[PATCH v5 2/7] IMA: update process_buffer_measurement to measure buffer hash

[Tushar Sugandhi]

On 2020-11-12 2:19 p.m., Mimi Zohar wrote:
> On Thu, 2020-11-12 at 13:47 -0800, Tushar Sugandhi wrote:
>>> On Sun, 2020-11-01 at 14:26 -0800, Tushar Sugandhi wrote:
>>>> process_buffer_measurement() currently only measures the input buffer.
>>>> In case of SeLinux policy measurement, the policy being measured could
>>>> be large (several MB). This may result in a large entry in IMA
>>>> measurement log.
>>>
>>> SELinux is an example of measuring large buffer data.  Please rewrite
>>> this patch description (and the other patch descriptions in this patch
>>> set) without using the example to describe its purpose [1].
>>>
>>> In this case, you might say,
>>>
>>> The original IMA buffer data measurement sizes were small (e.g. boot
>>> command line), but new buffer data measurement use cases are a lot
>>> larger.  Just as IMA measures the file data hash, not the file data,
>>> IMA should similarly support measuring the buffer data hash.
>>>
>> Sure. Thanks a lot for giving an example wording for us. Will update.
>>>>
>>>> Introduce a boolean parameter measure_buf_hash to support measuring
>>>> hash of a buffer, which would be much smaller, instead of the buffer
>>>> itself.
>>>
>>>> To use the functionality introduced in this patch, the attestation
>>>> client and the server changes need to go hand in hand. The
>>>> client/kernel would know what data is being measured as-is
>>>> (e.g. KEXEC_CMDLINE), and what data has it’s hash measured (e.g. SeLinux
>>>> Policy). And the attestation server should verify data/hash accordingly.
>>>>
>>>> Just like the data being measured in other cases, the attestation server
>>>> will know what are possible values of the large buffers being measured.
>>>> e.g. the possible valid SeLinux policy values that are being pushed to
>>>> the client. The attestation server will have to maintain the hash of
>>>> those buffer values.
>>>
>>> Each patch in the patch set builds upon the previous one.   (Think of
>>> it as a story, where each chapter builds upon the previous ones.)
>>> With rare exceptions, should patches reference subsequent patches. [2]
>>>
>>> [1] Refer to Documentation/process/submitting-patches.rst
>>> [2] Refer to the section "8) Commenting" in
>>> Documentation/process/coding-style.rst
>>>
>> I am not sure if you have any concerns about the last two paragraphs.
>> The description about the attestation client and server (the last two
>> paragraphs) was added for information/clarification purpose only, as per
>> your feedback on previous iterations. The subsequent patches don’t have
>> any code pertaining to attestation client/server.
>>
>> *Question*
>> Maybe the last two paragraphs are confusing/redundant. Could you please
>> let me know if I should remove the above two paragraphs altogether?
>> (starting with “To use the functionality introduced in this patch ...”)
>>
>> If we decide to keep the paragraphs, I will remove the specific examples
>> (KEXEC_CMDLINE, SeLinux etc.) as you mentioned elsewhere.
> 
> Instead of the above two paragraphs, perhaps explain how measuring the
> file data hash differs from measuring the buffer data hash.  Keep the
> explanation generic, short and simple.
> 
> Mimi
> 
Will do. Thanks for the quick response Mimi.
I also have some clarification questions on the other patches in this
series as well.

Would really appreciate if you could help us get clarification on those.

Thanks a lot again.

~Tushar