LSM: Lift LSM selection out of individual LSMs

As a prerequisite to adjusting LSM selection logic in the future, this
moves the selection logic up out of the individual major LSMs, making
their init functions only run when actually enabled. This considers all
LSMs enabled by default unless they specified an external "enable"
variable.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: John Johansen <john.johansen@canonical.com>

1 files changed, 71 insertions, 31 deletions

diff --git a/security/security.c b/security/security.c
index 6bc591f77b1a..c900d7a1441a 100644
--- a/security/security.c
+++ b/security/security.c
@@ -52,33 +52,96 @@ static __initdata bool debug;
                         pr_info(__VA_ARGS__);                   \
         } while (0)
 
+static bool __init is_enabled(struct lsm_info *lsm)
+{
+        if (!lsm->enabled || *lsm->enabled)
+                return true;
+
+        return false;
+}
+
+/* Mark an LSM's enabled flag. */
+static int lsm_enabled_true __initdata = 1;
+static int lsm_enabled_false __initdata = 0;
+static void __init set_enabled(struct lsm_info *lsm, bool enabled)
+{
+        /*
+         * When an LSM hasn't configured an enable variable, we can use
+         * a hard-coded location for storing the default enabled state.
+         */
+        if (!lsm->enabled) {
+                if (enabled)
+                        lsm->enabled = &lsm_enabled_true;
+                else
+                        lsm->enabled = &lsm_enabled_false;
+        } else if (lsm->enabled == &lsm_enabled_true) {
+                if (!enabled)
+                        lsm->enabled = &lsm_enabled_false;
+        } else if (lsm->enabled == &lsm_enabled_false) {
+                if (enabled)
+                        lsm->enabled = &lsm_enabled_true;
+        } else {
+                *lsm->enabled = enabled;
+        }
+}
+
+/* Is an LSM allowed to be initialized? */
+static bool __init lsm_allowed(struct lsm_info *lsm)
+{
+        /* Skip if the LSM is disabled. */
+        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_lsm) != 0)
+                return false;
+
+        return true;
+}
+
+/* Check if LSM should be initialized. */
+static void __init maybe_initialize_lsm(struct lsm_info *lsm)
+{
+        int enabled = lsm_allowed(lsm);
+
+        /* Record enablement (to handle any following exclusive LSMs). */
+        set_enabled(lsm, enabled);
+
+        /* If selected, initialize the LSM. */
+        if (enabled) {
+                int ret;
+
+                init_debug("initializing %s\n", lsm->name);
+                ret = lsm->init();
+                WARN(ret, "%s failed to initialize: %d\n", lsm->name, ret);
+        }
+}
+
 static void __init ordered_lsm_init(void)
 {
         struct lsm_info *lsm;
-        int ret;
 
         for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
                 if ((lsm->flags & LSM_FLAG_LEGACY_MAJOR) != 0)
                         continue;
 
-                init_debug("initializing %s\n", lsm->name);
-                ret = lsm->init();
-                WARN(ret, "%s failed to initialize: %d\n", lsm->name, ret);
+                maybe_initialize_lsm(lsm);
         }
 }
 
 static void __init major_lsm_init(void)
 {
         struct lsm_info *lsm;
-        int ret;
 
         for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
                 if ((lsm->flags & LSM_FLAG_LEGACY_MAJOR) == 0)
                         continue;
 
-                init_debug("initializing %s\n", lsm->name);
-                ret = lsm->init();
-                WARN(ret, "%s failed to initialize: %d\n", lsm->name, ret);
+                maybe_initialize_lsm(lsm);
         }
 }
 
@@ -169,29 +232,6 @@ static int lsm_append(char *new, char **result)
 }
 
 /**
- * security_module_enable - Load given security module on boot ?
- * @module: the name of the module
- *
- * Each LSM must pass this method before registering its own operations
- * to avoid security registration races. This method may also be used
- * to check if your LSM is currently loaded during kernel initialization.
- *
- * Returns:
- *
- * true if:
- *
- * - The passed LSM is the one chosen by user at boot time,
- * - or the passed LSM is configured as the default and the user did not
- *   choose an alternate LSM at boot time.
- *
- * Otherwise, return false.
- */
-int __init security_module_enable(const char *module)
-{
-        return !strcmp(module, chosen_lsm);
-}
-
-/**
  * security_add_hooks - Add a modules hooks to the hook lists.
  * @hooks: the hooks to add
  * @count: the number of hooks to add