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

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Message-ID: <6c2d2242-119c-2a8a-8062-6326fed6a45d@linux.microsoft.com>
Date:   Thu, 18 Feb 2021 14:05:12 -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

Hi Mimi,

On 2021-02-17 12:49 p.m., Tushar Sugandhi wrote:
> 
> 
> 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.
> 

I booted the system with "ima_policy=tcb" policy with/without my patch.
I also removed /etc/ima/ima-policy for testing these use-cases.
(so that it wouldn't override the policy generated by boot param 
"ima_policy=tcb").

I double checked the contents of the kernel policy:
#cat /sys/kernel/security/integrity/ima/policy
     dont_measure fsmagic=0x9fa0
     dont_measure fsmagic=0x62656572
     dont_measure fsmagic=0x64626720
     dont_measure fsmagic=0x1021994
     dont_measure fsmagic=0x1cd1
     dont_measure fsmagic=0x42494e4d
     dont_measure fsmagic=0x73636673
     dont_measure fsmagic=0xf97cff8c
     dont_measure fsmagic=0x43415d53
     dont_measure fsmagic=0x27e0eb
     dont_measure fsmagic=0x63677270
     dont_measure fsmagic=0x6e736673
     dont_measure fsmagic=0xde5e81e4
     measure func=MMAP_CHECK mask=MAY_EXEC
     measure func=BPRM_CHECK mask=MAY_EXEC
     measure func=FILE_CHECK mask=^MAY_READ euid=0
     measure func=FILE_CHECK mask=^MAY_READ uid=0
     measure func=MODULE_CHECK
     measure func=FIRMWARE_CHECK
     measure func=POLICY_CHECK

And then I compared the contents of the ascii_runtime_measurements with 
and without my patch.

And here are my findings:

(1) Files like systemd-udevd, x2go_sessions etc. get measured multiple
     times with the CONFIG_IMA_DISABLE_HTABLE=y.
     They only get measured once with the config "=n".

     10 668df8723f5a1f57a0afe3b50d44054d66363f3e ima-ng 
sha1:51f66e82421b93b21ad1e0a25e5efa4155c6a8e0 /lib/systemd/systemd-udevd
     10 668df8723f5a1f57a0afe3b50d44054d66363f3e ima-ng 
sha1:51f66e82421b93b21ad1e0a25e5efa4155c6a8e0 /lib/systemd/systemd-udevd

(2) There are lot more instances of /tmp/<random> measurement records
     with the CONFIG_IMA_DISABLE_HTABLE=y.
     Eg,

     10 33515851cfee4acbf24de9482ff018d33def1083 ima-ng 
sha1:da39a3ee5e6b4b0d3255bfef95601890afd80709 /tmp/oUWCVeypLR
     10 9d1dc0e1e54ee2e16308a824fc5780bd21b38208 ima-ng 
sha1:da39a3ee5e6b4b0d3255bfef95601890afd80709 /tmp/etX8dy7qqy
     10 8643a5543179b86c02d7e3e01e16b3bd2f8dbb9f ima-ng 
sha1:da39a3ee5e6b4b0d3255bfef95601890afd80709 /tmp/I4zTWEuyMf
     10 56e9547a4ed39036d2e790cfad78b467aa979e32 ima-ng 
sha1:da39a3ee5e6b4b0d3255bfef95601890afd80709 /tmp/Lh5wDm6_Ep

I believe both the observations are consistent with the expected outcome 
of the patch.

Please let me know if I should test any other scenario.

I will shortly post the v3 patch with updates to description and title 
as you suggested.

Thanks,
Tushar

> Thanks,
> Tushar
> 
>> thanks,
>>
>> Mimi
>>