LSM: Refactor "security=" in terms of enable/disable

For what are marked as the Legacy Major LSMs, make them effectively
exclusive when selected on the "security=" boot parameter, to handle
the future case of when a previously major LSMs become non-exclusive
(e.g. when TOMOYO starts blob-sharing).
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>

security/security.c

@@ -129,14 +129,6 @@ static bool __init lsm_allowed(struct lsm_info *lsm)
 	if (!is_enabled(lsm))
 		return false;
 
-	/* Skip major-specific checks if not a major LSM. */
-	if ((lsm->flags & LSM_FLAG_LEGACY_MAJOR) == 0)
-		return true;
-
-	/* Disabled if this LSM isn't the chosen one. */
-	if (strcmp(lsm->name, chosen_major_lsm) != 0)
-		return false;
-
 	return true;
 }
 
@@ -164,8 +156,28 @@ static void __init ordered_lsm_parse(const char *order, const char *origin)
 	struct lsm_info *lsm;
 	char *sep, *name, *next;
 
+	/* Process "security=", if given. */
 	if (!chosen_major_lsm)
 		chosen_major_lsm = CONFIG_DEFAULT_SECURITY;
+	if (chosen_major_lsm) {
+		struct lsm_info *major;
+
+		/*
+		 * To match the original "security=" behavior, this
+		 * explicitly does NOT fallback to another Legacy Major
+		 * if the selected one was separately disabled: disable
+		 * all non-matching Legacy Major LSMs.
+		 */
+		for (major = __start_lsm_info; major < __end_lsm_info;
+		     major++) {
+			if ((major->flags & LSM_FLAG_LEGACY_MAJOR) &&
+			    strcmp(major->name, chosen_major_lsm) != 0) {
+				set_enabled(major, false);
+				init_debug("security=%s disabled: %s\n",
+					   chosen_major_lsm, major->name);
+			}
+		}
+	}
 
 	sep = kstrdup(order, GFP_KERNEL);
 	next = sep;