[PATCH 2/2] SELinux: Measure state and hash of policy using IMA

[Lakshmi Ramasubramanian]

On 8/17/20 4:11 PM, Mimi Zohar wrote:
> On Mon, 2020-08-17 at 15:33 -0700, Lakshmi Ramasubramanian wrote:
>> On 8/17/20 3:00 PM, Casey Schaufler wrote:
>>> On 8/17/2020 2:31 PM, Mimi Zohar wrote:
>>>> On Thu, 2020-08-13 at 14:13 -0400, Stephen Smalley wrote:
>>>>> On Thu, Aug 13, 2020 at 2:03 PM Lakshmi Ramasubramanian
>>>>> <nramas at linux.microsoft.com> wrote:
>>>>>> On 8/13/20 10:58 AM, Stephen Smalley wrote:
>>>>>>> On Thu, Aug 13, 2020 at 1:52 PM Lakshmi Ramasubramanian
>>>>>>> <nramas at linux.microsoft.com> wrote:
>>>>>>>> On 8/13/20 10:42 AM, Stephen Smalley wrote:
>>>>>>>>
>>>>>>>>>> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
>>>>>>>>>> new file mode 100644
>>>>>>>>>> index 000000000000..f21b7de4e2ae
>>>>>>>>>> --- /dev/null
>>>>>>>>>> +++ b/security/selinux/measure.c
>>>>>>>>>> @@ -0,0 +1,204 @@
>>>>>>>>>> +static int selinux_hash_buffer(void *buf, size_t buf_len,
>>>>>>>>>> +                   void **buf_hash, int *buf_hash_len)
>>>>>>>>>> +{
>>>>>>>>>> +    struct crypto_shash *tfm;
>>>>>>>>>> +    struct shash_desc *desc = NULL;
>>>>>>>>>> +    void *digest = NULL;
>>>>>>>>>> +    int desc_size;
>>>>>>>>>> +    int digest_size;
>>>>>>>>>> +    int ret = 0;
>>>>>>>>>> +
>>>>>>>>>> +    tfm = crypto_alloc_shash("sha256", 0, 0);
>>>>>>>>>> +    if (IS_ERR(tfm))
>>>>>>>>>> +        return PTR_ERR(tfm);
>>>>>>>>> Can we make the algorithm selectable via kernel parameter and/or writing
>>>>>>>>> to a new selinuxfs node?
>>>>>>>> I can add a kernel parameter to select this hash algorithm.
>>>>>>> Also can we provide a Kconfig option for the default value like IMA does?
>>>>>>>
>>>>>> Would we need both - Kconfig and kernel param?
>>>>>>
>>>>>> The other option is to provide an IMA function to return the current
>>>>>> hash algorithm used for measurement. That way a consistent hash
>>>>>> algorithm can be employed by both IMA and the callers. Would that be better?
>>>>> This is why I preferred just passing the serialized policy buffer to
>>>>> IMA and letting it handle the hashing.  But apparently that approach
>>>>> wouldn't fly.  IMA appears to support both a Kconfig option for
>>>>> selecting a default algorithm and a kernel parameter for overriding
>>>>> it.  I assume the idea is that the distros can pick a reasonable
>>>>> default and then the end users can override that if they have specific
>>>>> requirements.  I'd want the same for SELinux.  If IMA is willing to
>>>>> export its hash algorithm to external components, then I'm willing to
>>>>> reuse that but not sure if that's a layering violation.
>>>> With the new ima_measure_critical_data() hook, I agree with you and
>>>> Casey it doesn't make sense for each caller to have to write their own
>>>> function.  Casey suggested exporting IMA's hash function or defining a
>>>> new common hash function.   There's nothing specific to IMA.
>>>
>>> Except that no one is going to use the function unless they're
>>> doing an IMA operation.
>>
>> Can we do the following instead:
>>
>> In ima_measure_critical_data() IMA hook, we can add another param for
>> the caller to indicate whether
>>
>>    => The contents of "buf" needs to be measured
>>       OR
>>    => Hash of the contents of "buf" needs to be measured.
>>
>> This way IMA doesn't need to export any new function to meet the hashing
>> requirement.
> 
> I'm not sure overloading the parameters is a good idea, but extending
> ima_measure_critical_data() to calculate a simple buffer hash should be
> fine.
> 

Sorry I wasn't clear - I didn't mean to say overload existing 
parameters, but extending the IMA hook to calculate the hash of the 
buffer - like the following:

int ima_measure_critical_data(const char *event_name,
                               const char *event_data_source,
                               const void *buf, int buf_len,
                               bool measure_buf_hash);

If measure_buf_hash is true, IMA will calculate the hash of contents of 
"buf" and measure the hash.
Else, IMA will measure the contents of "buf".

  -lakshmi