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

[Kees Cook]

On Wed, Oct 3, 2018 at 10:56 PM, John Johansen
<john.johansen@xxxxxxxxxxxxx> wrote:
> On 10/03/2018 01:36 PM, Kees Cook wrote:
>> I still think we should have all built LSMs enabled by default, with
>> CONFIG_LSM_DISABLE available to turn stuff off. CONFIG_LSM_ORDER
>
> and this as a distro ubuntu does not want.
> Ubuntu wants to make yes available by building them in, but does NOT
> want all the LSM enabled by default, not even necessarily all minor LSMs.
>
> As a distro we want a supported set as default, and users can opt-in
> to new LSMs. If a new LSM comes along we don't want it enabled by
> default, which happens Using the lsm disable approach.

Okay, but order still matters. Where, in the order, should a disabled
LSM go? It seems like the friendliest approach for an end-user would
be to do something like

lsm=+landlock

and it all works correctly. That user doesn't need to know about
ordering or the distro default LSMs. They just want to _add_ landlock.
They want all the other LSMs to still be present, and they want the
distro to have chosen where landlock is in the ordering.

>> I should also note that I don't want to leave CONFIG_DEFAULT_SECURITY
>> in, since it's just a way to disable all the other majors. I don't
>> like this because it will force LSMs to be disabled that don't need to
>> be once blob-sharing lands. The whole point of this series is to get
>> us away from fixed ordering and thinking about "major" vs "minor" and
>> towards "exclusive" or not, where we can continue to slowly chip away
>> at exclusivity without breaking anything.
>>
> sure we definitely want to get away form "major" vs "minor" and in
> generally even exclusive, except where to LSMs just can't live
> with each other.
>
> But that doesn't mean dropping something like default security. The
> mistake with the current DEFAULT_SECURITY was that it only applied
> to major LSMs, not the minor ones.

Right, we need to expand it to include a full description of ordering
and enablement.

How about this:

CONFIG_LSM specifies order and enablement status. For example:

CONFIG_LSM=yama,loadpin,apparmor,!selinux

This means init order is yama, loadpin, apparmor, selinux, but selinux
is disabled. Anything not listed in CONFIG_LSM but built in will be
disabled and ordered in link-order. (i.e. an implicit trailing
"!smack,!tomoyo".)

Then we add "lsm=" which understands modifiers "-", and "+".
"lsm=-apparmor,+selinux" wouldn't change ordering, but would disable
apparmor and enable selinux. "lsm=smack,loadpin" would enable only
smack and loadpin, in that order and disable everything else.

I don't want to overload "security=", but we can if we want. It would
be as above, but a trailing comma would be needed to trigger the
"ordering" behavior. e.g. "security=selinux" would disable all other
majors (retaining the current behavior), but "security=selinux," would
disable all other LSMs.

-Kees

-- 
Kees Cook
Pixel Security