On 4/29/2026 9:03 AM, Roberto Sassu wrote:
From: Roberto Sassu <[email protected]>
Introduction
============
The IMA measurements list is currently stored in the kernel memory.
Memory occupation grows linearly with the number of entries, 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 components.
Storing the IMA measurements list 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 allow user space
to remove the desired portion of the measurements list from the kernel.
Usage
=====
The IMA staging mechanism can be enabled from the kernel configuration
with the CONFIG_IMA_STAGING option.
If it is enabled, IMA duplicates the current 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, and
staged measurements are deleted upon confirmation.
This staging process is achieved with the following steps.
1. echo A > <original 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.
This submission proposes two ways for log trimming:
*Favour 1:* Staging with prompt
*Favour 2:* stage and delete N
Functionally, both approaches address the same problem, but *Favour 2
*is the
stronger design and should be preferred. There is no good reason to keep
*Favour 1.*
From a kernel implementation perspective, *Favour 2 *is more efficient
because it
minimizes the time spent holding the list lock (can’t be shorter). It
also substantially
reduces the amount of kernel-side logic, removing nearly half of the
code required
by the alternative approach.
From a user-space perspective, *Favour 2 *results in a much cleaner
model. It avoids
the need to track and reconcile both old and staged lists in user space
as well as
two lists (cur and staged) in the kernel space, which simplifies log
trimming logic
and reduces maintenance overhead. In addition, it preserves the existing
external
behavior by not exposing any staged list to user space.
Overall, *Favour 2 *provides the same functional result with lower
kernel complexity,
shorter kernel list lock hold time, and a simpler user-space interface.
For those
reasons, it is the preferable approach and *Favour 1* does not appear to
offer sufficient
justification to keep both implementations.
Steven
Management of Staged Measurements
=================================
Since with the staging mechanism measurement entries are removed from
the kernel, the user needs to save the staged ones in a storage and
concatenate them together, so that it can present them to remote
attestation agents as if staging was never done.
Patch set content
=================
Patches 1-8 are preparatory patches to quickly replace the hash table,
maintain separate counters for the different measurements list types,
mediate access to the measurements list interface, and simplify the staging
patches.
Patch 9 introduces the staging with prompt flavor. Patch 10 makes it
possible to flush the hash table when deleting all the staged measurements.
Patch 11 introduces the staging and deleting flavor. Patch 12 avoids
measurements entries to be stored twice if there is contention between the
measurements interfaces and kexec. Patch 13 adds the documentation of the
staging mechanism.
Changelog
=========
v4:
- Add write permission to the original measurement interface, and move
the A and N staging commands to that interface
- Explain better the two staging flavors and highlight that the staging
and delete only stages measurements internally
- Rename ima_queue_staged_delete_partial() to ima_queue_delete_partial()
- Replace ima_staged_measurements_prepended with per measurements list
flag to avoid copying staged and active list measurements twice
- Optimize the staging and deleting flavor by locklessly determining the
cut position in the active list, and immediately deleting entries
without explicit staging and splicing (suggested by Steven Chen)
v3:
- Add Kconfig option to enable the staging mechanism (suggested by Mimi)
- Change the meaning of BINARY_STAGED to be just the staged measurements
- Separate the two staging flavors in two different functions:
ima_queue_staged_delete_all() for staging with prompt,
ima_queue_staged_delete_partial() for staging and deleting
- Delete N entries without staging first (suggested by Mimi)
- Avoid duplicate staged entries if there is contention between the
measurements list interfaces and kexec
v2:
- New patch to move measurements and violation counters outside the
ima_h_table structure
- New patch to quickly replace the hash table
- Forbid partial deletion when flushing hash table (suggested by Mimi)
- Ignore ima_flush_htable if CONFIG_IMA_DISABLE_HTABLE is enabled
- BINARY_SIZE_* renamed to BINARY_* for better clarity
- Removed ima_measurements_staged_exist and testing list empty instead
- ima_queue_stage_trim() and ima_queue_delete_staged_trimmed() renamed to
ima_queue_stage() and ima_queue_delete_staged()
- New delete interval [1, ULONG_MAX - 1]
- Rename ima_measure_lock to ima_measure_mutex
- Move seq_open() and seq_release() outside the ima_measure_mutex lock
- Drop ima_measurements_staged_read() and use seq_read() instead
- Optimize create_securityfs_measurement_lists() changes
- New file name format with _staged suffix at the end of the file name
- Use _rcu list variant in ima_dump_measurement_list()
- Remove support for direct trimming and splice the remaining entries to
the active list (suggested by Mimi)
- Hot swap the hash table if flushing is requested
v1:
- Support for direct trimming without staging
- Support unstaging on kexec (requested by Gregory Lumen)
Roberto Sassu (13):
ima: Remove ima_h_table structure
ima: Replace static htable queue with dynamically allocated array
ima: Introduce per binary measurements list type ima_num_entries
counter
ima: Introduce per binary measurements list type binary_runtime_size
value
ima: Introduce _ima_measurements_start() and _ima_measurements_next()
ima: Mediate open/release method of the measurements list
ima: Use snprintf() in create_securityfs_measurement_lists
ima: Introduce ima_dump_measurement()
ima: Add support for staging measurements with prompt
ima: Add support for flushing the hash table when staging measurements
ima: Support staging and deleting N measurements entries
ima: Return error on deleting measurements already copied during kexec
doc: security: Add documentation of the IMA staging mechanism
.../admin-guide/kernel-parameters.txt | 4 +
Documentation/security/IMA-staging.rst | 163 +++++++++
Documentation/security/index.rst | 1 +
MAINTAINERS | 2 +
security/integrity/ima/Kconfig | 16 +
security/integrity/ima/ima.h | 32 +-
security/integrity/ima/ima_api.c | 2 +-
security/integrity/ima/ima_fs.c | 315 ++++++++++++++++--
security/integrity/ima/ima_init.c | 5 +
security/integrity/ima/ima_kexec.c | 53 ++-
security/integrity/ima/ima_queue.c | 283 ++++++++++++++--
11 files changed, 803 insertions(+), 73 deletions(-)
create mode 100644 Documentation/security/IMA-staging.rst
