linux-kernel - Re: [PATCH v5 6/7] IMA: add critical

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Date:   Fri, 6 Nov 2020 15:51:51 -0800
From:   Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Tushar Sugandhi <tusharsu@...ux.microsoft.com>,
        stephen.smalley.work@...il.com, casey@...aufler-ca.com,
        agk@...hat.com, snitzer@...hat.com, gmazyland@...il.com,
        paul@...l-moore.com
Cc:     tyhicks@...ux.microsoft.com, sashal@...nel.org, jmorris@...ei.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: Re: [PATCH v5 6/7] IMA: add critical_data to the built-in policy
 rules

On 11/6/20 7:37 AM, Lakshmi Ramasubramanian wrote:

Hi Mimi,

> 
>> Hi Lakshmi, Tushar,
>>
>> This patch defines a new critical_data builtin policy.  Please update
>> the Subject line.
>>
>> On Sun, 2020-11-01 at 14:26 -0800, Tushar Sugandhi wrote:
>>> From: Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
>>>
>>> The IMA hook to measure kernel critical data, namely
>>> ima_measure_critical_data(), could be called before a custom IMA policy
>>> is loaded. For example, SELinux calls ima_measure_critical_data() to
>>> measure its state and policy when they are initialized. This occurs
>>> before a custom IMA policy is loaded, and hence IMA hook will not
>>> measure the data. A built-in policy is therefore needed to measure
>>> critical data provided by callers before a custom IMA policy is loaded.
>>
>> ^Define a new critical data builtin policy to allow measuring early
>> kernel integrity critical data before a custom IMA policy is loaded.
> 
> I will add the above line in the patch description.
> 
>>
>> Either remove the references to SELinux or move this patch after the
>> subsequent patch which measures SELinux critical data.
> 
> I will remove the reference to SELinux.
> I think it would be better to have this patch before the SELinux 
> measurement patch.
> 
>>
>>>
>>> Add CRITICAL_DATA to built-in IMA rules if the kernel command line
>>> contains "ima_policy=critical_data". Set the IMA template for this rule
>>> to "ima-buf" since ima_measure_critical_data() measures a buffer.
>>>
>>> Signed-off-by: Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
>>
>>> ---
>>>   security/integrity/ima/ima_policy.c | 32 +++++++++++++++++++++++++++++
>>>   1 file changed, 32 insertions(+)
>>>
>>> diff --git a/security/integrity/ima/ima_policy.c 
>>> b/security/integrity/ima/ima_policy.c
>>> index ec99e0bb6c6f..dc8fe969d3fe 100644
>>> --- a/security/integrity/ima/ima_policy.c
>>> +++ b/security/integrity/ima/ima_policy.c
>>
>>> @@ -875,6 +884,29 @@ void __init ima_init_policy(void)
>>>                 ARRAY_SIZE(default_appraise_rules),
>>>                 IMA_DEFAULT_POLICY);
>>> +    if (ima_use_critical_data) {
>>> +        template = lookup_template_desc("ima-buf");
>>> +        if (!template) {
>>> +            ret = -EINVAL;
>>> +            goto out;
>>> +        }
>>> +
>>> +        ret = template_desc_init_fields(template->fmt,
>>> +                        &(template->fields),
>>> +                        &(template->num_fields));
>>
>> The default IMA template when measuring buffer data is "ima_buf".   Is
>> there a reason for allocating and initializing it here and not
>> deferring it until process_buffer_measurement()?
>>
> 
> You are right - good catch.
> I will remove the above and validate.
> 

process_buffer_measurement() allocates and initializes "ima-buf" 
template only when the parameter "func" is NONE. Currently, only 
ima_check_blacklist() passes NONE for func when calling 
process_buffer_measurement().

If "func" is anything other than NONE, ima_match_policy() picks
the default IMA template if the IMA policy rule does not specify a template.

We need to add "ima-buf" in the built-in policy for critical_data so 
that the default template is not used for buffer measurement.

Please let me know if I am missing something.

thanks,
  -lakshmi

>>
>>> +        if (ret)
>>> +            goto out;
>>> +
>>> +        critical_data_rules[0].template = template;
>>> +        add_rules(critical_data_rules,
>>> +              ARRAY_SIZE(critical_data_rules),
>>> +              IMA_DEFAULT_POLICY);
>>> +    }
>>> +
>>> +out:
>>> +    if (ret)
>>> +        pr_err("%s failed, result: %d\n", __func__, ret);
>>> +
>>>       ima_update_policy_flag();
>>>   }
>>
>