Re: [PATCH v0 2/2] IMA: Call queue functions to measure keys

Mimi Zohar
2019-12-03T00:02:44Z

Hi Lakshmi,

On Tue, 2019-11-26 at 18:52 -0800, Lakshmi Ramasubramanian wrote:
> Keys should be queued for measurement if custom IMA policies have
> not yet been applied. Keys queued for measurement, if any, need to be
> processed when custom IMA policies have been applied.

Please start with the problem description.  For example, measuring
keys requires loading a custom IMA policy.

>
> This patch adds the call to ima_queue_key_for_measurement() in
> the IMA hook function if ima_process_keys_for_measurement flag is set
> to false. And, the call to ima_process_queued_keys_for_measurement()
> when custom IMA policies have been applied in ima_update_policy().

This reads like pseudo code.  Please summarize the purpose of this
patch.

>
> NOTE:
> If the kernel is built with CONFIG_ASYMMETRIC_PUBLIC_KEY_SUBTYPE
> enabled then the IMA policy should be applied as custom IMA policies.
>
> Keys will be queued up until custom policies are applied and processed
> when custom policies have been applied.
>
> Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
> ---
> security/integrity/ima/ima_asymmetric_keys.c | 16 ++++++++++++++++
> security/integrity/ima/ima_policy.c | 12 ++++++++++++
> 2 files changed, 28 insertions(+)
>
> diff --git a/security/integrity/ima/ima_asymmetric_keys.c b/security/integrity/ima/ima_asymmetric_keys.c
> index 10deb77b22a0..adb7a307190f 100644
> --- a/security/integrity/ima/ima_asymmetric_keys.c
> +++ b/security/integrity/ima/ima_asymmetric_keys.c
> @@ -157,6 +157,8 @@ void ima_post_key_create_or_update(struct key *keyring, struct key *key,
> const void *payload, size_t payload_len,
> unsigned long flags, bool create)
> {
> + bool key_queued = false;
> +
> /* Only asymmetric keys are handled by this hook. */
> if (key->type != &key_type_asymmetric)
> return;
> @@ -164,6 +166,20 @@ void ima_post_key_create_or_update(struct key *keyring, struct key *key,
> if (!payload || (payload_len == 0))
> return;
>
> + if (!ima_process_keys_for_measurement)
> + key_queued = ima_queue_key_for_measurement(keyring,
> + payload,
> + payload_len);
> +
> + /*
> + * Need to check again if the key was queued or not because
> + * ima_process_keys_for_measurement could have flipped from
> + * false to true after it was checked above, but before the key
> + * could be queued by ima_queue_key_for_measurement().
> + */

You're describing a race condition.

> + if (key_queued)
> + return;
> +
> /*
> * keyring->description points to the name of the keyring
> * (such as ".builtin_trusted_keys", ".ima", etc.) to
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index 78b25f083fe1..a2e30a90f97d 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -812,6 +812,18 @@ void ima_update_policy(void)
> kfree(arch_policy_entry);
> }
> ima_update_policy_flag();
> +
> + /*
> + * Custom IMA policies have been setup.
> + * Process key(s) queued up for measurement now.
> + *
> + * NOTE:
> + * Custom IMA policies always overwrite builtin policies
> + * (policies compiled in code). If one wants measurement
> + * of asymmetric keys then it has to be configured in
> + * custom policies and updated here.
> + */

The "NOTE" is over commenting the code and belongs in the patch
description.

> + ima_process_queued_keys_for_measurement();

Overwriting the initial policy is highly recommended, but not everyone
defines a custom policy.  Should there be a time limit or some other
criteria before deleting the key measurement queue?

Mimi

> }
>
> /* Keep the enumeration in sync with the policy_tokens! */