KEYS: Separate the kernel signature checking keyring from module signing

Separate the kernel signature checking keyring from module signing so that it
can be used by code other than the module-signing code.

Signed-off-by: David Howells <dhowells@redhat.com>

init/Kconfig

@@ -1668,6 +1668,18 @@ config BASE_SMALL
 	default 0 if BASE_FULL
 	default 1 if !BASE_FULL
 
+config SYSTEM_TRUSTED_KEYRING
+	bool "Provide system-wide ring of trusted keys"
+	depends on KEYS
+	help
+	  Provide a system keyring to which trusted keys can be added.  Keys in
+	  the keyring are considered to be trusted.  Keys may be added at will
+	  by the kernel from compiled-in data and from hardware key stores, but
+	  userspace may only add extra keys if those keys can be verified by
+	  keys already in the keyring.
+
+	  Keys in this keyring are used by module signature checking.
+
 menuconfig MODULES
 	bool "Enable loadable module support"
 	option modules
@@ -1741,6 +1753,7 @@ config MODULE_SRCVERSION_ALL
 config MODULE_SIG
 	bool "Module signature verification"
 	depends on MODULES
+	select SYSTEM_TRUSTED_KEYRING
 	select KEYS
 	select CRYPTO
 	select ASYMMETRIC_KEY_TYPE