linux-kernel - Re: [PATCH 0/3] support for duplicate measurement of integrity critical data

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Date:   Tue, 9 Feb 2021 12:57:19 -0800
From:   Tushar Sugandhi <tusharsu@...ux.microsoft.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>, stephen.smalley.work@...il.com,
        casey@...aufler-ca.com, agk@...hat.com, snitzer@...hat.com,
        gmazyland@...il.com, paul@...l-moore.com
Cc:     tyhicks@...ux.microsoft.com, sashal@...nel.org, jmorris@...ei.org,
        nramas@...ux.microsoft.com, linux-integrity@...r.kernel.org,
        selinux@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: Re: [PATCH 0/3] support for duplicate measurement of integrity
 critical data



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
>