out of tree lsm's

[Casey Schaufler]

On 3/21/2017 9:06 AM, Peter Moody wrote:
> On Tue, Mar 21, 2017 at 8:36 AM, Casey Schaufler <casey at schaufler-ca.com> wrote:
>> On 3/21/2017 3:41 AM, Tetsuo Handa wrote:
>>> Tetsuo Handa wrote:
>>>> Casey Schaufler wrote:
>>>>>> right. sorry for the imprecise language; by site-specific I meant a "small" lsm.
>>>>>>
>>>>>> I would love to have the ability write a small lsm that I can build as
>>>>>> a module and load at boot eg. via initrd.
>>>>>>
>>>>>> AIUI, adding even a new "small" lsm requires kconfig patches, building
>>>>>> a new kernel, etc. I know there are objections to dynamically loadable
>>>>>> lsms and I was trying to find a compromise that made them easier to
>>>>>> work with.
>>>>> The stacking design criteria I'm working with
>>>>> include not doing anything that would prevent
>>>>> dynamic module loading. I do not plan to implement
>>>>> dynamic loading. Tetsuo has been a strong
>>>>> advocate of loadable modules. I would expect to
>>>>> see a proposal from him shortly after the
>>>>> general stacking lands, assuming it does.
>>>> But currently __lsm_ro_after_init which is planned to go to 4.12 is preventing
>>>> dynamic modules from loading. We need a legitimate interface for loadable modules like
>>>> http://lkml.kernel.org/r/201702152342.GBH04183.FOFJFHQOLMOtVS@I-love.SAKURA.ne.jp .
>>>> Requiring rodata=0 kernel command line option to allow dynamic modules is silly.
>>>>
>>> I think we need something like below change when allowing loadable modules.
>> I believe that a simpler approach would be to
>> add a separate list of dynamic hooks to supliment
>> the list of static hooks. If SELinux unloading is
>> desired the SELinux hooks would be put on the
>> dynamic list which would not be "hardened" with
>> _ro_after_init, where the rest of the static modules
>> would be.
> FWIW, I don't know if that would solve the case I was initially asking
> about since the out-of-tree lsm I was hoping to be able to access all
> of the standard security hooks with an out-of-tree module.

It would work fine. All I'm suggesting is that in addition
to security_hook_heads there would be a
security_hooks_heads_dynamic. The code in security.c would
be stretched to loop through both lists. Any locking or
other complexity associated with being dynamic would be
limited to the dynamic list.

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