[PATCH v3 0/2] IMA: Deferred measurement of keys

[PATCH v3 0/2] IMA: Deferred measurement of keys

[Lakshmi Ramasubramanian]

This patchset extends the previous version[1] by adding support for
deferred processing of keys.

With the patchset referenced above, the IMA subsystem supports
measuring asymmetric keys when the key is created or updated.
But keys created or updated before a custom IMA policy is loaded
are currently not measured. This includes keys added to, for instance,
.builtin_trusted_keys which happens early in the boot process.

This change adds support for queuing keys created or updated before
a custom IMA policy is loaded. The queued keys are processed when
a custom policy is loaded. Keys created or updated after a custom policy
is loaded are measured immediately (not queued).

If the kernel is built with both CONFIG_IMA and
CONFIG_ASYMMETRIC_PUBLIC_KEY_SUBTYPE enabled then the IMA policy
must be applied as a custom policy. Not providing a custom policy
in the above configuration would result in asymmeteric keys being queued
until a custom policy is loaded. This is by design.

[1] https://lore.kernel.org/linux-integrity/20191211164707.4698-1-nramas@linux.microsoft.com/

Testing performed:

  * Booted the kernel with this change.
  * Added .builtin_trusted_keys in "keyrings=" option in
    the IMA policy and verified the keys added to this
    keyring are measured.
  * Specified only func=KEY_CHECK and not "keyrings=" option,
    and verified the keys added to builtin_trusted_keys keyring
    are processed.
  * Added keys at runtime and verified they are measured
    if the IMA policy permitted.
      => For example, added keys to .ima keyring and verified.

Changelog:

  v3

  => Defined ima_process_keys flag to be static.
  => Set ima_process_keys with ima_keys_mutex held.
  => Added a comment in ima_process_queued_keys() function
     to state the use of temporary list for keys.

  v2

  => Rebased the changes to v5.5-rc1
  => Updated function names, variable names, and code comments
     to be less verbose.

  v1

  => Code cleanup

  v0

  => Based changes on v5.4-rc8
  => The following patchsets should be applied in that order
     https://lore.kernel.org/linux-integrity/1572492694-6520-1-git-send-email-zohar@linux.ibm.com
     https://lore.kernel.org/linux-integrity/20191204224131.3384-1-nramas@linux.microsoft.com/
  => Added functions to queue and dequeue keys, and process
     the queued keys when custom IMA policies are applied.


Lakshmi Ramasubramanian (2):
  IMA: Define workqueue for early boot key measurements
  IMA: Call workqueue functions to measure queued keys

 security/integrity/ima/ima.h                 |  15 +++
 security/integrity/ima/ima_asymmetric_keys.c | 127 +++++++++++++++++++
 security/integrity/ima/ima_policy.c          |   3 +
 3 files changed, 145 insertions(+)

-- 
2.17.1

[PATCH v3 1/2] IMA: Define workqueue for early boot "key" measurements

[Lakshmi Ramasubramanian]

Measuring keys requires a custom IMA policy to be loaded.
Keys created or updated before a custom IMA policy is loaded should
be queued and the keys should be processed after a custom policy
is loaded.

This patch defines workqueue for queuing keys when a custom IMA policy
has not yet been loaded.

A flag namely ima_process_keys is used to check if the key should be
queued or should be processed immediately.

Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
---
 security/integrity/ima/ima.h                 |  15 +++
 security/integrity/ima/ima_asymmetric_keys.c | 119 +++++++++++++++++++
 2 files changed, 134 insertions(+)

diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index f06238e41a7c..97f8a4078483 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -205,6 +205,21 @@ extern const char *const func_tokens[];
 
 struct modsig;
 
+#ifdef CONFIG_ASYMMETRIC_PUBLIC_KEY_SUBTYPE
+/*
+ * To track keys that need to be measured.
+ */
+struct ima_key_entry {
+	struct list_head list;
+	void *payload;
+	size_t payload_len;
+	char *keyring_name;
+};
+void ima_process_queued_keys(void);
+#else
+static inline void ima_process_queued_keys(void) {}
+#endif /* CONFIG_ASYMMETRIC_PUBLIC_KEY_SUBTYPE */
+
 /* LIM API function definitions */
 int ima_get_action(struct inode *inode, const struct cred *cred, u32 secid,
 		   int mask, enum ima_hooks func, int *pcr,
diff --git a/security/integrity/ima/ima_asymmetric_keys.c b/security/integrity/ima/ima_asymmetric_keys.c
index fea2e7dd3b09..a1781348ce69 100644
--- a/security/integrity/ima/ima_asymmetric_keys.c
+++ b/security/integrity/ima/ima_asymmetric_keys.c
@@ -14,6 +14,125 @@
 #include <keys/asymmetric-type.h>
 #include "ima.h"
 
+/*
+ * Flag to indicate whether a key can be processed
+ * right away or should be queued for processing later.
+ */
+static bool ima_process_keys;
+
+/*
+ * To synchronize access to the list of keys that need to be measured
+ */
+static DEFINE_MUTEX(ima_keys_mutex);
+static LIST_HEAD(ima_keys);
+
+static void ima_free_key_entry(struct ima_key_entry *entry)
+{
+	if (entry) {
+		kfree(entry->payload);
+		kfree(entry->keyring_name);
+		kfree(entry);
+	}
+}
+
+static struct ima_key_entry *ima_alloc_key_entry(
+	struct key *keyring,
+	const void *payload, size_t payload_len)
+{
+	int rc = 0;
+	struct ima_key_entry *entry;
+
+	entry = kzalloc(sizeof(*entry), GFP_KERNEL);
+	if (entry) {
+		entry->payload = kmemdup(payload, payload_len, GFP_KERNEL);
+		entry->keyring_name = kstrdup(keyring->description,
+					      GFP_KERNEL);
+		entry->payload_len = payload_len;
+	}
+
+	if ((entry == NULL) || (entry->payload == NULL) ||
+	    (entry->keyring_name == NULL)) {
+		rc = -ENOMEM;
+		goto out;
+	}
+
+	INIT_LIST_HEAD(&entry->list);
+
+out:
+	if (rc) {
+		ima_free_key_entry(entry);
+		entry = NULL;
+	}
+
+	return entry;
+}
+
+bool ima_queue_key(struct key *keyring, const void *payload,
+		   size_t payload_len)
+{
+	bool queued = false;
+	struct ima_key_entry *entry;
+
+	entry = ima_alloc_key_entry(keyring, payload, payload_len);
+	if (!entry)
+		return false;
+
+	mutex_lock(&ima_keys_mutex);
+	if (!ima_process_keys) {
+		list_add_tail(&entry->list, &ima_keys);
+		queued = true;
+	}
+	mutex_unlock(&ima_keys_mutex);
+
+	if (!queued)
+		ima_free_key_entry(entry);
+
+	return queued;
+}
+
+/*
+ * ima_process_queued_keys() - process keys queued for measurement
+ *
+ * This function sets ima_process_keys to true and processes queued keys.
+ * From here on keys will be processed right away (not queued).
+ */
+void ima_process_queued_keys(void)
+{
+	struct ima_key_entry *entry, *tmp;
+	LIST_HEAD(temp_ima_keys);
+
+	if (ima_process_keys)
+		return;
+
+	/*
+	 * To avoid holding the mutex when processing queued keys,
+	 * transfer the queued keys with the mutex held to a temp list,
+	 * release the mutex, and then process the queued keys from
+	 * the temp list.
+	 *
+	 * Since ima_process_keys is set to true, any new key will be
+	 * processed immediately and not be queued.
+	 */
+	INIT_LIST_HEAD(&temp_ima_keys);
+
+	mutex_lock(&ima_keys_mutex);
+
+	ima_process_keys = true;
+
+	list_for_each_entry_safe(entry, tmp, &ima_keys, list)
+		list_move_tail(&entry->list, &temp_ima_keys);
+
+	mutex_unlock(&ima_keys_mutex);
+
+	list_for_each_entry_safe(entry, tmp, &temp_ima_keys, list) {
+		process_buffer_measurement(entry->payload, entry->payload_len,
+					   entry->keyring_name, KEY_CHECK, 0,
+					   entry->keyring_name);
+		list_del(&entry->list);
+		ima_free_key_entry(entry);
+	}
+}
+
 /**
  * ima_post_key_create_or_update - measure asymmetric keys
  * @keyring: keyring to which the key is linked to
-- 
2.17.1

[PATCH v3 2/2] IMA: Call workqueue functions to measure queued keys

[Lakshmi Ramasubramanian]

Measuring keys requires a custom IMA policy to be loaded.
Keys should be queued for measurement if a custom IMA policy
is not yet loaded. Keys queued for measurement, if any, should be
processed when a custom IMA policy is loaded.

This patch updates the IMA hook function ima_post_key_create_or_update()
to queue the key if a custom IMA policy has not yet been loaded.
And, ima_update_policy() function, which is called when
a custom IMA policy is loaded, is updated to process queued keys.

Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
---
 security/integrity/ima/ima_asymmetric_keys.c | 8 ++++++++
 security/integrity/ima/ima_policy.c          | 3 +++
 2 files changed, 11 insertions(+)

diff --git a/security/integrity/ima/ima_asymmetric_keys.c b/security/integrity/ima/ima_asymmetric_keys.c
index a1781348ce69..132db04550e9 100644
--- a/security/integrity/ima/ima_asymmetric_keys.c
+++ b/security/integrity/ima/ima_asymmetric_keys.c
@@ -149,6 +149,8 @@ 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 queued = false;
+
 	/* Only asymmetric keys are handled by this hook. */
 	if (key->type != &key_type_asymmetric)
 		return;
@@ -156,6 +158,12 @@ void ima_post_key_create_or_update(struct key *keyring, struct key *key,
 	if (!payload || (payload_len == 0))
 		return;
 
+	if (!ima_process_keys)
+		queued = ima_queue_key(keyring, payload, payload_len);
+
+	if (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 a4dde9d575b2..04b9c6c555de 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -807,6 +807,9 @@ void ima_update_policy(void)
 		kfree(arch_policy_entry);
 	}
 	ima_update_policy_flag();
+
+	/* Custom IMA policy has been loaded */
+	ima_process_queued_keys();
 }
 
 /* Keep the enumeration in sync with the policy_tokens! */
-- 
2.17.1

Re: [PATCH v3 1/2] IMA: Define workqueue for early boot "key" measurements

[Mimi Zohar]

> +/*
> + * ima_process_queued_keys() - process keys queued for measurement
> + *
> + * This function sets ima_process_keys to true and processes queued keys.
> + * From here on keys will be processed right away (not queued).
> + */
> +void ima_process_queued_keys(void)
> +{
> +	struct ima_key_entry *entry, *tmp;
> +	LIST_HEAD(temp_ima_keys);
> +
> +	if (ima_process_keys)
> +		return;
> +
> +	/*
> +	 * To avoid holding the mutex when processing queued keys,
> +	 * transfer the queued keys with the mutex held to a temp list,
> +	 * release the mutex, and then process the queued keys from
> +	 * the temp list.
> +	 *
> +	 * Since ima_process_keys is set to true, any new key will be
> +	 * processed immediately and not be queued.
> +	 */
> +	INIT_LIST_HEAD(&temp_ima_keys);
> +
> +	mutex_lock(&ima_keys_mutex);

Don't you need a test here, before setting ima_process_keys?

	if (ima_process_keys)
		return;

Mimi

> +
> +	ima_process_keys = true;
> +
> +	list_for_each_entry_safe(entry, tmp, &ima_keys, list)
> +		list_move_tail(&entry->list, &temp_ima_keys);
> +
> +	mutex_unlock(&ima_keys_mutex);
> +
> +	list_for_each_entry_safe(entry, tmp, &temp_ima_keys, list) {
> +		process_buffer_measurement(entry->payload, entry->payload_len,
> +					   entry->keyring_name, KEY_CHECK, 0,
> +					   entry->keyring_name);
> +		list_del(&entry->list);
> +		ima_free_key_entry(entry);
> +	}
> +}
> +
> 

Re: [PATCH v3 1/2] IMA: Define workqueue for early boot "key" measurements

[Lakshmi Ramasubramanian]

On 12/12/19 5:55 PM, Mimi Zohar wrote:
>> +/*
>> + * ima_process_queued_keys() - process keys queued for measurement
>> + *
>> + * This function sets ima_process_keys to true and processes queued keys.
>> + * From here on keys will be processed right away (not queued).
>> + */
>> +void ima_process_queued_keys(void)
>> +{
>> +	struct ima_key_entry *entry, *tmp;
>> +	LIST_HEAD(temp_ima_keys);
>> +
>> +	if (ima_process_keys)
>> +		return;
>> +
>> +	/*
>> +	 * To avoid holding the mutex when processing queued keys,
>> +	 * transfer the queued keys with the mutex held to a temp list,
>> +	 * release the mutex, and then process the queued keys from
>> +	 * the temp list.
>> +	 *
>> +	 * Since ima_process_keys is set to true, any new key will be
>> +	 * processed immediately and not be queued.
>> +	 */
>> +	INIT_LIST_HEAD(&temp_ima_keys);
>> +
>> +	mutex_lock(&ima_keys_mutex);
> 
> Don't you need a test here, before setting ima_process_keys?
> 
> 	if (ima_process_keys)
> 		return;
> 
> Mimi

That check is done before the comment - at the start of 
ima_process_queued_keys().

+void ima_process_queued_keys(void)
+{
+	struct ima_key_entry *entry, *tmp;
+	LIST_HEAD(temp_ima_keys);
+
+	if (ima_process_keys)
+		return;

thanks,
  -lakshmi

Re: [PATCH v3 1/2] IMA: Define workqueue for early boot "key" measurements

[Mimi Zohar]

On Thu, 2019-12-12 at 17:59 -0800, Lakshmi Ramasubramanian wrote:
> On 12/12/19 5:55 PM, Mimi Zohar wrote:
> >> +/*
> >> + * ima_process_queued_keys() - process keys queued for measurement
> >> + *
> >> + * This function sets ima_process_keys to true and processes queued keys.
> >> + * From here on keys will be processed right away (not queued).
> >> + */
> >> +void ima_process_queued_keys(void)
> >> +{
> >> +	struct ima_key_entry *entry, *tmp;
> >> +	LIST_HEAD(temp_ima_keys);
> >> +
> >> +	if (ima_process_keys)
> >> +		return;
> >> +
> >> +	/*
> >> +	 * To avoid holding the mutex when processing queued keys,
> >> +	 * transfer the queued keys with the mutex held to a temp list,
> >> +	 * release the mutex, and then process the queued keys from
> >> +	 * the temp list.
> >> +	 *
> >> +	 * Since ima_process_keys is set to true, any new key will be
> >> +	 * processed immediately and not be queued.
> >> +	 */
> >> +	INIT_LIST_HEAD(&temp_ima_keys);
> >> +
> >> +	mutex_lock(&ima_keys_mutex);
> > 
> > Don't you need a test here, before setting ima_process_keys?
> > 
> > 	if (ima_process_keys)
> > 		return;
> > 
> > Mimi
> 
> That check is done before the comment - at the start of 
> ima_process_queued_keys().

The first test prevents taking the mutex unnecessarily.

Mimi


> +void ima_process_queued_keys(void)
> +{
> +	struct ima_key_entry *entry, *tmp;
> +	LIST_HEAD(temp_ima_keys);
> +
> +	if (ima_process_keys)
> +		return;
> 
> thanks,
>   -lakshmi

Re: [PATCH v3 1/2] IMA: Define workqueue for early boot "key" measurements

[Lakshmi Ramasubramanian]

On 12/12/2019 6:32 PM, Mimi Zohar wrote:

>>>
>>> Don't you need a test here, before setting ima_process_keys?
>>>
>>> 	if (ima_process_keys)
>>> 		return;
>>>
>>> Mimi
>>
>> That check is done before the comment - at the start of
>> ima_process_queued_keys().
> 
> The first test prevents taking the mutex unnecessarily.
> 
> Mimi

I am trying to understand your concern here. Could you please clarify?

  => If ima_process_keys is false
       -> With the mutex held, should check ima_process_keys again 
before setting?

Let's say 2 or more threads are racing in calling ima_process_queued_keys():

The 1st one will set ima_process_keys and process queued keys.

The 2nd and subsequent ones - even if they have gone past the initial 
check, will find an empty list of keys (the list "ima_keys") when they 
take the mutex. So they'll not process any keys.

thanks,
  -lakshmi

Re: [PATCH v3 1/2] IMA: Define workqueue for early boot "key" measurements

[Mimi Zohar]

On Thu, 2019-12-12 at 18:59 -0800, Lakshmi Ramasubramanian wrote:
> On 12/12/2019 6:32 PM, Mimi Zohar wrote:
> 
> >>>
> >>> Don't you need a test here, before setting ima_process_keys?
> >>>
> >>> 	if (ima_process_keys)
> >>> 		return;

> >> That check is done before the comment - at the start of
> >> ima_process_queued_keys().
> > 
> > The first test prevents taking the mutex unnecessarily.
> > 
> 
> I am trying to understand your concern here. Could you please clarify?
> 
>   => If ima_process_keys is false
>        -> With the mutex held, should check ima_process_keys again 
> before setting?
> 
> Let's say 2 or more threads are racing in calling ima_process_queued_keys():
> 
> The 1st one will set ima_process_keys and process queued keys.
> 
> The 2nd and subsequent ones - even if they have gone past the initial 
> check, will find an empty list of keys (the list "ima_keys") when they 
> take the mutex. So they'll not process any keys.

I just need to convince myself that this is correct.  Normally before
reading and writing a flag, there is some sort of locking.  With
taking the mutex before setting the flag, there is now only a lock
around the single writer.

Without taking a lock before reading the flag, will the queue always
be empty is the question.  If it is, then the comment is correct, but
the code assumes not and processes the list again.  Testing the flag
after taking the mutex just re-enforces the comment.

Bottom line, does reading the flag need to be lock protected?

Mimi

Re: [PATCH v3 1/2] IMA: Define workqueue for early boot "key" measurements

[Lakshmi Ramasubramanian]

On 12/13/19 5:06 AM, Mimi Zohar wrote:

> I just need to convince myself that this is correct.  Normally before
> reading and writing a flag, there is some sort of locking.  With
> taking the mutex before setting the flag, there is now only a lock
> around the single writer.
> 
> Without taking a lock before reading the flag, will the queue always
> be empty is the question.  If it is, then the comment is correct, but
> the code assumes not and processes the list again.  Testing the flag
> after taking the mutex just re-enforces the comment.
> 
> Bottom line, does reading the flag need to be lock protected?
> 
> Mimi
> 

I'll change this function to check the flag again after taking the lock 
and process only if the queue has entries. Will send an update today.

Please let me know if you have any concern in other functions in this 
file. I'll address them, if any, in today's update.

thanks,
  -lakshmi

Re: [PATCH v3 1/2] IMA: Define workqueue for early boot "key" measurements

[Mimi Zohar]

On Fri, 2019-12-13 at 08:06 -0500, Mimi Zohar wrote:
> On Thu, 2019-12-12 at 18:59 -0800, Lakshmi Ramasubramanian wrote:
> > On 12/12/2019 6:32 PM, Mimi Zohar wrote:
> > 
> > >>>
> > >>> Don't you need a test here, before setting ima_process_keys?
> > >>>
> > >>> 	if (ima_process_keys)
> > >>> 		return;
> 
> > >> That check is done before the comment - at the start of
> > >> ima_process_queued_keys().
> > > 
> > > The first test prevents taking the mutex unnecessarily.
> > > 
> > 
> > I am trying to understand your concern here. Could you please clarify?
> > 
> >   => If ima_process_keys is false
> >        -> With the mutex held, should check ima_process_keys again 
> > before setting?
> > 
> > Let's say 2 or more threads are racing in calling ima_process_queued_keys():
> > 
> > The 1st one will set ima_process_keys and process queued keys.
> > 
> > The 2nd and subsequent ones - even if they have gone past the initial 
> > check, will find an empty list of keys (the list "ima_keys") when they 
> > take the mutex. So they'll not process any keys.
> 
> I just need to convince myself that this is correct.  Normally before
> reading and writing a flag, there is some sort of locking.  With
> taking the mutex before setting the flag, there is now only a lock
> around the single writer.
> 
> Without taking a lock before reading the flag, will the queue always
> be empty is the question.  If it is, then the comment is correct, but
> the code assumes not and processes the list again.  Testing the flag
> after taking the mutex just re-enforces the comment.
> 
> Bottom line, does reading the flag need to be lock protected?

Reading the flag IS lock protected, just spread across two functions.
 For performance, ima_post_key_create_or_update() checks
ima_process_keys, before calling ima_queue_key(), which takes the
mutex before checking ima_process_keys again.

As long as both the reader and writer, take the mutex before checking
the flag, the locking is fine.  The additional check, before taking
the mutex, is simply for performance.

Mimi