powerpc/ima: Fix secure boot rules in ima arch policy

To prevent verifying the kernel module appended signature
twice (finit_module), once by the module_sig_check() and again by IMA,
powerpc secure boot rules define an IMA architecture specific policy
rule only if CONFIG_MODULE_SIG_FORCE is not enabled. This,
unfortunately, does not take into account the ability of enabling
"sig_enforce" on the boot command line (module.sig_enforce=1).

Including the IMA module appraise rule results in failing the
finit_module syscall, unless the module signing public key is loaded
onto the IMA keyring.

This patch fixes secure boot policy rules to be based on
CONFIG_MODULE_SIG instead.

Fixes: 4238fad3 ("powerpc/ima: Add support to initialize ima policy rules")
Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/r/1588342612-14532-1-git-send-email-nayna@linux.ibm.com

arch/powerpc/kernel/ima_arch.c

@@ -19,12 +19,12 @@ bool arch_ima_get_secureboot(void)
  * to be stored as an xattr or as an appended signature.
  *
  * To avoid duplicate signature verification as much as possible, the IMA
- * policy rule for module appraisal is added only if CONFIG_MODULE_SIG_FORCE
+ * policy rule for module appraisal is added only if CONFIG_MODULE_SIG
  * is not enabled.
  */
 static const char *const secure_rules[] = {
 	"appraise func=KEXEC_KERNEL_CHECK appraise_flag=check_blacklist appraise_type=imasig|modsig",
-#ifndef CONFIG_MODULE_SIG_FORCE
+#ifndef CONFIG_MODULE_SIG
 	"appraise func=MODULE_CHECK appraise_flag=check_blacklist appraise_type=imasig|modsig",
 #endif
 	NULL
@@ -50,7 +50,7 @@ static const char *const secure_and_trusted_rules[] = {
 	"measure func=KEXEC_KERNEL_CHECK template=ima-modsig",
 	"measure func=MODULE_CHECK template=ima-modsig",
 	"appraise func=KEXEC_KERNEL_CHECK appraise_flag=check_blacklist appraise_type=imasig|modsig",
-#ifndef CONFIG_MODULE_SIG_FORCE
+#ifndef CONFIG_MODULE_SIG
 	"appraise func=MODULE_CHECK appraise_flag=check_blacklist appraise_type=imasig|modsig",
 #endif
 	NULL