modsign: Allow external signing key to be specified

Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: David Howells <[email protected]>

Documentation/module-signing.txt

@@ -88,6 +88,22 @@ This has a number of options available:
      than being a module) so that modules signed with that algorithm can have
      their signatures checked without causing a dependency loop.
 
+ (4) "File name or PKCS#11 URI of module signing key" (CONFIG_MODULE_SIG_KEY)
+
+     Setting this option to something other than its default of
+     "signing_key.priv" will disable the autogeneration of signing keys and
+     allow the kernel modules to be signed with a key of your choosing.
+     The string provided should identify a file containing a private key
+     in PEM form, or — on systems where the OpenSSL ENGINE_pkcs11 is
+     appropriately installed — a PKCS#11 URI as defined by RFC7512.
+
+     If the PEM file containing the private key is encrypted, or if the
+     PKCS#11 token requries a PIN, this can be provided at build time by
+     means of the KBUILD_SIGN_PIN variable.
+
+     The corresponding X.509 certificate in DER form should still be placed
+     in a file named signing_key.x509 in the top-level build directory.
+
 
 =======================
 GENERATING SIGNING KEYS
@@ -100,8 +116,9 @@ it can be deleted or stored securely.  The public key gets built into the
 kernel so that it can be used to check the signatures as the modules are
 loaded.
 
-Under normal conditions, the kernel build will automatically generate a new
-keypair using openssl if one does not exist in the files:
+Under normal conditions, when CONFIG_MODULE_SIG_KEY is unchanged from its
+default of "signing_key.priv", the kernel build will automatically generate
+a new keypair using openssl if one does not exist in the files:
 
 	signing_key.priv
 	signing_key.x509
@@ -135,8 +152,12 @@ kernel sources tree and the openssl command.  The following is an example to
 generate the public/private key files:
 
 	openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
-	   -config x509.genkey -outform DER -out signing_key.x509 \
-	   -keyout signing_key.priv
+	   -config x509.genkey -outform PEM -out kernel_key.pem \
+	   -keyout kernel_key.pem
+
+The full pathname for the resulting kernel_key.pem file can then be specified
+in the CONFIG_MODULE_SIG_KEY option, and the certificate and key therein will
+be used instead of an autogenerated keypair.
 
 
 =========================
@@ -181,7 +202,7 @@ To manually sign a module, use the scripts/sign-file tool available in
 the Linux kernel source tree.  The script requires 4 arguments:
 
 	1.  The hash algorithm (e.g., sha256)
-	2.  The private key filename
+	2.  The private key filename or PKCS#11 URI
 	3.  The public key filename
 	4.  The kernel module to be signed
 

Makefile

@@ -870,7 +870,7 @@ INITRD_COMPRESS-$(CONFIG_RD_LZ4)   := lz4
 # export INITRD_COMPRESS := $(INITRD_COMPRESS-y)
 
 ifdef CONFIG_MODULE_SIG_ALL
-MODSECKEY = ./signing_key.priv
+MODSECKEY = $(CONFIG_MODULE_SIG_KEY)
 MODPUBKEY = ./signing_key.x509
 export MODPUBKEY
 mod_sign_cmd = scripts/sign-file $(CONFIG_MODULE_SIG_HASH) $(MODSECKEY) $(MODPUBKEY)

init/Kconfig

@@ -1948,6 +1948,20 @@ config MODULE_SIG_HASH
 	default "sha384" if MODULE_SIG_SHA384
 	default "sha512" if MODULE_SIG_SHA512
 
+config MODULE_SIG_KEY
+	string "File name or PKCS#11 URI of module signing key"
+	default "signing_key.priv"
+	depends on MODULE_SIG
+	help
+         Provide the file name of a private key in PKCS#8 PEM format, or
+         a PKCS#11 URI according to RFC7512. The corresponding X.509
+         certificate in DER form should be present in signing_key.x509
+         in the top-level build directory.
+
+         If this option is unchanged from its default "signing_key.priv",
+         then the kernel will automatically generate the private key and
+         certificate as described in Documentation/module-signing.txt
+
 config MODULE_COMPRESS
 	bool "Compress modules on installation"
 	depends on MODULES

kernel/Makefile

@@ -170,6 +170,10 @@ ifndef CONFIG_MODULE_SIG_HASH
 $(error Could not determine digest type to use from kernel config)
 endif
 
+# We do it this way rather than having a boolean option for enabling an
+# external private key, because 'make randconfig' might enable such a
+# boolean option and we unfortunately can't make it depend on !RANDCONFIG.
+ifeq ($(CONFIG_MODULE_SIG_KEY),"signing_key.priv")
 signing_key.priv signing_key.x509: x509.genkey
 	@echo "###"
 	@echo "### Now generating an X.509 key pair to be used for signing modules."
@@ -207,3 +211,4 @@ x509.genkey:
 	@echo >>x509.genkey "subjectKeyIdentifier=hash"
 	@echo >>x509.genkey "authorityKeyIdentifier=keyid"
 endif
+endif