certs: Trigger creation of RSA module signing key if it's not an RSA key

Address a kbuild issue where a developer created an ECDSA key for signing
kernel modules and then builds an older version of the kernel, when bi-
secting the kernel for example, that does not support ECDSA keys.

If openssl is installed, trigger the creation of an RSA module signing
key if it is not an RSA key.

Fixes: cfc411e ("Move certificate handling to its own directory")
Cc: David Howells <[email protected]>
Cc: David Woodhouse <[email protected]>
Signed-off-by: Stefan Berger <[email protected]>
Reviewed-by: Jarkko Sakkinen <[email protected]>
Tested-by: Jarkko Sakkinen <[email protected]>
Signed-off-by: Jarkko Sakkinen <[email protected]>

Author: stefanberger

certs/Makefile

@@ -57,11 +57,19 @@ endif
 redirect_openssl	= 2>&1
 quiet_redirect_openssl	= 2>&1
 silent_redirect_openssl = 2>/dev/null
+openssl_available       = $(shell openssl help 2>/dev/null && echo yes)
 
 # 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 ($(CONFIG_MODULE_SIG_KEY),"certs/signing_key.pem")
+
+ifeq ($(openssl_available),yes)
+X509TEXT=$(shell openssl x509 -in "certs/signing_key.pem" -text 2>/dev/null)
+
+$(if $(findstring rsaEncryption,$(X509TEXT)),,$(shell rm -f "certs/signing_key.pem"))
+endif
+
 $(obj)/signing_key.pem: $(obj)/x509.genkey
 	@$(kecho) "###"
 	@$(kecho) "### Now generating an X.509 key pair to be used for signing modules."