ima: ensure IMA_APPRAISE_MODSIG has necessary dependencies

IMA_APPRAISE_MODSIG is used for verifying the integrity of both kernel
and modules. Enabling IMA_APPRAISE_MODSIG without MODULES causes a build
break.

Ensure the build time kernel signing key is only generated if both
IMA_APPRAISE_MODSIG and MODULES are enabled.

Fixes: 0165f4ca22 ("ima: enable signing of modules with build time generated key")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

certs/Kconfig

@@ -4,7 +4,7 @@ menu "Certificates for signature checking"
 config MODULE_SIG_KEY
 	string "File name or PKCS#11 URI of module signing key"
 	default "certs/signing_key.pem"
-	depends on MODULE_SIG || IMA_APPRAISE_MODSIG
+	depends on MODULE_SIG || (IMA_APPRAISE_MODSIG && MODULES)
 	help
          Provide the file name of a private key/certificate in PEM format,
          or a PKCS#11 URI according to RFC7512. The file should contain, or

certs/Makefile

@@ -36,8 +36,10 @@ ifeq ($(CONFIG_MODULE_SIG),y)
 endif
 
 ifeq ($(CONFIG_IMA_APPRAISE_MODSIG),y)
+ifeq ($(CONFIG_MODULES),y)
 	SIGN_KEY = y
 endif
+endif
 
 ifdef SIGN_KEY
 ###############################################################################

certs/system_certificates.S

@@ -9,7 +9,8 @@
 system_certificate_list:
 __cert_list_start:
 __module_cert_start:
-#if defined(CONFIG_MODULE_SIG) || defined(CONFIG_IMA_APPRAISE_MODSIG)
+#if defined(CONFIG_MODULE_SIG) || (defined(CONFIG_IMA_APPRAISE_MODSIG) \
+			       && defined(CONFIG_MODULES))
 	.incbin "certs/signing_key.x509"
 #endif
 __module_cert_end: