Re: [RFC][PATCH v2 11/12] diglim: Remote Attestation

Mauro Carvalho Chehab
2021-07-28T12:47:39Z

Em Mon, 26 Jul 2021 18:36:59 +0200
Roberto Sassu <roberto.sassu@huawei.com> escreveu:

> Add more information about remote attestation with IMA and DIGLIM in
> Documentation/security/diglim/remote_attestation.rst.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
> Documentation/security/diglim/index.rst | 1 +
> .../security/diglim/remote_attestation.rst | 87 +++++++++++++++++++
> MAINTAINERS | 1 +
> 3 files changed, 89 insertions(+)
> create mode 100644 Documentation/security/diglim/remote_attestation.rst
>
> diff --git a/Documentation/security/diglim/index.rst b/Documentation/security/diglim/index.rst
> index 4771134c2f0d..0f28c5ad71c0 100644
> --- a/Documentation/security/diglim/index.rst
> +++ b/Documentation/security/diglim/index.rst
> @@ -10,3 +10,4 @@ Digest Lists Integrity Module (DIGLIM)
> introduction
> architecture
> implementation
> + remote_attestation
> diff --git a/Documentation/security/diglim/remote_attestation.rst b/Documentation/security/diglim/remote_attestation.rst
> new file mode 100644
> index 000000000000..83fd7581c460
> --- /dev/null
> +++ b/Documentation/security/diglim/remote_attestation.rst
> @@ -0,0 +1,87 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +Remote Attestation
> +==================
> +
> +When a digest list is added or deleted through the ``digest_list_add`` or
> +``digest_list_del`` interfaces, its buffer is sent to the IMA function
> +``ima_measure_critical_data()``. The primary reason for it is to calculate
> +the buffer digest, so that the digest list itself is searchable in the hash
> +table.
> +
> +``ima_measure_critical_data()`` can be also used to create a new
> +measurement entry each time this function is called, if there is an
> +appropriate rule in the IMA policy. Given that this function is called
> +during an addition or deletion of a digest list, a remote verifier can
> +infer from the measurement list precise information about what has been
> +uploaded to the kernel.
> +
> +To enable this functionality, the following rule must be added to the IMA
> +policy:
> +
> +::

As commented on other patches at this series, you can merge :: at the
previous text line, e. g.:

policy::

does the same as:

policy:

::

but it is nicer for text-only readers, IMO.

> +
> + measure func=CRITICAL_DATA label=diglim
> +
> +
> +When a file is uploaded, the workflow and the resulting IMA measurement
> +list are:
> +
> +.. code-block:: bash
> +
> + # echo $PWD/0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_list_add
> + # echo $PWD/0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_list_del
> + # cat /sys/kernel/security/integrity/ima/ascii_runtime_measurements
> + ...
> + 10 ima-buf sha256:<buffer digest> add_file_0-file_list-compact-cat <buffer>
> + 10 ima-buf sha256:<buffer digest> del_file_0-file_list-compact-cat <buffer>
> +
> +When a buffer is uploaded, the workflow and the resulting IMA measurement
> +list are:
> +
> +.. code-block:: bash
> +
> + # echo 0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_label
> + # cat 0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_list_add
> + # echo 0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_label
> + # cat 0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_list_del
> + # cat /sys/kernel/security/integrity/ima/ascii_runtime_measurements
> + ...
> + 10 ima-buf sha256:<buffer digest> add_buffer_0-file_list-compact-cat <buffer>
> + 10 ima-buf sha256:<buffer digest> del_buffer_0-file_list-compact-cat <buffer>
> +
> +In the second case, the digest list label must be set explicitly, as the
> +kernel cannot determine it by itself (in the first case it is derived from
> +the name of the file uploaded).
> +
> +The confirmation that the digest list has been processed by IMA can be
> +obtained by reading the ASCII representation of the digest list:
> +
> +.. code-block:: bash
> +
> + # cat /sys/kernel/security/integrity/diglim/digest_lists_loaded/sha256-<digest list digest>-0-file_list-compact-cat.ascii
> + actions: 1, version: 1, algo: sha256, type: 2, modifiers: 1, count: 1, datalen: 32
> + 87e5bd81850e11eeec2d3bb696b626b2a7f45673241cbbd64769c83580432869
> +
> +In this output, ``actions`` is set to 1 (``COMPACT_ACTION_IMA_MEASURED``
> +bit set).
> +
> +
> +DIGLIM guarantees that the information reported in the IMA measurement list
> +is complete. If digest list loading is not recorded, digest query results
> +are ignored by IMA. If the addition was recorded, deletion can be performed
> +only if also the deletion is recorded. This can be seen in the following
> +sequence of commands:
> +
> +.. code-block:: bash
> +
> + # echo 0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_label
> + # cat 0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_list_add
> + # echo 0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_label
> + # /tmp/cat 0-file_list-compact-cat > /sys/kernel/security/integrity/diglim/digest_list_del
> + diglim: actions mismatch, add: 1, del: 0
> + diglim: unable to upload generated digest list
> + /tmp/cat: write error: Invalid argument
> +
> +Digest list measurement is avoided with the execution of ``/tmp/cat``, for
> +which a dont_measure rule was previously added in the IMA policy.
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 0672128fae7f..a7c502685109 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -5461,6 +5461,7 @@ F: Documentation/security/diglim/architecture.rst
> F: Documentation/security/diglim/implementation.rst
> F: Documentation/security/diglim/index.rst
> F: Documentation/security/diglim/introduction.rst
> +F: Documentation/security/diglim/remote_attestation.rst
> F: include/linux/diglim.h
> F: include/uapi/linux/diglim.h
> F: security/integrity/diglim/diglim.h