[PATCH v4 6/8] ima: add build time policy

Mimi Zohar
2018-05-29T18:02:39Z

IMA by default does not measure, appraise or audit files, but can be
enabled at runtime by specifying a builtin policy on the boot command line
or by loading a custom policy.

This patch defines a build time policy, which verifies kernel modules,
firmware, kexec image, and/or the IMA policy signatures. This build time
policy is automatically enabled at runtime and persists after loading a
custom policy.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
---
security/integrity/ima/Kconfig | 58 +++++++++++++++++++++++++++++++++++++
security/integrity/ima/ima_policy.c | 46 +++++++++++++++++++++++++++--
2 files changed, 101 insertions(+), 3 deletions(-)

diff --git a/security/integrity/ima/Kconfig b/security/integrity/ima/Kconfig
index 6a8f67714c83..004919d9bf09 100644
--- a/security/integrity/ima/Kconfig
+++ b/security/integrity/ima/Kconfig
@@ -156,6 +156,64 @@ config IMA_APPRAISE
<http://linux-ima.sourceforge.net>
If unsure, say N.

+config IMA_APPRAISE_BUILD_POLICY
+ bool "IMA build time configured policy rules"
+ depends on IMA_APPRAISE && INTEGRITY_ASYMMETRIC_KEYS
+ default n
+ help
+ This option defines an IMA appraisal policy at build time, which
+ is enforced at run time without having to specify a builtin
+ policy name on the boot command line. The build time appraisal
+ policy rules persist after loading a custom policy.
+
+ Depending on the rules configured, this policy may require kernel
+ modules, firmware, the kexec kernel image, and/or the IMA policy
+ to be signed. Unsigned files might prevent the system from
+ booting or applications from working properly.
+
+config IMA_APPRAISE_REQUIRE_FIRMWARE_SIGS
+ bool "Appraise firmware signatures"
+ depends on IMA_APPRAISE_BUILD_POLICY
+ default n
+ help
+ This option defines a policy requiring all firmware to be signed,
+ including the regulatory.db. If both this option and
+ CFG80211_REQUIRE_SIGNED_REGDB are enabled, then both signature
+ verification methods are necessary.
+
+config IMA_APPRAISE_REQUIRE_KEXEC_SIGS
+ bool "Appraise kexec kernel image signatures"
+ depends on IMA_APPRAISE_BUILD_POLICY
+ default n
+ help
+ Enabling this rule will require all kexec'ed kernel images to
+ be signed and verified by a public key on the trusted IMA
+ keyring.
+
+ Kernel image signatures can not be verified by the original
+ kexec_load syscall. Enabling this rule will prevent its
+ usage.
+
+config IMA_APPRAISE_REQUIRE_MODULE_SIGS
+ bool "Appraise kernel modules signatures"
+ depends on IMA_APPRAISE_BUILD_POLICY
+ default n
+ help
+ Enabling this rule will require all kernel modules to be signed
+ and verified by a public key on the trusted IMA keyring.
+
+ Kernel module signatures can only be verified by IMA-appraisal,
+ via the finit_module syscall. Enabling this rule will prevent
+ the usage of the init_module syscall.
+
+config IMA_APPRAISE_REQUIRE_POLICY_SIGS
+ bool "Appraise IMA policy signature"
+ depends on IMA_APPRAISE_BUILD_POLICY
+ default n
+ help
+ Enabling this rule will require the IMA policy to be signed and
+ and verified by a key on the trusted IMA keyring.
+
config IMA_APPRAISE_BOOTPARAM
bool "ima_appraise boot parameter"
depends on IMA_APPRAISE
diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index c27f6993b07a..3c0bc8a1a88e 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -49,6 +49,7 @@

int ima_policy_flag;
static int temp_ima_appraise;
+static int build_ima_appraise __ro_after_init;

#define MAX_LSM_RULES 6
enum lsm_rule_types { LSM_OBJ_USER, LSM_OBJ_ROLE, LSM_OBJ_TYPE,
@@ -162,6 +163,25 @@ static struct ima_rule_entry default_appraise_rules[] __ro_after_init = {
#endif
};

+static struct ima_rule_entry build_appraise_rules[] __ro_after_init = {
+#ifdef CONFIG_IMA_APPRAISE_REQUIRE_MODULE_SIGS
+ {.action = APPRAISE, .func = MODULE_CHECK,
+ .flags = IMA_FUNC | IMA_DIGSIG_REQUIRED},
+#endif
+#ifdef CONFIG_IMA_APPRAISE_REQUIRE_FIRMWARE_SIGS
+ {.action = APPRAISE, .func = FIRMWARE_CHECK,
+ .flags = IMA_FUNC | IMA_DIGSIG_REQUIRED},
+#endif
+#ifdef CONFIG_IMA_APPRAISE_REQUIRE_KEXEC_SIGS
+ {.action = APPRAISE, .func = KEXEC_KERNEL_CHECK,
+ .flags = IMA_FUNC | IMA_DIGSIG_REQUIRED},
+#endif
+#ifdef CONFIG_IMA_APPRAISE_REQUIRE_POLICY_SIGS
+ {.action = APPRAISE, .func = POLICY_CHECK,
+ .flags = IMA_FUNC | IMA_DIGSIG_REQUIRED},
+#endif
+};
+
static struct ima_rule_entry secure_boot_rules[] __ro_after_init = {
{.action = APPRAISE, .func = MODULE_CHECK,
.flags = IMA_FUNC | IMA_DIGSIG_REQUIRED},
@@ -435,7 +455,7 @@ void ima_update_policy_flag(void)
ima_policy_flag |= entry->action;
}

- ima_appraise |= temp_ima_appraise;
+ ima_appraise |= (build_ima_appraise | temp_ima_appraise);
if (!ima_appraise)
ima_policy_flag &= ~IMA_APPRAISE;
}
@@ -488,8 +508,8 @@ void __init ima_init_policy(void)
}

/*
- * Insert the appraise rules requiring file signatures, prior to
- * any other appraise rules.
+ * Insert the builtin "secure_boot" policy rules requiring file
+ * signatures, prior to any other appraise rules.
*/
for (i = 0; i < secure_boot_entries; i++) {
list_add_tail(&secure_boot_rules[i].list, &ima_default_rules);
@@ -497,6 +517,26 @@ void __init ima_init_policy(void)
ima_appraise_flag(secure_boot_rules[i].func);
}

+ /*
+ * Insert the build time appraise rules requiring file signatures
+ * for both the initial and custom policies, prior to other appraise
+ * rules.
+ */
+ for (i = 0; i < ARRAY_SIZE(build_appraise_rules); i++) {
+ struct ima_rule_entry *entry;
+
+ if (!secure_boot_entries)
+ list_add_tail(&build_appraise_rules[i].list,
+ &ima_default_rules);
+
+ entry = kmemdup(&build_appraise_rules[i], sizeof(*entry),
+ GFP_KERNEL);
+ if (entry)
+ list_add_tail(&entry->list, &ima_policy_rules);
+ build_ima_appraise |=
+ ima_appraise_flag(build_appraise_rules[i].func);
+ }
+
for (i = 0; i < appraise_entries; i++) {
list_add_tail(&default_appraise_rules[i].list,
&ima_default_rules);
--
2.7.5