Re: [PATCH] ima: Handle -ESTALE returned by ima_filter_rule_match()

["Guozihua (Scott)" <guozihua@huawei.com>]

On 2022/8/30 9:20, Mimi Zohar wrote:
> On Sat, 2022-08-27 at 17:57 +0800, Guozihua (Scott) wrote:
>> On 2022/8/25 21:02, Mimi Zohar wrote:
>>> On Wed, 2022-08-24 at 09:56 +0800, Guozihua (Scott) wrote:
>>>> On 2022/8/24 9:26, Mimi Zohar wrote:
>>>>> On Tue, 2022-08-23 at 21:28 +0800, Guozihua (Scott) wrote:
>>>>>> On 2022/8/23 21:21, Mimi Zohar wrote:
>>>>>>> On Tue, 2022-08-23 at 16:12 +0800, Guozihua (Scott) wrote:
>>>>>>>>> The question is whether we're waiting for the SELinux policy to change
>>>>>>>>> from ESTALE or whether it is the number of SELinux based IMA policy
>>>>>>>>> rules or some combination of the two.  Retrying three times seems to be
>>>>>>>>> random.  If SELinux waited for ESTALE to change, then it would only be
>>>>>>>>> dependent on the time it took to update the SELinux based IMA policy
>>>>>>>>> rules.
>>>>>>>>
>>>>>>>> We are waiting for ima_lsm_update_rules() to finish re-initializing all
>>>>>>>> the LSM based rules.
>>>>>>>
>>>>>>> Fine.  Hopefully retrying a maximum of 3 times is sufficient.
>>>>>>>
>>>>>> Well, at least this should greatly reduce the chance of this issue from
>>>>>> happening.
>>>>>
>>>>> Agreed
>>>>>
>>>>>> This would be the best we I can think of without locking and
>>>>>> busy waiting. Maybe we can also add delays before we retry. Maybe you
>>>>>> got any other thought in mind?
>>>>>
>>>>> Another option would be to re-introduce the equivalent of the "lazy"
>>>>> LSM update on -ESTALE, but without updating the policy rule, as the
>>>>> notifier callback will eventually get to it.
>>>>>
>>>>
>>>> For this to happen we would need a way to tell when we are able to
>>>> continue with the retry though.
>>>
>>> Previously with the lazy update, on failure security_filter_rule_init()
>>> was called before the retry.  To avoid locking or detecting when to
>>> continue, another option would be to call to
>>> security_filter_rule_init() with a local copy of the rule.  The retry
>>> would be based on a local copy of the rule.
>>>
>>> Eventually the registered callback will complete, so we don't need to
>>> be concerned about updating the actual rules.
>>
>> Is it possible to cause race condition though? With this, the notifier
>> path seems to be unnecessary.
> 
> I don't see how there would be a race condition.  The notifier callback
> is the normal method of updating the policy rules.  Hopefully -ESTALE
> isn't something that happens frequently.

The notifier callback uses RCU to update rules, I think we should mimic 
that behavior if we are to update individual rules in the matching logic.

-- 
Best
GUO Zihua