SELinux "filtering" capabilities?

[Casey Schaufler]

On 4/19/2017 4:58 AM, Stephen Smalley wrote:
> On Tue, 2017-04-18 at 15:37 -0700, Casey Schaufler wrote:
>> I don't expect anyone else to have run into this
>> as I am working with SELinux and Smack on the same
>> machine at the same time. While there are a number
>> of interactions that I can explain, I have one that
>> is perplexing me. I assume something rational is
>> going on, but I am having trouble tracking it down.
>>
>> A process with CAP_MAC_ADMIN can change its Smack label
>> by writing the new label to /proc/self/attr/smack/current.*
>> If I have both SELinux and Smack enabled the write fails
>> with -EPERM, indicating that the process lacks CAP_MAC_ADMIN.
>> Instrumenting the Smack code verifies that, even though the
>> process reports having CAP_MAC_ADMIN, the capability is gone
>> in smack_setprocattr().
>>
>> It seem that this could be happening in the write() path,
>> or perhaps an artifact of SELinux not knowing something
>> special about smackfs. I don't see anything obvious.
>> Unfortunately, it is going to be somewhat difficult for
>> me to claim that I have SELinux and Smack working, if not
>> together, at least begrudgingly on the same system.
>>
>> ----
>> * The smack subdir of attr isn't upstream yet, but I hope
>>   to get it there real soon.
> Since smack_privileged() calls capable() rather than cap_capable(), the
> CAP_MAC_ADMIN check will trigger a check by all enabled security
> modules, including SELinux.  SELinux will then apply a check against
> policy as to whether the current process is allowed mac_admin
> permission.  This is only allowed to a handful of domains (not even
> unconfined_t) because to SELinux, CAP_MAC_ADMIN means the ability to
> set or get a raw, uninterpreted security context that may be unknown to
> the currently loaded security policy.
>
> I suspect that smack_privileged() should call cap_capable() instead.
>
Brilliant. Did the trick. Thank you.

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