linux-kernel - Re: [PATCH v2] IMA: support for duplicate data measurement

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Date:   Wed, 17 Feb 2021 12:49:37 -0800
From:   Tushar Sugandhi <tusharsu@...ux.microsoft.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     tyhicks@...ux.microsoft.com, sashal@...nel.org, jmorris@...ei.org,
        nramas@...ux.microsoft.com, linux-integrity@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] IMA: support for duplicate data measurement



On 2021-02-17 12:39 p.m., Mimi Zohar wrote:
> On Wed, 2021-02-17 at 10:53 -0800, Tushar Sugandhi wrote:
>> Thanks for the feedback Mimi.
>> Appreciate it.
>>
>> On 2021-02-17 7:03 a.m., Mimi Zohar wrote:
>>> Hi Tushar,
>>>
>>> The Subject line could be improved.  Perhaps something like - "IMA:
>>> support for duplicate measurement records"
>>>
>> Will do.
>>
>>> On Tue, 2021-02-16 at 18:46 -0800, Tushar Sugandhi wrote:
>>>> IMA does not measure duplicate data since TPM extend is a very expensive
>>>> operation.  However, in some cases, 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 measurements
>>>> on the system.
>>>
>>> This patch description is written from your specific usecase
>>> perspective, but it impacts file and buffer data measurements as well,
>>> not only critical data measurements.  In all of these situations, with
>>> this patch a new measurement record is added/appended to the
>>> measurement list.  Please re-write the patch description making it more
>>> generic.
>>>
>>> For example, I would start with something like, "IMA does not include
>>> duplicate file, buffer or critical data measurement records ..."
>>>
>> Agreed.
>> I will generalize the description further and send the v3 for review.
> 
> It would be good to boot with the ima_policy=tcb policy with/without
> your patch and account for the different number of measurements.   Are
> all the differences related to duplicate measurements - original file
> hash -> new file hash -> original file hash - similar to what you
> described.
> 
Thanks for the ima_policy=tcb pointer.

I tested my patch with:
  - duplicate buffer content for "measure func=CRITICAL_DATA"
  - and reading the same file twice with "measure func=FILE_CHECK 
mask=MAY_READ"

In both the above use cases, IMA is measuring the duplicate entries with 
the patch, and not measuring the duplicate entries w/o the patch.

I will test the "ima_policy=tcb" boot-scenario as you suggested, before 
posting the next version.

Thanks,
Tushar

> thanks,
> 
> Mimi
>