[PATCH] SELinux: Measure state and hash of policy using IMA

[Lakshmi Ramasubramanian]

On 9/8/20 4:58 AM, Stephen Smalley wrote:
> On Tue, Sep 8, 2020 at 12:44 AM Lakshmi Ramasubramanian
> <nramas at linux.microsoft.com> wrote:
>>
>> On 9/7/20 3:32 PM, Stephen Smalley wrote:
>>
>>>> Signed-off-by: Lakshmi Ramasubramanian <nramas at linux.microsoft.com>
>>>> Suggested-by: Stephen Smalley <stephen.smalley.work at gmail.com>
>>>> Reported-by: kernel test robot <lkp at intel.com> # error: implicit declaration of function 'vfree'
>>>> Reported-by: kernel test robot <lkp at intel.com> # error: implicit declaration of function 'crypto_alloc_shash'
>>>> Reported-by: kernel test robot <lkp at intel.com> # sparse: symbol 'security_read_selinux_policy' was not declared. Should it be static?
>>>
>>> Not sure these Reported-by lines are useful since they were just on
>>> submitted versions of the patch not on an actual merged commit.
>>
>> I'll remove them when I update the patch.
>>
>>>
>>>> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
>>>> new file mode 100644
>>>> index 000000000000..caf9107937d9
>>>> --- /dev/null
>>>> +++ b/security/selinux/measure.c
>>> <snip>
>>>> +void selinux_measure_state(struct selinux_state *state, bool policy_mutex_held)
>>>> +{
>>> <snip>
>>>> +
>>>> +       if (!policy_mutex_held)
>>>> +               mutex_lock(&state->policy_mutex);
>>>> +
>>>> +       rc = security_read_policy_kernel(state, &policy, &policy_len);
>>>> +
>>>> +       if (!policy_mutex_held)
>>>> +               mutex_unlock(&state->policy_mutex);
>>>
>>> This kind of conditional taking of a mutex is generally frowned upon
>>> in my experience.
>>> You should likely just always take the mutex in the callers of
>>> selinux_measure_state() instead.
>>> In some cases, it may be the caller of the caller.  Arguably selinuxfs
>>> could be taking it around all state modifying operations (e.g.
>>> enforce, checkreqprot) not just policy modifying ones although it
>>> isn't strictly for that purpose.
>>
>> Since currently policy_mutex is not used to synchronize access to state
>> variables (enforce, checkreqprot, etc.) I am wondering if
>> selinux_measure_state() should measure only state if policy_mutex is not
>> held by the caller - similar to how we skip measuring policy if
>> initialization is not yet completed.
> 
> No, we want to measure policy whenever there is a policy to measure.
> Just move the taking of the mutex to the callers of
> selinux_measure_state() so that it can be unconditional.
> 

Will do.

  -lakshmi