MODSIGN: Use the same digest for the autogen key sig as for the module sig

Use the same digest type for the autogenerated key signature as for the module
signature so that the hash algorithm is guaranteed to be present in the kernel.

Without this, the X.509 certificate loader may reject the X.509 certificate so
generated because it was self-signed and the signature will be checked against
itself - but this won't work if the digest algorithm must be loaded as a
module.

The symptom is that the key fails to load with the following message emitted
into the kernel log:

MODSIGN: Problem loading in-kernel X.509 certificate (-65)

the error in brackets being -ENOPKG. What you should see is something like:

MODSIGN: Loaded cert 'Magarathea: Glacier signing key: 9588321144239a119d3406d4c4cf1fbae1836fa0'

Note that this doesn't apply to certificates that are not self-signed as we
don't check those currently as they require the parent CA certificate to be
available.

Reported-by: Rusty Russell <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Rusty Russell <[e-mail]>

kernel/Makefile

···
 # fail and that the kernel may be used afterwards.
 #
 ###############################################################################
+sign_key_with_hash :=
+ifeq ($(CONFIG_MODULE_SIG_SHA1),y)
+sign_key_with_hash := -sha1
+endif
+ifeq ($(CONFIG_MODULE_SIG_SHA224),y)
+sign_key_with_hash := -sha224
+endif
+ifeq ($(CONFIG_MODULE_SIG_SHA256),y)
+sign_key_with_hash := -sha256
+endif
+ifeq ($(CONFIG_MODULE_SIG_SHA384),y)
+sign_key_with_hash := -sha384
+endif
+ifeq ($(CONFIG_MODULE_SIG_SHA512),y)
+sign_key_with_hash := -sha512
+endif
+ifeq ($(sign_key_with_hash),)
+$(error Could not determine digest type to use from kernel config)
+endif
+
 signing_key.priv signing_key.x509: x509.genkey
 	@echo "###"
 	@echo "### Now generating an X.509 key pair to be used for signing modules."
···
 	@echo "###"
 	@echo "###     rngd -r /dev/hwrandom"
 	@echo "###"
-	openssl req -new -nodes -utf8 -sha1 -days 36500 -batch \
+	openssl req -new -nodes -utf8 $(sign_key_with_hash) -days 36500 -batch \
 		-x509 -config x509.genkey \
 		-outform DER -out signing_key.x509 \
 		-keyout signing_key.priv