[PATCH v7 12/12] doc: security: Add documentation of exporting and deleting IMA measurements

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu at huawei.com>

Add the documentation of exporting and deleting IMA measurements in
Documentation/security/IMA-export-delete.rst.

Also add the missing Documentation/security/IMA-templates.rst file in
MAINTAINERS.

Link: https://github.com/linux-integrity/linux/issues/1
Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
---
 Documentation/security/IMA-export-delete.rst | 203 +++++++++++++++++++
 Documentation/security/index.rst             |   1 +
 MAINTAINERS                                  |   2 +
 3 files changed, 206 insertions(+)
 create mode 100644 Documentation/security/IMA-export-delete.rst

diff --git a/Documentation/security/IMA-export-delete.rst b/Documentation/security/IMA-export-delete.rst
new file mode 100644
index 000000000000..1600ead03b03
--- /dev/null
+++ b/Documentation/security/IMA-export-delete.rst
@@ -0,0 +1,203 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+IMA Measurements Export and Delete
+==================================
+
+
+Introduction
+============
+
+The IMA measurements list is currently stored in the kernel memory. Memory
+occupation grows linearly with the number of records, and can become a
+problem especially in environments with reduced resources.
+
+While there is an advantage in keeping the IMA measurements list in kernel
+memory, so that it is always available for reading from the securityfs
+interfaces, storing it elsewhere would make it possible to free precious
+memory for other kernel usage.
+
+The IMA measurements list needs to be retained and safely stored for new
+attestation servers to validate it. Assuming the IMA measurements list is
+properly saved, storing it outside the kernel does not introduce security
+issues, since its integrity is anyway protected by the TPM.
+
+Hence, the new IMA staging mechanism is introduced to export IMA
+measurements to user space and delete them from kernel space.
+
+Staging consists in atomically moving the current measurements list to a
+temporary list, so that measurements can be deleted afterwards. The staging
+operation locks the hot path (racing with addition of new measurements) for
+a very short time, only for swapping the list pointers. Deletion of the
+measurements instead is done locklessly, away from the hot path.
+
+There are two flavors of the staging mechanism. In the staging with prompt,
+all current measurements are staged, read and deleted upon confirmation. In
+the staging and deleting flavor, N measurements are staged from the
+beginning of the current measurements list and immediately deleted without
+confirmation.
+
+
+Management of Staged Measurements
+=================================
+
+Since with the staging mechanism measurement records are removed from the
+kernel, the staged measurements need to be saved in a storage and
+concatenated together, so that they can be presented during remote
+attestation as if staging was never done. This task can be accomplished by
+a remote attestation agent modified to support staging, or a system
+service.
+
+Coordination is necessary in the case where there are multiple actors
+requesting measurements to be staged.
+
+In the staging with prompt case, the measurement interfaces can be accessed
+only by one actor (writer) at a time, so the others will get an error until
+the former closes it. Since the actors don't care about N, when they gain
+access to the interface, they will get all the staged measurements at the
+time of their request.
+
+In the case of staging and deleting, coordination is more important, since
+there is the risk that two actors unaware of each other compute the value N
+on the current measurements list and request IMA to stage N twice.
+
+
+Remote Attestation Agent Workflow
+=================================
+
+Remote attestation agents can be configured to always present all the
+measurements to the remote verifiers or, alternatively, to only provide the
+measurements that have not been verified yet by the remote verifiers.
+
+In the latter case, determining which measurements need to be sent and
+verified must solely depend on the remote verifier. The remote attestation
+agent can proactively send partial measurements, at the condition that they
+are the ones that the remote verifier needs.
+
+An agent can rely on one of the supported staging methods to proactively
+send to a remote verifier the measurements since the previous request up
+to the ones that verify the TPM quote obtained in the current request.
+The workflow with each staging method is the following.
+
+With staging with prompt, the agent stages the current measurements list,
+reads and stores the measurements in a storage and immediately requests
+IMA to delete the staged measurements from kernel memory. Afterwards, it
+calculates N by replaying the PCR extend on the stored measurements until
+the calculated PCRs match the quoted PCRs. It then keeps the measurements
+in excess for the next attestation request.
+
+At the next attestation request, the agent performs the same steps above,
+and concatenates the new measurements to the ones in excess from the
+previous request. Also in this case, the agent replays the PCR extend until
+it matches the currently quoted PCRs, keeps the measurements in excess and
+presents the new N measurement records to the remote attestation server.
+
+With the staging and deleting method, the agent reads the current
+measurements list, calculates N and requests IMA to delete only those. The
+measurements in excess are kept in the IMA measurements list and can be
+retrieved at the next remote attestation request.
+
+While keeping only the excess measurements in the storage could be
+sufficient to serve the requests of a remote verifier, it is advised to
+keep all the obtained measurements locally, as they might be needed for the
+attestation with a different remote verifier.
+
+
+Usage
+=====
+
+The IMA staging mechanism can be enabled from the kernel configuration with
+the CONFIG_IMA_STAGING option. This option prevents inadvertently removing
+the IMA measurement list on systems which do not properly save it.
+
+If the option is enabled, IMA duplicates the current securityfs
+measurements interfaces (both binary and ASCII), by adding the ``_staged``
+file suffix. Both the original and the staging interfaces gain the write
+permission for the root user and group, but require the process to have
+CAP_SYS_ADMIN set.
+
+The staging mechanism supports two flavors.
+
+
+Staging with prompt
+~~~~~~~~~~~~~~~~~~~
+
+The current measurements list is moved to a temporary staging area,
+allowing it to be saved to external storage, before being deleted upon
+confirmation.
+
+This staging process is achieved with the following steps.
+
+ 1. ``echo A > <_staged interface>``: the user requests IMA to stage the
+    entire measurements list;
+ 2. ``cat <_staged interface>``: the user reads the staged measurements;
+ 3. ``echo D > <_staged interface>``: the user requests IMA to delete
+    staged measurements.
+
+
+Staging and deleting
+~~~~~~~~~~~~~~~~~~~~
+
+N measurements are staged to a temporary staging area, and immediately
+deleted without further confirmation.
+
+This staging process is achieved with the following steps.
+
+ 1. ``cat <original interface>``: the user reads the current measurements
+    list and determines what the value N for staging should be;
+ 2. ``echo N > <original interface>``: the user requests IMA to delete N
+    measurements from the current measurements list.
+
+
+Interface Access
+================
+
+In order to avoid the IMA measurements list being suddenly truncated by the
+staging mechanism during a read, or having multiple concurrent staging, a
+semaphore-like locking scheme has been implemented on all the measurements
+list interfaces.
+
+Multiple readers can access concurrently the original and staged
+interfaces, and they can be in mutual exclusion with one writer. In order
+to see the same state across all the measurement interfaces, the same
+writer is allowed to open multiple interfaces for write or read/write.
+
+If an illegal access occurs, the open to the measurements list interface is
+denied.
+
+
+Kexec
+=====
+
+In the event a kexec() system call occurs between staging and deleting, the
+staged measurement records are marshalled before the current measurements
+list, so that they are both available when the secondary kernel starts.
+
+If measurement is suspended before requesting to delete staged or current
+measurements, IMA returns an error to user space to let it know that
+marshalling is already in progress, so that it does not save the
+measurements twice.
+
+IMA also disallows staging when suspending measurement, to avoid the
+situation where neither measurements are carried over to the secondary
+kernel, nor they are saved by user space to the storage.
+
+
+Hash table
+==========
+
+By default, the template digest of staged measurement records are kept in
+kernel memory (only template data are freed), to be able to detect
+duplicate records independently of staging.
+
+The new kernel option ``ima_flush_htable`` has been introduced to
+explicitly request a complete deletion of the staged measurements, for
+maximum kernel memory saving. If the option has been specified, duplicate
+records are still avoided on records of the current measurements list,
+but there can be duplicates between different groups of staged
+measurements.
+
+Flushing the hash table is supported only for the staging with prompt
+flavor. For the staging and deleting flavor, it would have been necessary
+to lock the hot path adding new measurements for the time needed to remove
+each selected measurement individually.
diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst
index 3e0a7114a862..00650dcf38cb 100644
--- a/Documentation/security/index.rst
+++ b/Documentation/security/index.rst
@@ -8,6 +8,7 @@ Security Documentation
    credentials
    snp-tdx-threat-model
    IMA-templates
+   IMA-export-delete
    keys/index
    lsm
    lsm-development
diff --git a/MAINTAINERS b/MAINTAINERS
index 461a3eed6129..70ff6bae3493 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -12752,6 +12752,8 @@ R:	Eric Snowberg <eric.snowberg at oracle.com>
 L:	linux-integrity at vger.kernel.org
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity.git
+F:	Documentation/security/IMA-export-delete.rst
+F:	Documentation/security/IMA-templates.rst
 F:	include/linux/secure_boot.h
 F:	security/integrity/
 F:	security/integrity/ima/
-- 
2.43.0