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SubjectRe: [PATCH 0/3] support for duplicate measurement of integrity critical data
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On 2021-02-09 10:53 a.m., Mimi Zohar wrote:
> On Tue, 2021-02-09 at 10:23 -0800, Tushar Sugandhi wrote:
>>> On Mon, 2021-02-08 at 15:22 -0500, Mimi Zohar wrote:
>>>> On Fri, 2021-01-29 at 16:45 -0800, Tushar Sugandhi wrote:
>>>>> IMA does not measure duplicate buffer data since TPM extend is a very
>>>>> expensive operation. However, in some cases for integrity critical
>>>>> data, the measurement of duplicate data is necessary to accurately
>>>>> determine the current state of the system. Eg, SELinux state changing
>>>>> from 'audit', to 'enforcing', and back to 'audit' again. In this
>>>>> example, currently, IMA will not measure the last state change to
>>>>> 'audit'. This limits the ability of attestation services to accurately
>>>>> determine the current state of the integrity critical data on the
>>>>> system.
>>>>>
>>>>> This series addresses this gap by providing the ability to measure
>>>>> duplicate entries for integrity critical data, driven by policy.
>>>>
>>>> The same reason for re-measuring buffer data is equally applicable to
>>>> files. In both cases, the file or the buffer isn't re-measured if it
>>>> already exists in the htable. Please don't limit this patch set to
>>>> just buffer data.
>>>
>> Agreed. I wasn't sure if you wanted the support for files, or other
>> buffer measurement scenarios, except critical data. So I started the
>> implementation with supporting just critical data. Happy to extend it
>> to files and other buffer measurement scenarios as you suggested.
>>
>>> Instead of making the change on a per measurement rule basis, disabling
>>> "htable" would be the simplest way of forcing re-measurements. All
>>> that would be needed is a new Kconfig (e.g. CONFIG_IMA_DISABLE_HTABLE)
>>> and the associated test in ima_add_template_entry().
>>>
>> Agreed. Earlier I wasn't sure if you wanted allow_dup support for all
>> the scenarios. Now that it is clear, I will implement it as you
>> suggested. Thank you so much for the pointers. Appreciate it.
>
> There are two different solutions - per measurement rule, disabling
> htable - being discussed. Disabling htable requires miminumal
> changes. Which version are you thinking of implementing?
>
I am thinking of implementing "disabling 'htable' using a new Kconfig
(e.g. CONFIG_IMA_DISABLE_HTABLE)". That is, not using the var
ima_htable or ima_lookup_digest_entry() if that CONFIG is set.
So the duplicate measurements are allowed when the CONFIG is set.
This would cover all the measurement scenarios through a single CONFIG
setting.

I am not planning to implement it as a "per measurement rule".

Sorry it wasn't clear in my earlier response.

Thanks,
Tushar

> thanks,
>
> Mimi
>

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