kexec_file: split KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE

This is a preparatory patch for kexec_file_load() lockdown.  A locked down
kernel needs to prevent unsigned kernel images from being loaded with
kexec_file_load().  Currently, the only way to force the signature
verification is compiling with KEXEC_VERIFY_SIG.  This prevents loading
usigned images even when the kernel is not locked down at runtime.

This patch splits KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE.
Analogous to the MODULE_SIG and MODULE_SIG_FORCE for modules, KEXEC_SIG
turns on the signature verification but allows unsigned images to be
loaded.  KEXEC_SIG_FORCE disallows images without a valid signature.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Matthew Garrett <mjg59@google.com>
cc: kexec@lists.infradead.org
Signed-off-by: James Morris <jmorris@namei.org>

security/integrity/ima/Kconfig

@@ -160,7 +160,7 @@ config IMA_APPRAISE
 
 config IMA_ARCH_POLICY
         bool "Enable loading an IMA architecture specific policy"
-        depends on KEXEC_VERIFY_SIG || IMA_APPRAISE && INTEGRITY_ASYMMETRIC_KEYS
+        depends on KEXEC_SIG || IMA_APPRAISE && INTEGRITY_ASYMMETRIC_KEYS
         default n
         help
           This option enables loading an IMA architecture specific policy

security/integrity/ima/ima_main.c

@@ -541,7 +541,7 @@ int ima_load_data(enum kernel_load_data_id id)
 
 	switch (id) {
 	case LOADING_KEXEC_IMAGE:
-		if (IS_ENABLED(CONFIG_KEXEC_VERIFY_SIG)
+		if (IS_ENABLED(CONFIG_KEXEC_SIG)
 		    && arch_ima_get_secureboot()) {
 			pr_err("impossible to appraise a kernel image without a file descriptor; try using kexec_file_load syscall.\n");
 			return -EACCES;