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Subject: Re: [PATCH v5 2/7] IMA: update process_buffer_measurement to
 measure buffer hash
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Tushar Sugandhi <tusharsu@linux.microsoft.com>,
        stephen.smalley.work@gmail.com, casey@schaufler-ca.com,
        agk@redhat.com, snitzer@redhat.com, gmazyland@gmail.com,
        paul@paul-moore.com
Cc:     tyhicks@linux.microsoft.com, sashal@kernel.org, jmorris@namei.org,
        nramas@linux.microsoft.com, linux-integrity@vger.kernel.org,
        selinux@vger.kernel.org, linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, dm-devel@redhat.com
Date:   Thu, 05 Nov 2020 09:30:26 -0500
In-Reply-To: <20201101222626.6111-3-tusharsu@linux.microsoft.com>
References: <20201101222626.6111-1-tusharsu@linux.microsoft.com>
         <20201101222626.6111-3-tusharsu@linux.microsoft.com>
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Hi Tushar,

Please don't include the filename in the Subject line[1].   The Subject
line should be a summary phrase describing the patch.   In this case,
it is adding support for measuring the buffer data hash.

On Sun, 2020-11-01 at 14:26 -0800, Tushar Sugandhi wrote:
> process_buffer_measurement() currently only measures the input buffer.
> In case of SeLinux policy measurement, the policy being measured could
> be large (several MB). This may result in a large entry in IMA
> measurement log.

SELinux is an example of measuring large buffer data.  Please rewrite
this patch description (and the other patch descriptions in this patch
set) without using the example to describe its purpose [1].

In this case, you might say,

The original IMA buffer data measurement sizes were small (e.g. boot
command line), but new buffer data measurement use cases are a lot
larger.  Just as IMA measures the file data hash, not the file data,
IMA should similarly support measuring the buffer data hash.

> 
> Introduce a boolean parameter measure_buf_hash to support measuring
> hash of a buffer, which would be much smaller, instead of the buffer
> itself.

> To use the functionality introduced in this patch, the attestation
> client and the server changes need to go hand in hand. The
> client/kernel would know what data is being measured as-is
> (e.g. KEXEC_CMDLINE), and what data has it’s hash measured (e.g. SeLinux
> Policy). And the attestation server should verify data/hash accordingly.
> 
> Just like the data being measured in other cases, the attestation server
> will know what are possible values of the large buffers being measured.
> e.g. the possible valid SeLinux policy values that are being pushed to
> the client. The attestation server will have to maintain the hash of
> those buffer values.

Each patch in the patch set builds upon the previous one.   (Think of
it as a story, where each chapter builds upon the previous ones.)  
With rare exceptions, should patches reference subsequent patches. [2]

[1] Refer to Documentation/process/submitting-patches.rst
[2] Refer to the section "8) Commenting" in
Documentation/process/coding-style.rst

thanks,

Mimi