LSM: Prepare for reorganizing "security=" logic

This moves the string handling for "security=" boot parameter into
a stored pointer instead of a string duplicate. This will allow
easier handling of the string when switching logic to use the coming
enable/disable infrastructure.
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: John Johansen <john.johansen@canonical.com>

security/security.c

@@ -34,9 +34,6 @@
 
 #define MAX_LSM_EVM_XATTR	2
 
-/* Maximum number of letters for an LSM name string */
-#define SECURITY_NAME_MAX	10
-
 /* How many LSMs were built into the kernel? */
 #define LSM_COUNT (__end_lsm_info - __start_lsm_info)
 
@@ -45,9 +42,8 @@ static ATOMIC_NOTIFIER_HEAD(lsm_notifier_chain);
 
 char *lsm_names;
 /* Boot-time LSM user choice */
-static __initdata char chosen_lsm[SECURITY_NAME_MAX + 1] =
-	CONFIG_DEFAULT_SECURITY;
 static __initdata const char *chosen_lsm_order;
+static __initdata const char *chosen_major_lsm;
 
 static __initconst const char * const builtin_lsm_order = CONFIG_LSM;
 
@@ -138,7 +134,7 @@ static bool __init lsm_allowed(struct lsm_info *lsm)
 		return true;
 
 	/* Disabled if this LSM isn't the chosen one. */
-	if (strcmp(lsm->name, chosen_lsm) != 0)
+	if (strcmp(lsm->name, chosen_major_lsm) != 0)
 		return false;
 
 	return true;
@@ -168,6 +164,9 @@ static void __init ordered_lsm_parse(const char *order, const char *origin)
 	struct lsm_info *lsm;
 	char *sep, *name, *next;
 
+	if (!chosen_major_lsm)
+		chosen_major_lsm = CONFIG_DEFAULT_SECURITY;
+
 	sep = kstrdup(order, GFP_KERNEL);
 	next = sep;
 	/* Walk the list, looking for matching LSMs. */
@@ -257,12 +256,12 @@ int __init security_init(void)
 }
 
 /* Save user chosen LSM */
-static int __init choose_lsm(char *str)
+static int __init choose_major_lsm(char *str)
 {
-	strncpy(chosen_lsm, str, SECURITY_NAME_MAX);
+	chosen_major_lsm = str;
 	return 1;
 }
-__setup("security=", choose_lsm);
+__setup("security=", choose_major_lsm);
 
 /* Explicitly choose LSM initialization order. */
 static int __init choose_lsm_order(char *str)