[PATCH] LSM: Ignore "security=" when "lsm=" is specified

[Tetsuo Handa]

Kees Cook wrote:
> To avoid potential confusion, explicitly ignore "security=" when "lsm=" is
> used on the command line, and report that it is happening.

To maintain the existing behavior of CONFIG_DEFAULT_SECURITY, I also suggest this change.
This saves e.g. Ubuntu users who are using only AppArmor from explicitly specifying
security=apparmor when they don't want to enable other LSM_FLAG_LEGACY_MAJOR modules.

---
 security/Kconfig    | 37 +++++++++++++++++++++++++++++++++++++
 security/security.c |  5 ++++-
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/security/Kconfig b/security/Kconfig
index 9555f49..6a40995 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -250,5 +250,42 @@ config LSM
 
 	  If unsure, leave this as the default.
 
+choice
+	prompt "Default exclusive security module"
+	default DEFAULT_SECURITY_SELINUX if SECURITY_SELINUX
+	default DEFAULT_SECURITY_SMACK if SECURITY_SMACK
+	default DEFAULT_SECURITY_TOMOYO if SECURITY_TOMOYO
+	default DEFAULT_SECURITY_APPARMOR if SECURITY_APPARMOR
+	default DEFAULT_SECURITY_DAC
+
+	help
+	  The security module where only one of these modules should be enabled if
+	  neither the "security=" parameter nor the "lsm=" parameter is specified.
+
+	config DEFAULT_SECURITY_SELINUX
+		bool "SELinux" if SECURITY_SELINUX=y
+
+	config DEFAULT_SECURITY_SMACK
+		bool "Simplified Mandatory Access Control" if SECURITY_SMACK=y
+
+	config DEFAULT_SECURITY_TOMOYO
+		bool "TOMOYO" if SECURITY_TOMOYO=y
+
+	config DEFAULT_SECURITY_APPARMOR
+		bool "AppArmor" if SECURITY_APPARMOR=y
+
+	config DEFAULT_SECURITY_DAC
+		bool "Unix Discretionary Access Controls"
+
+endchoice
+
+config DEFAULT_SECURITY
+	string
+	default "selinux" if DEFAULT_SECURITY_SELINUX
+	default "smack" if DEFAULT_SECURITY_SMACK
+	default "tomoyo" if DEFAULT_SECURITY_TOMOYO
+	default "apparmor" if DEFAULT_SECURITY_APPARMOR
+	default "" if DEFAULT_SECURITY_DAC
+
 endmenu
 
diff --git a/security/security.c b/security/security.c
index e6153ed..c44e3cd 100644
--- a/security/security.c
+++ b/security/security.c
@@ -294,8 +294,11 @@ static void __init ordered_lsm_init(void)
 			chosen_major_lsm = NULL;
 		}
 		ordered_lsm_parse(chosen_lsm_order, "cmdline");
-	} else
+	} else {
+		if (!chosen_major_lsm)
+			chosen_major_lsm = CONFIG_DEFAULT_SECURITY;
 		ordered_lsm_parse(builtin_lsm_order, "builtin");
+	}
 
 	for (lsm = ordered_lsms; *lsm; lsm++)
 		prepare_lsm(*lsm);
-- 
1.8.3.1