[RFC][PATCH] ima: Use sequence number to wait for policy updates

From: Roberto Sassu
Date: Tue May 07 2024 - 05:29:22 EST


From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

Maintain a global sequence number, and set it to individual policy rules,
when they are created.

When a rule is stale, wait for the global sequence number to increase,
which happens when the LSM policy has been fully updated.

Finally, restart the ima_match_policy() loop, which should not encounter
the stale rule anymore.

On policy update, already increase the rule sequence number, so that a wait
can be implemented for the next policy update.

Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
---
security/integrity/ima/ima_policy.c | 65 +++++++++++++++--------------
1 file changed, 33 insertions(+), 32 deletions(-)

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index c0556907c2e6..4f8275e216fe 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -51,9 +51,12 @@
#define INVALID_PCR(a) (((a) < 0) || \
(a) >= (sizeof_field(struct ima_iint_cache, measured_pcrs) * 8))

+DECLARE_WAIT_QUEUE_HEAD(wait_queue_t);
+
int ima_policy_flag;
static int temp_ima_appraise;
static int build_ima_appraise __ro_after_init;
+static int global_seqno;

atomic_t ima_setxattr_allowed_hash_algorithms;

@@ -122,6 +125,7 @@ struct ima_rule_entry {
struct ima_rule_opt_list *keyrings; /* Measure keys added to these keyrings */
struct ima_rule_opt_list *label; /* Measure data grouped under this label */
struct ima_template_desc *template;
+ int seqno;
};

/*
@@ -442,6 +446,8 @@ static int ima_lsm_update_rule(struct ima_rule_entry *entry)
if (!nentry)
return -ENOMEM;

+ nentry->seqno++;
+
list_replace_rcu(&entry->list, &nentry->list);
synchronize_rcu();
/*
@@ -497,6 +503,8 @@ int ima_lsm_policy_change(struct notifier_block *nb, unsigned long event,
return NOTIFY_DONE;

ima_lsm_update_rules();
+ global_seqno++;
+ wake_up(&wait_queue_t);
return NOTIFY_OK;
}

@@ -560,18 +568,16 @@ static bool ima_match_rule_data(struct ima_rule_entry *rule,
* @mask: requested action (MAY_READ | MAY_WRITE | MAY_APPEND | MAY_EXEC)
* @func_data: func specific data, may be NULL
*
- * Returns true on rule match, false on failure.
+ * Returns 1 on rule match, 0 on mismatch, -ESTALE on stale policy.
*/
-static bool ima_match_rules(struct ima_rule_entry *rule,
- struct mnt_idmap *idmap,
- struct inode *inode, const struct cred *cred,
- u32 secid, enum ima_hooks func, int mask,
- const char *func_data)
+static int ima_match_rules(struct ima_rule_entry *rule,
+ struct mnt_idmap *idmap,
+ struct inode *inode, const struct cred *cred,
+ u32 secid, enum ima_hooks func, int mask,
+ const char *func_data)
{
int i;
- bool result = false;
struct ima_rule_entry *lsm_rule = rule;
- bool rule_reinitialized = false;

if ((rule->flags & IMA_FUNC) &&
(rule->func != func && func != POST_SETATTR))
@@ -642,7 +648,6 @@ static bool ima_match_rules(struct ima_rule_entry *rule,
return false;
}

-retry:
switch (i) {
case LSM_OBJ_USER:
case LSM_OBJ_ROLE:
@@ -663,27 +668,13 @@ static bool ima_match_rules(struct ima_rule_entry *rule,
break;
}

- if (rc == -ESTALE && !rule_reinitialized) {
- lsm_rule = ima_lsm_copy_rule(rule);
- if (lsm_rule) {
- rule_reinitialized = true;
- goto retry;
- }
- }
- if (!rc) {
- result = false;
- goto out;
- }
+ if (!rc)
+ return false;
+ else if (rc == -ESTALE)
+ return rc;
}
- result = true;

-out:
- if (rule_reinitialized) {
- for (i = 0; i < MAX_LSM_RULES; i++)
- ima_filter_rule_free(lsm_rule->lsm[i].rule);
- kfree(lsm_rule);
- }
- return result;
+ return true;
}

/*
@@ -741,12 +732,12 @@ int ima_match_policy(struct mnt_idmap *idmap, struct inode *inode,
const char *func_data, unsigned int *allowed_algos)
{
struct ima_rule_entry *entry;
- int action = 0, actmask = flags | (flags << 1);
+ int action = 0, rc, actmask = flags | (flags << 1);
struct list_head *ima_rules_tmp;

if (template_desc && !*template_desc)
*template_desc = ima_template_desc_current();
-
+retry:
rcu_read_lock();
ima_rules_tmp = rcu_dereference(ima_rules);
list_for_each_entry_rcu(entry, ima_rules_tmp, list) {
@@ -754,9 +745,18 @@ int ima_match_policy(struct mnt_idmap *idmap, struct inode *inode,
if (!(entry->action & actmask))
continue;

- if (!ima_match_rules(entry, idmap, inode, cred, secid,
- func, mask, func_data))
+ rc = ima_match_rules(entry, idmap, inode, cred, secid,
+ func, mask, func_data);
+ if (!rc)
continue;
+ else if (rc == -ESTALE) {
+ rcu_read_unlock();
+
+ wait_event_interruptible(wait_queue_t,
+ (global_seqno == entry->seqno + 1));
+
+ goto retry;
+ }

action |= entry->flags & IMA_NONACTION_FLAGS;

@@ -1153,6 +1153,7 @@ static int ima_lsm_rule_init(struct ima_rule_entry *entry,
result = 0;
}

+ entry->seqno = global_seqno;
return result;
}

--
2.34.1