Re: MLS dominance check behavior on el7

From: Joshua Brindle <joshua.brindle@crunchydata.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: SELinux <selinux@tycho.nsa.gov>, Ted X Toth <txtoth@gmail.com>
Subject: Re: MLS dominance check behavior on el7
Date: Tue, 11 Sep 2018 13:39:51 -0400	[thread overview]
Message-ID: <CAGB+Vh5MF+p4wLNONp5Vozv_d85BJ=nK8vqiHj1Aq=Y_2WkPrg@mail.gmail.com> (raw)
In-Reply-To: <bb05c207-2c43-5af2-6ca0-c2b04cac0521@tycho.nsa.gov>

On Tue, Sep 11, 2018 at 1:33 PM, Stephen Smalley <sds@tycho.nsa.gov> wrote:
> On 09/11/2018 12:53 PM, Joshua Brindle wrote:
>>
>> On Tue, Sep 11, 2018 at 10:41 AM, Stephen Smalley <sds@tycho.nsa.gov>
>> wrote:
>>>
>>> On 09/10/2018 06:30 PM, Ted Toth wrote:
>>>>
>>>>
>>>> mcstrans mcscolor.c also uses the same logic I'd been using to check
>>>> dominance so this too will no longer function as expected on el7. Do you
>>>> any
>>>> suggestions for doing a 'generic' (one not tied to a specific resource
>>>> class) dominance check in lieu of context contains?
>>>
>>>
>>>
>>> You should probably define your own permission with its own constraint to
>>> avoid depending on the base policy's particular constraint definitions.
>>> Certainly for your own code.  For mcstrans, mcscolor probably ought to be
>>> switched to using at least a separate permission in the context class if
>>> not
>>> its own class to avoid overloading the meaning with pam_selinux's usage
>>> (or
>>> vice versa, but likely harder to change pam_selinux at this point).
>>>
>>
>> Isn't the actual question what the GLB of the 2 contexts is, rather
>> than what permissions one has on the other? It seems like a hack to
>> use permissions to figure out dominance.
>>
>> Would a libselinux interface to determine glb and lub of 2 contexts
>> make sense? Or maybe add a default_range glb and lub option and then
>> calculate it using relabel?
>
>
> At least as used in mcstrans, it appears to be a way of matching which entry
> from the colors configuration to use.  So it is just a "Can context C1 use
> the colors specified for context C2?" question.  It just happens that the
> way they are deciding that for the MLS part is through the dominance
> relation.  And determining whether context C1 dominates context C2 is
> something only the kernel security server or libsepol with the same policy
> file loaded into it can answer, not libselinux or anything else.
>

I meant an libselinux as in a library interface to some file in
selinuxfs to calculate the glb.

If it is 'permission to see a color' that makes sense, I was thinking
the source context and target context have a glb that maps to the
color to be shown.