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

Roberto Sassu
2021-07-26T16:41:18Z

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:
+
+::
+
+ 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
--
2.25.1