[RFC][PATCH 02/12] digest_lists: Overview

[Roberto Sassu]

This patch adds an overview of Huawei Digest Lists to
Documentation/security/digest_lists.rst.

Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
---
 Documentation/security/digest_lists.rst | 228 ++++++++++++++++++++++++
 Documentation/security/index.rst        |   1 +
 MAINTAINERS                             |   7 +
 3 files changed, 236 insertions(+)
 create mode 100644 Documentation/security/digest_lists.rst

diff --git a/Documentation/security/digest_lists.rst b/Documentation/security/digest_lists.rst
new file mode 100644
index 000000000000..8980be7836f8
--- /dev/null
+++ b/Documentation/security/digest_lists.rst
@@ -0,0 +1,228 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================
+Huawei Digest Lists
+===================
+
+Introduction
+============
+
+Huawei Digest Lists is a simple in kernel database for storing file and
+metadata digests and for reporting to its users (e.g. Integrity Measurement
+Architecture or IMA) whether the digests are in the database or not. The
+choice of placing it in the kernel and not in a user space process is
+explained later in the Security Assumptions section.
+
+The database is populated by directly uploading the so called digest lists,
+a set of digests concatenated together and preceded by a header including
+information about them (e.g. whether the file or metadata with a given
+digest is immutable or not). Digest lists are stored in the kernel memory
+as they are, the kernel just builds indexes to easily lookup digests.
+
+The kernel supports only one digest list format called ``compact``. However,
+alternative formats (e.g. the RPM header) can be supported through a user
+space parser that, by invoking the appropriate library (more can be added),
+converts the original digest list to the compact format and uploads it to
+the kernel.
+
+The database keeps track of whether digest lists have been processed in
+some way (e.g. measured or appraised by IMA). This is important for example
+for remote attestation, so that remote verifiers understand what has been
+loaded in the database.
+
+It is a transactional database, i.e. it has the ability to roll back to the
+beginning of the transaction if an error occurred during the addition of a
+digest list (the deletion operation always succeeds). This capability has
+been tested with an ad-hoc fault injection mechanism capable of simulating
+failures during the operations.
+
+Finally, the database exposes to user space, through securityfs, the digest
+lists currently loaded, the number of digests added, a query interface and
+an interface to set digest list labels.
+
+
+Binary Integrity
+----------------
+
+Integrity is a fundamental security property in information systems.
+Integrity could be described as the condition in which a generic
+component is just after it has been released by the entity that created it.
+
+One way to check whether a component is in this condition (called binary
+integrity) is to calculate its digest and to compare it with a reference
+value (i.e. the digest calculated in controlled conditions, when the
+component is released).
+
+IMA, a software part of the integrity subsystem, can perform such
+evaluation and execute different actions:
+
+- store the digest in an integrity-protected measurement list, so that it
+  can be sent to a remote verifier for analysis;
+- compare the calculated digest with a reference value (usually protected
+  with a signature) and deny operations if the file is found corrupted;
+- store the digest in the system log.
+
+
+Contribution
+------------
+
+Huawei Digest Lists facilitates the provisioning of reference values for
+system and application files from software vendors, to the kernel.
+
+Possible sources for digest lists are:
+
+- RPM headers;
+- Debian repository metadata.
+
+These sources are already usable without any modification required for
+Linux vendors.
+
+If digest list sources are signed (usually they are, like the ones above),
+remote verifiers could identify their provenance, or Linux vendors could
+prevent the loading of unsigned ones or those signed with an untrusted key.
+
+
+Possible Usages
+---------------
+
+Provisioned reference values can be used (e.g. by IMA) to make
+integrity-related decisions (allow list or deny list).
+
+Possible usages for IMA are:
+
+- avoid recording measurement of files whose digest is found in the
+  pre-provisioned reference values:
+
+  - reduces the number of TPM operations (PCR extend);
+  - could make a TPM PCR predictable, as the PCR would not be affected by
+    the temporal sequence of executions if binaries are known
+    (no measurement);
+
+- exclusively grant access to files whose digest is found in the
+  pre-provisioned reference values:
+
+  - faster verification time (fewer signature verifications);
+  - no need to generate a signature for every file.
+
+
+Security Assumptions
+--------------------
+
+Since digest lists are stored in the kernel memory, they are no susceptible
+to attacks by user space processes.
+
+A locked-down kernel, that accepts only verified kernel modules, will allow
+digest lists to be added or deleted only though a well-defined and
+monitored interface. In this situation, the root user is assumed to be
+untrusted, i.e. it cannot subvert without being detected the mandatory
+policy stating which files are accessible by the system.
+
+Adoption
+--------
+
+A former version of Huawei Digest Lists is used in the following OSes:
+
+- openEuler 20.09
+  https://github.com/openeuler-mirror/kernel/tree/openEuler-20.09
+
+- openEuler 21.03
+  https://github.com/openeuler-mirror/kernel/tree/openEuler-21.03
+
+Originally, Huawei Digest Lists was part of IMA. In this version,
+it has been redesigned as a standalone module with an API that makes
+its functionality accessible by IMA and, eventually, other subsystems.
+
+User Space Support
+------------------
+
+Digest lists can be generated and managed with ``digest-list-tools``:
+
+https://github.com/openeuler-mirror/digest-list-tools
+
+It includes two main applications:
+
+- ``gen_digest_lists``: generates digest lists from files in the
+  filesystem or from the RPM database (more digest list sources can be
+  supported);
+- ``manage_digest_lists``: converts and uploads digest lists to the
+  kernel.
+
+
+Simple Usage Example
+--------------------
+
+1. Digest list generation (RPM headers and their signature are copied to
+   the specified directory):
+
+.. code-block:: bash
+
+ # mkdir /etc/digest_lists
+ # gen_digest_lists -t file -f rpm+db -d /etc/digest_lists -o add
+
+
+2. Digest list upload with the user space parser:
+
+.. code-block:: bash
+
+ # manage_digest_lists -p add-digest -d /etc/digest_lists
+
+3. First digest list query:
+
+.. code-block:: bash
+
+ # echo sha256-$(sha256sum /bin/cat) > /sys/kernel/security/integrity/digest_lists/digest_query
+ # cat /sys/kernel/security/integrity/digest_lists/digest_query
+   sha256-[...]-0-file_list-rpm-coreutils-8.32-18.fc33.x86_64 (actions: 0): version: 1, algo: sha256, type: 2, modifiers: 1, count: 106, datalen: 3392
+
+4. Second digest list query:
+
+.. code-block:: bash
+
+ # echo sha256-$(sha256sum /bin/zip) > /sys/kernel/security/integrity/digest_lists/digest_query
+ # cat /sys/kernel/security/integrity/digest_lists/digest_query
+   sha256-[...]-0-file_list-rpm-zip-3.0-27.fc33.x86_64 (actions: 0): version: 1, algo: sha256, type: 2, modifiers: 1, count: 4, datalen: 128
+
+
+Architecture
+============
+
+This section introduces the high level architecture.
+
+::
+
+ 5. add/delete from hash table and add refs to digest list
+        +---------------------------------------------+
+        |                            +-----+   +-------------+         +--+
+        |                            | key |-->| digest refs |-->...-->|  |
+        V                            +-----+   +-------------+         +--+
+ +-------------+                     +-----+   +-------------+
+ | digest list |                     | key |-->| digest refs |
+ |  (compact)  |                     +-----+   +-------------+
+ +-------------+                     +-----+   +-------------+
+        ^ 4. copy to                 | key |-->| digest refs |
+        |    kernel memory           +-----+   +-------------+ kernel space
+ --------------------------------------------------------------------------
+        ^                                          ^             user space
+        |<----------------+       3b. upload       |
+ +-------------+   +------------+                  | 6. query digest
+ | digest list |   | user space | 2b. convert
+ |  (compact)  |   |   parser   |
+ +-------------+   +------------+
+ 1a. upload               ^       1b. read
+                          |
+                   +------------+
+                   | RPM header |
+                   +------------+
+
+
+As mentioned before, digest lists can be uploaded directly if they are in
+the compact format (step 1a) or can be uploaded indirectly by the user
+space parser if they are in an alternative format (steps 1b-3b).
+
+During upload, the kernel makes a copy of the digest list to the kernel
+memory (step 4), and creates the necessary structures to index the digests
+(hash table and an array of digest list references to locate the digests in
+the digest list) (step 5).
+
+Finally, digests can be searched from user space through a securityfs file
+(step 6) or by the kernel itself.
diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst
index 16335de04e8c..80877b520403 100644
--- a/Documentation/security/index.rst
+++ b/Documentation/security/index.rst
@@ -17,3 +17,4 @@ Security Documentation
    tpm/index
    digsig
    landlock
+   digest_lists
diff --git a/MAINTAINERS b/MAINTAINERS
index 8c5ee008301a..cba3d82fee43 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8381,6 +8381,13 @@ W:	http://www.st.com/
 F:	Documentation/devicetree/bindings/iio/humidity/st,hts221.yaml
 F:	drivers/iio/humidity/hts221*
 
+HUAWEI DIGEST LISTS
+M:	Roberto Sassu <roberto.sassu@xxxxxxxxxx>
+L:	linux-integrity@xxxxxxxxxxxxxxx
+S:	Supported
+T:	git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity.git
+F:	Documentation/security/digest_lists.rst
+
 HUAWEI ETHERNET DRIVER
 M:	Bin Luo <luobin9@xxxxxxxxxx>
 L:	netdev@xxxxxxxxxxxxxxx
-- 
2.25.1