[PATCH security-next v3 17/29] LSM: Introduce CONFIG_LSM_ENABLE

[Kees Cook]

To provide a set of default-enabled LSMs at boot, this introduces the
new CONFIG_LSM_ENABLE. A value of "all" means all builtin LSMs are
enabled by default. Any unlisted LSMs will be implicitly disabled
(excepting those with LSM-specific CONFIGs for enabling/disabling).

The behavior of the LSM-specific CONFIGs for SELinux are AppArmor
unchanged: the default-enabled state for those LSMs remains controlled
through their LSM-specific "enable" CONFIGs.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/linux/lsm_hooks.h | 2 +-
 security/Kconfig          | 8 ++++++++
 security/security.c       | 4 +++-
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 95798f212dbf..ab23f1bc6d77 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -2044,7 +2044,7 @@ extern void security_add_hooks(struct security_hook_list *hooks, int count,
 struct lsm_info {
 	const char *name;	/* Populated automatically. */
 	unsigned long flags;	/* Optional: flags describing LSM */
-	int *enabled;		/* Optional: NULL means enabled. */
+	int *enabled;		/* Optional: NULL checks CONFIG_LSM_ENABLE */
 	int (*init)(void);
 };
 
diff --git a/security/Kconfig b/security/Kconfig
index 27d8b2688f75..71306b046270 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -276,5 +276,13 @@ config DEFAULT_SECURITY
 	default "apparmor" if DEFAULT_SECURITY_APPARMOR
 	default "" if DEFAULT_SECURITY_DAC
 
+config LSM_ENABLE
+	string "LSMs to enable at boot time"
+	default "all"
+	help
+	  A comma-separate list of LSMs to enable by default at boot. The
+	  default is "all", to enable all LSM modules at boot. Any LSMs
+	  not listed here will be disabled by default.
+
 endmenu
 
diff --git a/security/security.c b/security/security.c
index a8107d54b3d3..7ecb9879a863 100644
--- a/security/security.c
+++ b/security/security.c
@@ -45,6 +45,8 @@ char *lsm_names;
 static __initdata char chosen_lsm[SECURITY_NAME_MAX + 1] =
 	CONFIG_DEFAULT_SECURITY;
 
+static __initconst const char * const builtin_lsm_enable = CONFIG_LSM_ENABLE;
+
 static bool debug __initdata;
 #define init_debug(...)						\
 	do {							\
@@ -182,7 +184,7 @@ static void __init parse_lsm_enable(const char *str,
 static void __init prepare_lsm_enable(void)
 {
 	/* Prepare defaults. */
-	parse_lsm_enable("all", default_enabled, true);
+	parse_lsm_enable(builtin_lsm_enable, default_enabled, true);
 }
 
 /**
-- 
2.17.1