[PATCH v11 0/6] KEYS: Measure keys when they are created or updated

Lakshmi Ramasubramanian
2019-12-11T16:47:35Z

Keys created or updated in the system are currently not measured.
Therefore an attestation service, for instance, would not be able to
attest whether or not the trusted keys keyring(s), for instance, contain
only known good (trusted) keys.

IMA measures system files, command line arguments passed to kexec,
boot aggregate, etc. It can be used to measure keys as well.
But there is no mechanism available in the kernel for IMA to
know when a key is created or updated.

This change aims to address measuring keys created or updated
in the system.

To achieve the above the following changes have been made:

- Added a new IMA hook namely, ima_post_key_create_or_update, which
measures the key. This IMA hook is called from key_create_or_update
function. The key measurement can be controlled through IMA policy.

A new IMA policy function KEY_CHECK has been added to measure keys.
"keyrings=" option can be specified for KEY_CHECK to limit
measuring the keys loaded onto the specified keyrings only.

uid can be specified to further restrict key measurement for keys
created by specific user.

# measure keys loaded onto any keyring
measure func=KEY_CHECK

# measure keys loaded onto the IMA keyring only for root user
measure func=KEY_CHECK uid=0 keyring=".ima"

# measure keys on the BUILTIN and IMA keyrings into a different PCR
measure func=KEY_CHECK keyring=".builtin_trusted_keys|.ima" pcr=11

Testing performed:

* Booted the kernel with this change.
* When KEY_CHECK policy is set IMA measures keys loaded
onto any keyring (keyrings= option not specified).
* Keys are not measured when KEY_CHECK is not set.
* When keyrings= option is specified for KEY_CHECK then only the keys
loaded onto a keyring specified in the option is measured.
* When uid is specified in the policy the key is measured
only when the current user id matches the one given in the policy.
* Added a new key to a keyring.
=> Added keys to .ima and .evm keyrings.
* Added the same key again.
=> Add the same key to .ima and .evm keyrings.

Change Log:

v11:

=> Rebased the changes to v5.5-rc1
=> Check user id (uid) before checking keyring in ima_match_keyring()
=> Updated patch descriptions per Mimi's feedback.

v10:

=> Added check for user id (uid) in ima_match_keyring()
=> Updated ima_match_keyring() function to use strsep() to
check for keyring match.
=> Edited key measurement validation description.

v9:

=> Changed the measured key data from just the public key to
the entire payload passed to key_create_or_update() function.
This payload is the certificate from which the key is created
or updated by key_create_or_update() function.
=> Added check in process_buffer_measurement() to return
immediately if ima_policy_flag is set to zero.

v8:

=> Updated ima_match_keyring() function to check for
whole keyring name match.
=> Used CONFIG_ASYMMETRIC_PUBLIC_KEY_SUBTYPE instead of
CONFIG_KEYS to build ima_asymmetric_keys.c and enable
the IMA hook to measure keys since this config handles
the required build time dependencies better.
=> Updated patch description to illustrate verification
of key measurement.

v7:

=> Removed CONFIG_IMA_MEASURE_ASYMMETRIC_KEYS option and used
CONFIG_KEYS instead for ima_asymmetric_keys.c
=> Added the patches related to "keyrings=" option support to
this patch set.

v6:

=> Rebased the changes to v5.4-rc7
=> Renamed KEYRING_CHECK to KEY_CHECK per Mimi's suggestion.
=> Excluded the patches that add support for limiting key
measurement to specific keyrings ("keyrings=" option
for "measure func=KEY_CHECK" in the IMA policy).
Also, excluded the patches that add support for deferred
processing of keys (queue support).
These patches will be added in separate patch sets later.

v5:

=> Reorganized the patches to add measurement of keys through
the IMA hook without any queuing and then added queuing support.
=> Updated the queuing functions to minimize code executed inside mutex.
=> Process queued keys after custom IMA policies have been applied.

v4:

=> Rebased the changes to v5.4-rc3
=> Applied the following dependent patch set first
and then added new changes.
https://lore.kernel.org/linux-integrity/1572492694-6520-1-git-send-email-zohar@linux.ibm.com
=> Refactored the patch set to separate out changes related to
func KEYRING_CHECK and options keyrings into different patches.
=> Moved the functions to queue and dequeue keys for measurement
from ima_queue.c to a new file ima_asymmetric_keys.c.
=> Added a new config namely CONFIG_IMA_MEASURE_ASYMMETRIC_KEYS
to compile ima_asymmetric_keys.c

v3:

=> Added KEYRING_CHECK for measuring keys. This can optionally specify
keyrings to measure.
=> Updated ima_get_action() and related functions to return
the keyrings if specified in the policy.
=> process_buffer_measurement() function is updated to take keyring
as a parameter. The key will be measured if the policy includes
the keyring in the list of measured keyrings. If the policy does not
specify any keyrings then all keys are measured.

v2:

=> Per suggestion from Mimi reordered the patch set to first
enable measuring keys added or updated in the system.
And, then scope the measurement to keys added to
builtin_trusted_keys keyring through ima policy.
=> Removed security_key_create_or_update function and instead
call ima hook, to measure the key, directly from
key_create_or_update function.

v1:

=> LSM function for key_create_or_update. It calls ima.
=> Added ima hook for measuring keys
=> ima measures keys based on ima policy.

v0:

=> Added LSM hook for key_create_or_update.
=> Measure keys added to builtin or secondary trusted keys keyring.

Lakshmi Ramasubramanian (6):
IMA: Check IMA policy flag
IMA: Add KEY_CHECK func to measure keys
IMA: Define an IMA hook to measure keys
KEYS: Call the IMA hook to measure keys
IMA: Add support to limit measuring keys
IMA: Read keyrings= option from the IMA policy

Documentation/ABI/testing/ima_policy | 16 +++-
include/linux/ima.h | 14 +++
security/integrity/ima/Makefile | 1 +
security/integrity/ima/ima.h | 9 +-
security/integrity/ima/ima_api.c | 8 +-
security/integrity/ima/ima_appraise.c | 4 +-
security/integrity/ima/ima_asymmetric_keys.c | 58 ++++++++++++
security/integrity/ima/ima_main.c | 12 ++-
security/integrity/ima/ima_policy.c | 95 ++++++++++++++++++--
security/keys/key.c | 10 +++
10 files changed, 207 insertions(+), 20 deletions(-)
create mode 100644 security/integrity/ima/ima_asymmetric_keys.c

--
2.17.1