When a signed policy is not mandatory, userspace can write IMA policy rules
directly to the securityfs policy file:
echo -e "measure func=BPRM_CHECK mask=MAY_EXEC\n" \
"audit func=BPRM_CHECK mask=MAY_EXEC\n" \
> /sys/kernel/security/ima/policy
or by cat'ing the entire IMA custom policy file:
cat ima-policy-file > /sys/kernel/security/ima/policy
Because these rules originate from userspace and cross the userspace/kernel
trust boundary, measure the raw write buffer before parsing, regardless of
whether the new policy will be accepted or not. This can be caught when
'measure func=POLICY_CHECK' is enabled (e.g., ima_policy=tcb). The
measurement template is forced to ima-buf.
This follows the "measure & load" paradigm, exposing potential bugs in
the policy code and detecting attempts to corrupt IMA. It also completes
the POLICY_CHECK hook, which already measures partial policy load by file.
To verify the template data hash value, convert the buffer policy data
to binary:
grep "ima_policy_written" \
/sys/kernel/security/integrity/ima/ascii_runtime_measurements | \
tail -1 | cut -d' ' -f 6 | xxd -r -p | sha256sum
Suggested-by: Roberto Sassu <[email protected]>
Signed-off-by: Enrico Bravi <[email protected]>
---
security/integrity/ima/ima.h | 1 +
security/integrity/ima/ima_fs.c | 1 +
security/integrity/ima/ima_main.c | 22 ++++++++++++++++++++++
security/integrity/ima/ima_policy.c | 3 +++
4 files changed, 27 insertions(+)
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index befa221716e5..8d60dcf9c689 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -455,6 +455,7 @@ void *ima_policy_next(struct seq_file *m, void *v, loff_t
*pos);
void ima_policy_stop(struct seq_file *m, void *v);
int ima_policy_show(struct seq_file *m, void *v);
void ima_measure_loaded_policy(void);
+int ima_measure_raw_policy(const char *buf, size_t buf_len);
/* Appraise integrity measurements */
#define IMA_APPRAISE_ENFORCE 0x01
diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
index 65e7812c702f..fed97eb625dd 100644
--- a/security/integrity/ima/ima_fs.c
+++ b/security/integrity/ima/ima_fs.c
@@ -356,6 +356,7 @@ static ssize_t ima_write_policy(struct file *file, const
char __user *buf,
1, 0);
result = -EACCES;
} else {
+ ima_measure_raw_policy(data, datalen);
result = ima_parse_add_rule(data);
}
mutex_unlock(&ima_write_mutex);
diff --git a/security/integrity/ima/ima_main.c
b/security/integrity/ima/ima_main.c
index 5cea53fc36df..5277e1f680f0 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -1221,6 +1221,28 @@ int ima_measure_critical_data(const char *event_label,
}
EXPORT_SYMBOL_GPL(ima_measure_critical_data);
+/**
+ * ima_measure_raw_policy - Measure the raw policy write buffer
+ * @buf: pointer to the buffer containing the raw policy data
+ * @buf_len: size of the buffer
+ *
+ * Measure the raw policy buffer sent to the IMA policy securityfs file. The
+ * buffer is written from userspace, and the measurement is performed before
+ * parsing it. This measurement includes any data written on the policy file
+ * such as malformed policy rules and comments.
+ *
+ * Return 0 on success, a negative value otherwise.
+ */
+int ima_measure_raw_policy(const char *buf, size_t buf_len)
+{
+ if (!buf || !buf_len)
+ return -EINVAL;
+
+ return process_buffer_measurement(&nop_mnt_idmap, NULL, buf, buf_len,
+ "ima_policy_written", POLICY_CHECK, 0,
+ NULL, false, NULL, 0);
+}
+
#ifdef CONFIG_INTEGRITY_ASYMMETRIC_KEYS
/**
diff --git a/security/integrity/ima/ima_policy.c
b/security/integrity/ima/ima_policy.c
index a65b7e4b64d6..d6d249190705 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -543,6 +543,8 @@ static bool ima_match_rule_data(struct ima_rule_entry *rule,
opt_list = rule->label;
break;
+ case POLICY_CHECK:
+ return true;
default:
return false;
}
@@ -591,6 +593,7 @@ static bool ima_match_rules(struct ima_rule_entry *rule,
switch (func) {
case KEY_CHECK:
case CRITICAL_DATA:
+ case POLICY_CHECK:
return ((rule->func == func) &&
ima_match_rule_data(rule, func_data, cred));
default:
--
2.52.0
