Security: Introduce security= boot parameter

Add the security= boot parameter. This is done to avoid LSM
registration clashes in case of more than one bult-in module.

User can choose a security module to enable at boot. If no
security= boot parameter is specified, only the first LSM
asking for registration will be loaded. An invalid security
module name will be treated as if no module has been chosen.

LSM modules must check now if they are allowed to register
by calling security_module_enable(ops) first. Modify SELinux
and SMACK to do so.

Do not let SMACK register smackfs if it was not chosen on
boot. Smackfs assumes that smack hooks are registered and
the initial task security setup (swapper->security) is done.

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Acked-by: James Morris <jmorris@namei.org>

1 files changed, 37 insertions, 1 deletions

diff --git a/security/security.c b/security/security.c
index 2ef593ec70f3..dd0c6baed494 100644
--- a/security/security.c
+++ b/security/security.c
@@ -17,6 +17,8 @@
 #include <linux/kernel.h>
 #include <linux/security.h>
 
+/* Boot-time LSM user choice */
+static __initdata char chosen_lsm[SECURITY_NAME_MAX + 1];
 
 /* things that live in dummy.c */
 extern struct security_operations dummy_security_ops;
@@ -67,13 +69,47 @@ int __init security_init(void)
         return 0;
 }
 
+/* Save user chosen LSM */
+static int __init choose_lsm(char *str)
+{
+        strncpy(chosen_lsm, str, SECURITY_NAME_MAX);
+        return 1;
+}
+__setup("security=", choose_lsm);
+
+/**
+ * security_module_enable - Load given security module on boot ?
+ * @ops: a pointer to the struct security_operations that is to be checked.
+ *
+ * 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.
+ *
+ * Return true if:
+ *      -The passed LSM is the one chosen by user at boot time,
+ *      -or user didsn't specify a specific LSM and we're the first to ask
+ *       for registeration permissoin,
+ *      -or the passed LSM is currently loaded.
+ * Otherwise, return false.
+ */
+int __init security_module_enable(struct security_operations *ops)
+{
+        if (!*chosen_lsm)
+                strncpy(chosen_lsm, ops->name, SECURITY_NAME_MAX);
+        else if (strncmp(ops->name, chosen_lsm, SECURITY_NAME_MAX))
+                return 0;
+
+        return 1;
+}
+
 /**
  * register_security - registers a security framework with the kernel
  * @ops: a pointer to the struct security_options that is to be registered
  *
  * This function is to allow a security module to register itself with the
  * kernel security subsystem.  Some rudimentary checking is done on the @ops
- * value passed to this function.
+ * value passed to this function. You'll need to check first if your LSM
+ * is allowed to register its @ops by calling security_module_enable(@ops).
  *
  * If there is already a security module registered with the kernel,
  * an error will be returned.  Otherwise 0 is returned on success.