Re: [dm-devel] [PATCH 0/7] device mapper target measurements using IMA

[Tushar Sugandhi <tusharsu@linux.microsoft.com>]

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Hi Mike,

On 7/20/21 2:27 PM, Mike Snitzer wrote:
> On Mon, Jul 12 2021 at  8:48P -0400,
> Tushar Sugandhi <tusharsu@linux.microsoft.com> wrote:
>
>> For a given system, various external services/infrastructure tools
>> (including the attestation service) interact with it - both during the
>> setup and during rest of the system run-time.  They share sensitive data
>> and/or execute critical workload on that system.  The external services
>> may want to verify the current run-time state of the relevant kernel
>> subsystems before fully trusting the system with business-critical
>> data/workload.
>>
>> Device mapper is one such kernel subsystem that plays a critical role on
>> a given system by providing various important functionalities to the
>> block devices with various target types like crypt, verity, integrity
>> etc.  Each of these target types’ functionalities can be configured with
>> various attributes.  The attributes chosen to configure these target types
>> can significantly impact the security profile of the block device,
>> and in-turn, of the system itself.  For instance, the type of encryption
>> algorithm and the key size determines the strength of encryption for a
>> given block device.
>>
>> Therefore, verifying the current state of various block devices as well
>> as their various target attributes is crucial for external services
>> before fully trusting the system with business-critical data/workload.
>>
>> IMA provides the necessary functionality for device mapper to measure the
>> state and configuration of various block devices -
>>    - BY device mapper itself, from within the kernel,
>>    - in a tamper resistant way,
>>    - and re-measured - triggered on state/configuration change.
>>
>> This patch series uses this IMA functionality, by calling the function
>> ima_measure_critical_data(), when a block device state is changed (e.g.
>> on device create, resume, rename, remove etc.)  It measures the device
>> state and configuration and stores it in IMA logs, so that it can be
>> used by external services for managing the system.
> I needed to EXPORT_SYMBOL_GPL(ima_measure_critical_data); otherwise I
> couldn't compile.. not sure but I can only imagine you compile DM
> (and all targets) to be builtin?

I believe the EXPORT_SYMBOL_GPL(ima_measure_critical_data) is now needed 
because of the move “#ifdef CONFIG_IMA from dm-ima.c to dm-ima.h”, and 
the associated change in the Makefile
+ifeq ($(CONFIG_IMA),y)
dm-mod-objs += dm-ima.o
+endif
Earlier I needed to export symbol only if I was calling 
ima_measure_critical_data() directly from various dm-*.c files.
With my earlier implementation, it wasn’t needed when it was called from 
the wrapper dm_ima_measure_data() in dm-ima.c.

> In addition to fixing that (in first table load patch) I changed
> various things along the way while I reviewed each patch.

Thank you so much for making the necessary changes Mike. Really 
appreciate it.

> Things that I recall are:
> - moved #ifdef CONFIG_IMA from dm-ima.c to dm-ima.h
> - fixed various typos and whitespace
> - consistently prepend "," in STATUSTYPE_IMA's DMEMIT()s as opposed to
>    having a mix or pre and postfix throughout targets
> - fixed what seemed like malformed STATUSTYPE_IMA handling for
>    dm-multipath -- it was DMEMIT(";") for each dm-mpath's pathgroup
> - added some fields to dm-mpath, renamed some IMA names in name=value
>    pairs to be more clear.

I went through the changes you made. They look fine. In addition to the 
above changes, you also fixed a warning in dm-raid.c. (initializing the 
recovery variable)
Thanks again. :)

> I've staged the result in linux-next via linux-dm.git's dm-5.15
> branch, see:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-5.15
>
> I've compiled tested both with and without CONFIG_IMA set.  But
> haven't actually tested the code.

No worries. I will test the changesthoroughly again.

> Please send any incremental fixes relative to the dm-5.15 branch and
> I'll get them folded in where appropriate.

Ok. I will send incremental patches against dm-5.15 as you've suggested. 
Thanks again for the review and all the help so far. Really appreciate 
it. ~Tushar

> Thanks,
> Mike

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