x86/ima: Check EFI_RUNTIME_SERVICES before using

Checking efi_enabled(EFI_BOOT) is not sufficient to ensure that
EFI runtime services are available, e.g. if efi=noruntime is used.

Without this, I get an oops on a PREEMPT_RT kernel where efi=noruntime is
the default.

Fixes: 399574c6 ("x86/ima: retry detecting secure boot mode")
Cc: stable@vger.kernel.org  (linux-5.0)
Signed-off-by: Scott Wood <swood@redhat.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

arch/x86/kernel/ima_arch.c

@@ -18,6 +18,11 @@ static enum efi_secureboot_mode get_sb_mode(void)
 
 	size = sizeof(secboot);
 
+	if (!efi_enabled(EFI_RUNTIME_SERVICES)) {
+		pr_info("ima: secureboot mode unknown, no efi\n");
+		return efi_secureboot_mode_unknown;
+	}
+
 	/* Get variable contents into buffer */
 	status = efi.get_variable(efi_SecureBoot_name, &efi_variable_guid,
 				  NULL, &size, &secboot);