[PATCH security-next v5 22/30] LSM: Separate idea of "major" LSM from "exclusive" LSM

[Kees Cook]

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 <keesc...@chromium.org>
Reviewed-by: Casey Schaufler <ca...@schaufler-ca.com>
---
 include/linux/lsm_hooks.h  |  1 +
 security/apparmor/lsm.c    |  2 +-
 security/security.c        | 12 ++++++++++++
 security/selinux/hooks.c   |  2 +-
 security/smack/smack_lsm.c |  2 +-
 security/tomoyo/tomoyo.c   |  2 +-
 6 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index b6b05d351eb4..e88c725a43b8 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -2040,6 +2040,7 @@ 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. */
diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index bc56b058dc75..ebf63461a79c 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -1602,7 +1602,7 @@ 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,
 };
diff --git a/security/security.c b/security/security.c
index 2c754968f98b..65f1fa733e4b 100644
--- a/security/security.c
+++ b/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);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 71a10fedecb3..0f8d7bb88197 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -7194,7 +7194,7 @@ 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,
 };
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index f243044d5a55..92e4baa342f8 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -4881,6 +4881,6 @@ 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,
 };
diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
index a46f6bc1e97c..daff7d7897ad 100644
--- a/security/tomoyo/tomoyo.c
+++ b/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,
 };
-- 
2.17.1