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

[John Johansen]

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 ;)

>>>> 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.

>> Auditing is why apparmor's internal stacking is not bail on first
>> fail.
> 
> Within a security module I get that. But we've already got the
> priority wrong for audit in general, because you only get to the
> LSM if the traditional code approves. Every guidance I ever got

true

> said you should do the MAC checks first, because you're much more
> concerned about getting audit records about MAC failures than DAC.
> 

yep, wouldn't that be nice to have

>>>> Should "lsm=" imply implicit enable/disable? (I think no: unlisted
>>>> LSMs are implicitly auto-appended to the explicit list)
>>> If you want to add something that isn't there instead of making
>>> it explicit you want "lsm.enable=" not "lsm=".
>>>
>>>> So then we could have "lsm.enable=..." and "lsm.disable=...".
>>>>
>>>> If builtin list was:
>>>> capability,yama,loadpin,integrity,{selinux,smack,tomoyo,apparmor}
>>>> then:
>>>>
>>>>     lsm.disable=loadpin lsm=smack
>>> Methinks this should be lsm.disable=loadpin lsm.enable=smack
>>>
>> that would only work if order is not important
> 
> It works unless you want to change the order at boot, and
> I still don't see a use case for that.

see above

> 
>>>> 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 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


>>> In the "lsm=" case "apparmor.enabled=0" should be equivalent to leaving
>>> apparmor off the list, but it's up to the AppArmor code to do that.
>>>> If "lsm.enable=apparmor apparmor.enabled=0" is specified the explict wish
>>> of the security module is used, but it's up to the AppArmor code to do that.
>>>
>> current behavior
> 
> That's right.
> 
>>> If "lsm.disable=apparmor apparmor.enabled=1" is specified the infrastructure
>>> should have shut down AppArmor before it looked to see the "apparmor.enabled=1",
>>> so it will remain disabled.
>>>
>> yep, current behavior
> 
> 2 for 2!
> 
> 
>>> If "lsm.enable=apparmor lsm.disable=apparmor" is specified the last value
>>> specified is used giving "lsm.disable=apparmor".
>>>
>> makes sense
> 
> 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