ima: require signed IMA policy when UEFI secure boot is enabled

With commit 099f26f2 ("integrity: machine keyring CA
configuration"), users are able to add custom IMA CA keys via
MOK.  This allows users to sign their own IMA polices without
recompiling the kernel. For the sake of security, mandate signed IMA
policy when UEFI secure boot is enabled.

Note this change may affect existing users/tests i.e users won't be able
to load an unsigned IMA policy when the IMA architecture specific policy
is configured and UEFI secure boot is enabled.
Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Coiby Xu <coxu@redhat.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_efi.c

@@ -57,6 +57,9 @@ static const char * const sb_arch_rules[] = {
 	"measure func=KEXEC_KERNEL_CHECK",
 #if !IS_ENABLED(CONFIG_MODULE_SIG)
 	"appraise func=MODULE_CHECK appraise_type=imasig",
+#endif
+#if IS_ENABLED(CONFIG_INTEGRITY_MACHINE_KEYRING) && IS_ENABLED(CONFIG_IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY)
+	"appraise func=POLICY_CHECK appraise_type=imasig",
 #endif
 	"measure func=MODULE_CHECK",
 	NULL