LSM: Separate idea of "major" LSM from "exclusive" LSM

In order to both support old "security=" Legacy Major LSM selection, and
handling real exclusivity, this creates LSM_FLAG_EXCLUSIVE and updates
the selection logic to handle them.
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>

include/linux/lsm_hooks.h

@@ -2043,6 +2043,7 @@ extern void security_add_hooks(struct security_hook_list *hooks, int count,
 				char *lsm);
 
 #define LSM_FLAG_LEGACY_MAJOR	BIT(0)
+#define LSM_FLAG_EXCLUSIVE	BIT(1)
 
 struct lsm_info {
 	const char *name;	/* Required. */

security/apparmor/lsm.c

@@ -1723,7 +1723,7 @@ static int __init apparmor_init(void)
 
 DEFINE_LSM(apparmor) = {
 	.name = "apparmor",
-	.flags = LSM_FLAG_LEGACY_MAJOR,
+	.flags = LSM_FLAG_LEGACY_MAJOR | LSM_FLAG_EXCLUSIVE,
 	.enabled = &apparmor_enabled,
 	.init = apparmor_init,
 };

security/security.c

@@ -49,6 +49,7 @@ static __initconst const char * const builtin_lsm_order = CONFIG_LSM;
 
 /* Ordered list of LSMs to initialize. */
 static __initdata struct lsm_info **ordered_lsms;
+static __initdata struct lsm_info *exclusive;
 
 static __initdata bool debug;
 #define init_debug(...)						\
@@ -129,6 +130,12 @@ static bool __init lsm_allowed(struct lsm_info *lsm)
 	if (!is_enabled(lsm))
 		return false;
 
+	/* Not allowed if another exclusive LSM already initialized. */
+	if ((lsm->flags & LSM_FLAG_EXCLUSIVE) && exclusive) {
+		init_debug("exclusive disabled: %s\n", lsm->name);
+		return false;
+	}
+
 	return true;
 }
 
@@ -144,6 +151,11 @@ static void __init maybe_initialize_lsm(struct lsm_info *lsm)
 	if (enabled) {
 		int ret;
 
+		if ((lsm->flags & LSM_FLAG_EXCLUSIVE) && !exclusive) {
+			exclusive = lsm;
+			init_debug("exclusive chosen: %s\n", lsm->name);
+		}
+
 		init_debug("initializing %s\n", lsm->name);
 		ret = lsm->init();
 		WARN(ret, "%s failed to initialize: %d\n", lsm->name, ret);

security/selinux/hooks.c

@@ -6989,7 +6989,7 @@ void selinux_complete_init(void)
    all processes and objects when they are created. */
 DEFINE_LSM(selinux) = {
 	.name = "selinux",
-	.flags = LSM_FLAG_LEGACY_MAJOR,
+	.flags = LSM_FLAG_LEGACY_MAJOR | LSM_FLAG_EXCLUSIVE,
 	.enabled = &selinux_enabled,
 	.init = selinux_init,
 };

security/smack/smack_lsm.c

@@ -4809,6 +4809,6 @@ static __init int smack_init(void)
  */
 DEFINE_LSM(smack) = {
 	.name = "smack",
-	.flags = LSM_FLAG_LEGACY_MAJOR,
+	.flags = LSM_FLAG_LEGACY_MAJOR | LSM_FLAG_EXCLUSIVE,
 	.init = smack_init,
 };

security/tomoyo/tomoyo.c

@@ -550,6 +550,6 @@ static int __init tomoyo_init(void)
 
 DEFINE_LSM(tomoyo) = {
 	.name = "tomoyo",
-	.flags = LSM_FLAG_LEGACY_MAJOR,
+	.flags = LSM_FLAG_LEGACY_MAJOR | LSM_FLAG_EXCLUSIVE,
 	.init = tomoyo_init,
 };