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

[Casey Schaufler]

On 9/17/2018 3:36 PM, John Johansen wrote:
> On 09/17/2018 02:57 PM, Casey Schaufler wrote:
>> On 9/17/2018 12:55 PM, John Johansen wrote:
>>> On 09/17/2018 12:23 PM, Casey Schaufler wrote:
>>>> On 9/17/2018 11:14 AM, Kees Cook wrote:
>>>>>> Keep security=$lsm with the existing exclusive behavior.
>>>>>> Add lsm=$lsm1,...,$lsmN which requires a full list of modules
>>>>>>
>>>>>> If you want to be fancy (I don't!) you could add
>>>>>>
>>>>>> lsm.add=$lsm1,...,$lsmN which adds the modules to the stack
>>>>>> lsm.delete=$lsm1,...,$lsmN which deletes modules from the stack
>>>>> We've got two issues: ordering and enablement. It's been strongly
>>>>> suggested that we should move away from per-LSM enable/disable flags
>>>>> (to which I agree).
>>>> I also agree. There are way too many ways to turn off some LSMs.
>>>>
>>> I wont disagree, but its largely because we didn't have this discussion
>>> when we should have.
>> True that.
>>
>>
>>>>> If ordering should be separate from enablement (to
>>>>> avoid the "booted kernel with new LSM built in, but my lsm="..." line
>>>>> didn't include it so it's disabled case), then I think we need to
>>>>> split the logic (otherwise we just reinvented "security=" with similar
>>>>> problems).
>>>> We could reduce the problem by declaring that LSM ordering is
>>>> not something you can specify on the boot line. I can see value
>>>> in specifying it when you build the kernel, but your circumstances
>>>> would have to be pretty strange to change it at boot time.
>>>>
>>> if there is LSM ordering the getting
>>>
>>>   lsm=B,A,C
>>>
>>> is not the behavior I would expect from specifying
>>>
>>>   lsm=A,B,C
>> Right. You'd expect that they'd be used in the order specified.
>>
> and yet you argue for something different ;)

A foolish consistency is the hobgoblin of little minds.
Or, more to the point in this case, I don't see a way to
accomplish the ends well, so I'm casting about for something
that no one hates too badly.

>>>>> Should "lsm=" allow arbitrary ordering? (I think yes.)
>>>> I say no. Assume you can specify it at build time. When would
>>>> you want to change the order? Why would you?
>>>>
>>> because maybe you care about the denial message from one LSM more than
>>> you do from another. Since stacking is bail on first fail the order
>>> could be important from an auditing POV
>> I understand that a distribution would want to specify the order
>> for support purposes and that a developer would want to specify
>> the order to ensure reproducible behavior. But they are going to
>> be controlling their kernel builds. I'm not suggesting that the
>> order shouldn't be capable of build time specification. What I
>> don't see is a reason to rearrange it at boot time.
>>
> Because not all users have the same priority as the distro. It can
> also aid in debugging and testing of LSMs in a stacked situation.

My assumption is that specifying the LSM order on the boot line
by hand is going to be pretty rare. So it doesn't have to be easy,
it just needs to be sane.

> ... <snip>
>>>>> becomes
>>>>>
>>>>>     capability,smack,yama,integrity
>>>>>
>>>>> and
>>>>>
>>>>>     CONFIG_SECURITY_LOADPIN_DEFAULT_ENABLED=n
>>>>>     selinux.enable=0 lsm.add=loadpin lsm.disable=smack,tomoyo lsm=integrity
>>>> Do you mean
>>>> 	selinux.enable=0 lsm.enable=loadpin lsm.disable=smack,tomoyo lsm.enable=integrity
>>>> 	selinux.enable=0 lsm.enable=loadpin,integrity lsm.disable=smack,tomoyo
>>>> 	selinux.enable=0 lsm.enable=loadpin lsm.enable=integrity lsm.disable=smack lsm.disable=tomoyo
>>>>
>>>>> becomes
>>>>>
>>>>>     capability,integrity,yama,loadpin,apparmor
>>>>>
>>>>>
>>>>> If "lsm=" _does_ imply enablement, then how does it interact with
>>>>> per-LSM disabling? i.e. what does "apparmor.enabled=0
>>>>> lsm=yama,apparmor" mean? If it means "turn on apparmor" how do I turn
>>>>> on a CONFIG-default-off LSM without specifying all the other LSMs too?
>>>> There should either be one option "lsm=", which is an explicit list or
>>>> two, "lsm.enable=" and "lsm.disable", which modify the built in default.
>>>>
>>> maybe but this breaks with current behavior as their is a mismatch between
>>> how the major lsms do selection/enablement and the minor ones.
>> Which is why you have to continue supporting "security=".
>>
> I would argue that switching to lsm= isn't exactly a fix either as we have
> the whole minor lsm problem that we are currently debating.

I'm finding it hard to argue for "lsm=" because it's too clumsy.


>>> I personally would prefer
>>>
>>>   lsm=
>>>
>>> but that breaks how the minor lsms are currently enable
>> I don't know if I'd say "breaks", but it would require change.
>>
> depends how you look at it. Its a change to how its interacted with but so
> is switching to lsm=
>
> or making the minor module kconfig automatically add the current minor
> lsms to a default lsm selection list, and making $lsm.disable behave
> like apparmor or selinux=0.
>
> we got it wrong early on, so now we have to live with something not
> as clean as it could have been

It's not the first time and won't be the last.

 

>>> ... <snip>
>> The rules for modification are pretty obvious. The downside is, as
>> you point out, that they don't address ordering. Maybe we address that
>> directly:
>>
>> 	lsm.order=*,tomoyo
>>
>> 		TOMOYO should be last.
>>
>> 	lsm.order=apparmor,*
>>
>> 		AppArmor should be first.
>>
>>
>> 	lsm.order=*,sara,selinux,*
>>
>> 		SELinux should come directly after SARA but we otherwise don't care.
>>
>> 	lsm.order=smack,*,landlock,*
>>
>> 		Smack should be first and LandLock should come sometime later.
>>
>> 	lsm.order=*,yama,*
>>
>> 		Is meaningless.
>>
>> Modules not listed may go anywhere there is a "*" in the order.
>> An lsm.order= without a "*" is an error, and ignored.
>> If a module is specified in lsm.order but not built in it is ignored.
>> If a module is specified but disabled it is ignored.
>> The capability module goes first regardless.
>>
> I don't mind using lsm.order if we must but really do not like the '*'
> idea. It makes this way more complicated than it needs to be

We could arbitrarily say that anything unspecified goes after what
shows up in lsm.order (like lsm.order=yama,smack,* )