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

[Lakshmi Ramasubramanian]

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.

  -lakshmi

> 
>>    Should
>> the common hash function be prefixed with "security_"?
> 
> Yuck. That prefix is for interfaces that are exported outside the
> security sub-system. We're talking about a function that is provided
> for use within the security sub-system, but not for any one particular
> security module or non-module feature. We're currently using the lsm_
> prefix for interfaces used within the security subsystem, so I suggest
> lsm_hash_brown_potatoes() might be the way to go.


> 
>>
>> Like when we add a new security hook call, the new LSM call is separate
>> from any other change.   Please break up this patch with the SELinux
>> specific pieces separated from the ima_measure_critical_data() call as
>> much as possible.
>>