legacy major", would have been disabled by
> the "security=" not matching it.)
> 

To me, "security=selinux" means SELinux and nothing else, so I think that
all of these params are inviting a lot of confusion.

Sorry, I don't have a good answer for this.

> 
> The LSM order needs to be defined externally to enablement because
> something may become enabled when not listed in the order.
> 
> Now, maybe I misunderstood your earlier suggestion, and what you meant
> was to do something like:
> 
> CONFIG_LSM="yama,apparmor,!selinux"
> 
> to mean "put selinux here in the order, but don't enable it". Then the
> problem becomes what happens to an LSM that has been built in but not
> listed in CONFIG_LSM?
> 
> Related to that, this means that when new LSMs are added, they will
> need to be added to any custom CONFIG_LSM= or lsm= parameters. If
> that's really how we have to go, I'll accept it, but I think it's a
> bit unfriendly. :P
> 
> Another reason I don't like it is because it requires users to know
> about all the LSMs to make changes. One LSM can't be added/removed
> without specifying ALL of the LSMs. (i.e. there is no trivial way to
> enable/disable a single LSM without it growing its own enable/disable
> code as in SELinux/AppArmor. I'd hoped to make that easier for both
> users and developers.) Again, I can live with it, but I think it's
> unfriendly.
> 
> I just want to have a direct I can go that meets all the requirements.
> :) I'm fine to ignore my sense of aesthetics if everyone can agree on
> the code.


-- 
~Randy