[PATCH 16/18] LSM: Allow arbitrary LSM ordering

[Casey Schaufler]

On 9/17/2018 4:28 PM, Kees Cook wrote:
> On Mon, Sep 17, 2018 at 4:26 PM, John Johansen
> <john.johansen at canonical.com> wrote:
>> On 09/17/2018 04:20 PM, Kees Cook wrote:
>>> On Mon, Sep 17, 2018 at 4:10 PM, Mickaël Salaün <mic at digikod.net> wrote:
>>>> Landlock, because it target unprivileged users, should only be called
>>>> after all other major (access-control) LSMs. The admin or distro must
>>>> not be able to change that order in any way. This constraint doesn't
>>>> apply to current LSMs, though.
>>> Good point! It will be easy to add LSM_ORDER_LAST, though, given the
>>> machinery introduced in this series.
>>>
>> And when we have two LSMs that want to use that?
> We'll cross that bridge when we come to it, but perhaps "last
> exclusive"? (lsm.enable/disable to choose)

If we define restrictions on use of LSM_ORDER_LAST like we have
for LSM_ORDER_FIRST (only for capabilities) before anyone starts
abusing it we may be OK. Since an LSM_ORDER_LAST has to know that
it can't count on getting called (a non-last module may return -EACCES)
I don't see any way that having multiple LSM_ORDER_LAST modules in
any given order would be a real problem. Of course, a module could be
doing state management that *really* requires it be last, but that
would be a badly designed module and someone sensible would NAK it.