LSM: Revive CONFIG_DEFAULT_SECURITY_* for "make oldconfig"

Commit 70b62c2 ("LoadPin: Initialize as ordered LSM") removed
CONFIG_DEFAULT_SECURITY_{SELINUX,SMACK,TOMOYO,APPARMOR,DAC} from
security/Kconfig and changed CONFIG_LSM to provide a fixed ordering as a
default value. That commit expected that existing users (upgrading from
Linux 5.0 and earlier) will edit CONFIG_LSM value in accordance with
their CONFIG_DEFAULT_SECURITY_* choice in their old kernel configs. But
since users might forget to edit CONFIG_LSM value, this patch revives
the choice (only for providing the default value for CONFIG_LSM) in order
to make sure that CONFIG_LSM reflects CONFIG_DEFAULT_SECURITY_* from their
old kernel configs.

Note that since TOMOYO can be fully stacked against the other legacy
major LSMs, when it is selected, it explicitly disables the other LSMs
to avoid them also initializing since TOMOYO does not expect this
currently.

Reported-by: Jakub Kicinski <[email protected]>
Reported-by: Randy Dunlap <[email protected]>
Fixes: 70b62c2 ("LoadPin: Initialize as ordered LSM")
Co-developed-by: Tetsuo Handa <[email protected]>
Signed-off-by: Tetsuo Handa <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Acked-by: Casey Schaufler <[email protected]>
Signed-off-by: James Morris <[email protected]>

security/Kconfig

@@ -239,8 +239,46 @@ source "security/safesetid/Kconfig"
 
 source "security/integrity/Kconfig"
 
+choice
+	prompt "First legacy 'major LSM' to be initialized"
+	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
+	  This choice is there only for converting CONFIG_DEFAULT_SECURITY
+	  in old kernel configs to CONFIG_LSM in new kernel configs. Don't
+	  change this choice unless you are creating a fresh kernel config,
+	  for this choice will be ignored after CONFIG_LSM has been set.
+
+	  Selects the legacy "major security module" that will be
+	  initialized first. Overridden by non-default CONFIG_LSM.
+
+	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 LSM
 	string "Ordered list of enabled LSMs"
+	default "yama,loadpin,safesetid,integrity,smack,selinux,tomoyo,apparmor" if DEFAULT_SECURITY_SMACK
+	default "yama,loadpin,safesetid,integrity,apparmor,selinux,smack,tomoyo" if DEFAULT_SECURITY_APPARMOR
+	default "yama,loadpin,safesetid,integrity,tomoyo" if DEFAULT_SECURITY_TOMOYO
+	default "yama,loadpin,safesetid,integrity" if DEFAULT_SECURITY_DAC
 	default "yama,loadpin,safesetid,integrity,selinux,smack,tomoyo,apparmor"
 	help
 	  A comma-separated list of LSMs, in initialization order.